Учебно-методический комплекс дисциплины " Basis of biochemistry " Для специальности

Function of Enzymes

Biological Functions of Enzymes:

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  Enzymes perform a wide variety of functions in living organisms. 
  
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  They are major components in signal transduction and cell regulation, kinases and phosphatases help in this function. 
  
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  They take part in movement with the help of the protein myosin which aids in muscle contraction.
  
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  Also other ATPases in the cell membrane acts as ion pumps in active transport mechanism. 
  
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  Enzymes present in the viruses are for infecting cell. 
  
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  Enzymes play a important role in the digestive activity of the enzymes. 
  
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  Amylases and proteases are enzyme sthat breakdown large molecules into absorbable molecules. 
  
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  Variuos enzymes owrk together in a order forming metabolic pathways. Example: Glycolysis. 
  

Industrial Application of Enzymes: 

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  Food Processing - Amylases enzymes from fungi and plants are used in production of sugars from starch in making corn-syrup.
  
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  Catalyze enzyme is used in breakdown of starch into sugar, and in baking fermentation process of yeast raises the dough.
  
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  Proteases enzyme help in manufacture of biscuits in lowering the protein level. 
  
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  Baby foods - Trypsin enzyme is used in pre-digestion of baby foods. 
  
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  Brewing industry - Enzymes from barley are widely used in brewing industries. 
  
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  Amylases, glucanases, proteases, betaglucanases, arabinoxylases, amyloglucosidase, acetolactatedecarboxylases are used in prodcution of beer industries. 
  
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  Fruit juices - Enzymes like cellulases,pectinases help are used in clarifying fruit juices. 
  
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  Dairy Industry - Renin is used inmanufacture of cheese. Lipases are used in ripening blue-mold cheese. Lactases breaks down lactose to glucose and galactose. 
  
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  Meat Tenderizes - Papain is used to soften meat. 
  
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  Starch Industry - Amylases, amyloglucosidases and glycoamylases converts starch into glucose and syrups. 
  
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  Glucose isomerases - production enhanced sweetening properties and lowering calorific values.
  
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  Paper industry - Enzymes like amylases, xylanases, cellulases and liginases lower the viscosity, and removes lignin to soften paper. 
  
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  Biofuel Industry - Enzymes like cellulases are used in breakdown of cellulose into sugars which can be fermented.
  
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  Biological detergent - proteases, amylases, lipases, cellulases, asist in removal of protein stains, oily  stains and acts as fabric conditioners. 
  
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  Rubber Industry - Catalase enzyme converts latex into foam rubber. 
  
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  Molecular Biology - Restriction enzymes, DNA ligase and polymerases are used in genetic engineering, pharmacology, agriculture, medicine, PCR techniques, and are also important in forensic science.
  

Examples of Enzymes

A few well known examples of enzymes are as follows: Lipases, Amylases, Maltases, Pepsin, Protease, Catalases, Maltase, Sucrase, Pepsin, Renin, Catalases, 

A few examples of foods that are rich in enzymes:

Enzymes are available in the food we eat. Foods that are canned, or processed food like irradiation,drying, and freezing make the foods enzyme dead. Refined foods are void of any sort of nutrition. Food that is whole, uncooked and unpasteurized milk will provide enough enzymes. There are two basic ways to increase enzyme intake. First is to eat more fresh foods, cooking tends to kill enzymes. Raw fruits and vegetables are a good source of enzymes. Fermented food like yoghurt, intake improves body's enzyme status. The other way to increase enzyme status of the body is by intake of enzyme supplements.

Here is a list of foods rich in enzymes - Apples, apricots, asparagus, avocado, banana, beans, beets, broccoli, cabbage, carrots, celery, cherries, cucumber, figs, garlic, ginger, grapes, green barley grass,kiwi fruit, etc.

Modul 5. Vitamins

The definition, constitution and classification of vitamins and their role in enzymatic reactions and in exchange processes.
Vitamins are natural substances found in plants and animals and known as Essential nutrients for human beings. The name vitamin is obtained from "vital amines" as it was originally thought that these substances were all amines. Human body uses these substances to stay healthy and support its many functions. There are two types of vitamins: water-soluble and fat-soluble.
The body needs vitamins to stay healthy and a varied diet usually gives you all the vitamins you need. Vitamins do not provide energy (calories) directly, but they do help regulate energy-producing processes. With the exception of vitamin D and K, vitamins cannot be synthesized by the human body and must be obtained from the diet. Vitamins have to come from food because they are not manufactured or formed by the body.

There are several roles for vitamins and trace minerals in diseases:

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  Deficiencies of vitamins and minerals may be caused by disease states such as mal absorption;
  
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  Deficiency and excess of vitamins and minerals can cause disease in and of themselves (e.g., vitamin A intoxication and liver disease);
  
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  Vitamins and minerals in high doses may be used as drugs (e.g., niacin for hypercholesterolemia).
  

Vitamins are essential for the normal growth and development of a multi-cellular organism. The developing fetus requires certain vitamins and minerals to be present at certain times. If there is serious deficiency in one or more of these nutrients, a child may develop a deficiency disease. Deficiencies of vitamins are classified as either primary or secondary.

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  Primary Deficiency: A primary deficiency occurs when you do not get enough of the vitamin in the food you eat.
  
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  Secondary Deficiency: A secondary deficiency may be due to an underlying disorder that prevents or limits the absorption or use of the vitamin.
  

Types of Vitamins

Vitamins, one of the most essential nutrients required by the body and can be broadly classified into two main categories i.e., water-soluble vitamins and fat-soluble vitamins.

Water-Soluble Vitamins

B-complex Vitamins

Eight of the water-soluble vitamins are known as the vitamin B-complex group: thiamin (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), vitamin B6 (pyridoxine), folate (folic acid), vitamin B12, biotin and pantothenic acid. The B vitamins are widely distributed in foods,and their influence is felt in many parts of the body. They function as coenzymes that help the body obtain energy from food. The B vitamins are also important for normal appetite, good vision, and healthy skin, nervous system, and red blood cell formation.

Thiamin: Vitamin B1

What is Thiamin. Thiamin, or vitamin B1, helps to release energy from foods, promotes normal appetite, and is important in maintaining proper nervous system function.

Food Sources for Thiamin. Sources include peas, pork, liver, and legumes. Most commonly, thiamin is found in whole grains and fortified grain products such as cereal, and enriched products like bread, pasta, rice, and tortillas. The process of enrichment adds back nutrients that are lost when grains are processed. Among the nutrients added during the enrichment process are thiamin (B1), niacin (B3), riboflavin (B2), folate and iron.

Thiamin Deficiency. Under-consumption of thiamin is rare in the United States due to wide availability of enriched grain products. However, low calorie diets as well as diets high in refined and processed carbohydrates may place one at risk for thiamin deficiency. Alcoholics are especially prone to thiamin deficiency because excess alcohol consumption often replaces food or meals. Symptoms of thiamin deficiency include: mental confusion, muscle weakness, wasting, water retention (edema), impaired growth, and the disease known as beriberi. Thiamin deficiency is currently not a problem in the United States.

Too much Thiamin. No problems with overconsumption are known for thiamin.

Riboflavin: Vitamin B2

What is Riboflavin. Riboflavin, or vitamin B2, helps to release energy from foods, promotes good vision, and healthy skin. It also helps to convert the amino acid tryptophan (which makes up protein) into niacin.

Food Sources for Riboflavin. Sources include liver, eggs, dark green vegetables, legumes, whole and enriched grain products, and milk. Ultraviolet light is known to destroy riboflavin, which is why most milk is packaged in opaque containers instead of clear.

Riboflavin Deficiency. Under consumption of riboflavin is rare in the United States. However, it has been known to occur with alcoholism, malignancy, hyperthyroidism, and in the elderly. Symptoms of deficiency include cracks at the corners of the mouth, dermatitis on nose and lips, light sensitivity, cataracts, and a sore, red tongue.

Too much Riboflavin. No problems with overconsumption are known for riboflavin.

Niacin: Vitamin B3, Nicotinamide, Nicotinic Acid.

What is Niacin. Niacin, or vitamin B3, is involved in energy production, normal enzyme function, digestion, promoting normal appetite, healthy skin, and nerves.

Food Sources for Niacin. Sources include liver, fish, poultry, meat, peanuts, whole and enriched grain products.

Niacin Deficiency. Niacin deficiency is not a problem in the United States. However, it is known to occur with alcoholism, protein malnourishment, low calorie diets, and diets high in refined carbohydrates. Pellagra is the disease state that occurs as a result of severe niacin deficiency. Symptoms include cramps, nausea, mental confusion, and skin problems.

Too much Niacin. Consuming large doses of niacin supplements may cause flushed skin, rashes, or liver damage. Over consumption of niacin is not a problem if it is obtained through food.

Vitamin B6: Pyridoxine, Pyridoxal, Pyridoxamine

What is Vitamin B6. Vitamin B6, otherwise known as pyridoxine, pyridoxal or pyridoxamine, aids in protein metabolism and red blood cell formation. It is also involved in the body’s production of chemicals such as insulin and hemoglobin.

Food Sources for Vitamin B6. Sources include pork, meats, whole grains and cereals, legumes, and green, leafy vegetables.

Vitamin B6 Deficiency. Deficiency symptoms include skin disorders, dermatitis, cracks at corners of mouth, anemia, kidney stones, and nausea. A vitamin B6 deficiency in infants can cause mental confusion.

Too much Vitamin B6. Over consumption is rare, but excess doses of vitamin B6 over time have been known to result in nerve damage.

Folate: Folic Acid, Folacin

What is Folate. Folate, also known as folic acid or folacin, aids in protein metabolism, promoting red blood cell formation, and lowering the risk for neural tube birth defects. Folate may also play a role in controlling homocysteine levels, thus reducing the risk for coronary heart disease.

Food Sources for Folate. Sources of folate include liver, kidney, dark green leafy vegetables, meats, fish, whole grains, fortified grains and cereals, legumes, and citrus fruits. Not all whole grain products are fortified with folate. Check the nutrition label to see if folic acid has been added.

Folate Deficiency. Folate deficiency affects cell growth and protein production, which can lead to overall impaired growth. Deficiency symptoms also include anemia and diarrhea. A folate deficiency in women who are pregnant or of child bearing age may result in the delivery of a baby with neural tube defects such as spina bifida.

Too much Folate. Over consumption of folate offers no known benefits, and may mask B12 deficiency as well as interfere with some medications.

Vitamin B12: Cobalamin

What is B12. Vitamin B12, also known as cobalamin, aids in the building of genetic material, production of normal red blood cells, and maintenance of the nervous system.

Food Sources for Vitamin B12. Vitamin B12 can only be found only in foods of animal origin such as meats, liver, kidney, fish, eggs, milk and milk products, oysters, shellfish. Some fortified foods may contain vitamin B12.

Vitamin B12 Deficiency. Vitamin B12 deficiency most commonly affects strict vegetarians (those who eat no animal products), infants of vegan mothers, and the elderly. Symptoms of deficiency include anemia, fatigue, neurological disorders, and degeneration of nerves resulting in numbness and tingling. In order to prevent vitamin B12 deficiency, a dietary supplement should be taken. Some people develop a B12 deficiency because they cannot absorb the vitamin through their stomach lining. This can be treated through vitamin B12 injections.

Too much Vitamin B12. No problems with overconsumption of vitamin B12 are known.

Biotin

What is Biotin. Biotin helps release energy from carbohydrates and aids in the metabolism of fats, proteins and carbohydrates from food.

Food Sources for Biotin. Sources of Biotin include liver, kidney, egg yolk, milk, most fresh vegetables, yeast breads and cereals. Biotin is also made by intestinal bacteria.

Biotin Deficiency. Biotin deficiency is uncommon under normal circumstances, but symptoms include fatigue, loss of appetite, nausea, vomiting, depression, muscle pains, heart abnormalities and anemia.

Too much Biotin. No problems with overconsumption are known for Biotin.

Pantothenic Acid

What is Pantothenic Acid. Pantothenic Acid is involved in energy production, and aids in the formation of hormones and the metabolism of fats, proteins, and carbohydrates from food.

Food Sources for Pantothenic Acid. Sources include liver, kidney, meats, egg yolk, whole grains, and legumes. Pantothenic Acid is also made by intestinal bacteria.

Pantothenic Acid Deficiency. Pantothenic Acid deficiency is uncommon due to its wide availability in most foods.

Too much Pantothenic Acid. No problems with overconsumption are known for Pantothenic Acid. Rarely, diarrhea and water retention will occur with excessive amounts.

Vitamin C: Ascorbic Acid, Ascorbate

What is Vitamin C

The body needs vitamin C, also known as ascorbic acid or ascorbate, to remain in proper working condition. Vitamin C benefits the body by holding cells together through collagen synthesis; collagen is a connective tissue that holds muscles, bones, and other tissues together. Vitamin C also aids in wound healing, bone and tooth formation, strengthening blood vessel walls, improving immune system function, increasing absorption and utilization of iron, and acting as an antioxidant.

Since our bodies cannot produce or store vitamin C, an adequate daily intake of this nutrient is essential for optimum health. Vitamin C works with vitamin E as an antioxidant, and plays a crucial role in neutralizing free radicals throughout the body. An antioxidant can be a vitamin, mineral, or a carotenoid, present in foods, that slows the oxidation process and acts to repair damage to cells of the body. Studies suggest that vitamin C may reduce the risk of certain cancers, heart disease, and cataracts. Research continues to document the degree of these effects.

Food Sources for Vitamin C. Consuming vitamin C-rich foods is the best method to ensure an adequate intake of this vitamin. While many common plant foods contain vitamin C, the best sources are citrus fruits. For example, one orange, a kiwi fruit, 6 oz. of grapefruit juice or 1/3 cup of chopped sweet red pepper each supply enough vitamin C for one day.

Vitamin C Deficiency. Although rare in the United States, severe vitamin C deficiency may result in the disease known as scurvy, causing a loss of collagen strength throughout the body. Loss of collagen results in loose teeth, bleeding and swollen gums, and improper wound healing. More commonly, vitamin C deficiency presents as a secondary deficiency in alcoholics, the elderly, and in smokers.

The following conditions have been shown to increase vitamin C requirements (Table 1):

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  Environmental stress, such as air and noise pollution
  
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  Use of certain drugs, such as oral contraceptives
  
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  Tissue healing of wounds
  
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  Growth (children from 0- 12 months, and pregnant women)
  
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  Fever and infection
  
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  Smoking.
  

Too Much Vitamin C. Despite being a water-soluble vitamin that the body excretes when in excess, vitamin C overdoses have been shown to cause kidney stones, gout, diarrhea, and rebound scurvy.

Fat-Soluble Vitamins

The fat-soluble vitamins, A, D, E, and K, are stored in the body for long periods of time and generally pose a greater risk for toxicity when consumed in excess than water-soluble vitamins. Eating a normal, well-balanced diet will not lead to toxicity in otherwise healthy individuals. However, taking vitamin supplements that contain megadoses of vitamins A, D, E and K may lead to toxicity. The body only needs small amounts of any vitamin.

While diseases caused by a lack of fat-soluble vitamins are rare in the United States, symptoms of mild deficiency can develop without adequate amounts of vitamins in the diet. Additionally, some health problems may decrease the absorption of fat, and in turn, decrease the absorption of vitamins A, D, E and K. Consult a medical professional about any potential health problems that may interfere with vitamin absorption.

Vitamin A: Retinol

What is Vitamin A

Vitamin A, also called retinol, has many functions in the body. In addition to helping the eyes adjust to light changes, vitamin A plays an important role in bone growth, tooth development, reproduction, cell division, gene expression, and regulation of the immune system. The skin, eyes, and mucous membranes of the mouth, nose, throat and lungs depend on vitamin A to remain moist. Vitamin A is also an important antioxidant that may play a role in the prevention of certain cancers.

Food Sources for Vitamin A

Eating a wide variety of foods is the best way to ensure that the body gets enough vitamin A. The retinol, retinal, and retinoic acid forms of vitamin A are supplied primarily by foods of animal origin such as dairy products, fish and liver. Some foods of plant origin contain the antioxidant, betacarotene, which the body converts to vitamin A. Beta-carotene, comes from fruits and vegetables, especially those that are orange or dark green in color. Vitamin A sources also include carrots, pumpkin, winter squash, dark green leafy vegetables and apricots, all of which are rich in beta-carotene.

Compared to vitamin A, it takes twice the amount of carotene rich foods to meet the body’s vitamin A requirements, so one may need to increase consumption of carotene containing plant foods.

Recent studies indicate that vitamin A requirements may be increased due to hyperthyroidism, fever, infection, cold, and exposure to excessive amounts of sunlight. Those that consume excess alcohol or have renal disease should also increase intake of vitamin A.

Vitamin A Deficiency

Vitamin A deficiency in the United States is rare, but the disease that results is known as xerophthalmia. It most commonly occurs in developing nations usually due to malnutrition. Since vitamin A is stored in the liver, it may take up to 2 years for signs of deficiency to appear. Night blindness and very dry, rough skin may indicate a lack of vitamin A. Other signs of possible vitamin A deficiency include decreased resistance to infections, faulty tooth development, and slower bone growth.

Too much Vitamin A

In the United States, toxic or excess levels of vitamin A are more of a concern than deficiencies. The Tolerable Upper Intake Level (UL) for adults is 3,000 mcg RAE (Table 2). It would be difficult to reach this level consuming food alone, but some multivitamin supplements contain high doses of vitamin A. If you take a multivitamin, check the label to be sure the majority of vitamin A provided is in the form of beta-carotene, which appears to be safe. Symptoms of vitamin A toxicity include dry, itchy skin, headache, nausea, and loss of appetite. Signs of severe overuse over a short period of time include dizziness, blurred vision and slowed growth. Vitamin A toxicity also can cause severe birth defects and may increase the risk for hip fractures.

Vitamin D

What is Vitamin D

Vitamin D plays a critical role in the body’s use of calcium and phosphorous. It works by increasing the amount of calcium absorbed from the small intestine, helping to form and maintain bones. Vitamin D benefits the body by playing a role in immunity and controlling cell growth. Children especially need adequate amounts of vitamin D to develop strong bones and healthy teeth.

Food Sources for Vitamin D

The primary food sources of vitamin D are milk and other dairy products fortified with vitamin D. Vitamin D is also found in oily fish (e.g., herring, salmon and sardines) as well as in cod liver oil. In addition to the vitamin D provided by food, we obtain vitamin D through our skin which produces vitamin D in response to sunlight.

Vitamin D Deficiency

Symptoms of vitamin D deficiency in growing children include rickets (long, soft bowed legs) and flattening of the back of the skull. Vitamin D deficiency in adults may result in osteomalacia (muscle and bone weakness), and osteoporosis (loss of bone mass).

Recently published data introduces a concern that some adults and children may be more prone to developing vitamin D deficiency due to an increase in sunscreen use. In addition, those that live in inner cities, wear clothing that covers most of the skin, or live in northern climates where little sun is seen in the winter are also prone to vitamin D deficiency. Since most foods have very low vitamin D levels (unless they are enriched) a deficiency may be more likely to develop without adequate exposure to sunlight. Adding fortified foods to the diet such as milk, and for adults including a supplement, are effective at ensuring adequate vitamin D intake and preventing low vitamin D levels.

Vitamin D deficiency has been associated with increased risk of common cancers, autoimmune diseases, hypertension, and infectious disease. In the absence of adequate sun exposure, at least 800 to 1,000 IU of vitamin D3 may be needed to reach the circulating level required to maximize vitamin D’s benefits.

Who is at Risk — These populations may require extra vitamin D in the form of supplements or fortified foods:

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  Exclusively breast-fed infants: Human milk only provides 25 IU of vitamin D per liter. All breast-fed and partially breast-fed infants should be given a vitamin D supplement of 400 IU/day
  
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  Dark Skin: Those with dark pigmented skin synthesize less vitamin D upon exposure to sunlight compared to those with light pigmented skin.
  
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  Elderly: This population has a reduced ability to synthesize vitamin D upon exposure to sunlight, and is also more likely to stay indoors and wear sunscreen which blocks vitamin D synthesis.
  
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  Covered and protected skin: Those that cover all of their skin with clothing while outside, and those that wear sunscreen with an SPF factor of 8, block most of the synthesis of vitamin D from sunlight.
  
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  Disease: Fat malabsorption syndromes, inflammatory bowel disease (IBD), and obesity are all known to result in a decreased ability to absorb and/or use vitamin D in fat stores.
  

Vitamin E: Tocopherol

Vitamin E benefits the body by acting as an antioxidant, and protecting vitamins A and C, red blood cells, and essential fatty acids from destruction. Research from decades ago suggested that taking antioxidant supplements, vitamin E in particular, might help prevent heart disease and cancer. However, newer findings indicate that people who take antioxidant and vitamin E supplements are not better protected against heart disease and cancer than non-supplement users. Many studies show a link between regularly eating an antioxidant rich diet full of fruits and vegetables, and a lower risk for heart disease, cancer, and several other diseases. Essentially, recent research indicates that to receive the full benefits of antioxidants and phytonutrients in the diet, one should consume these compounds in the form of fruits and vegetables, not as supplements.

Food Sources for Vitamin E

About 60 percent of vitamin E in the diet comes from vegetable oil (soybean, corn, cottonseed, and safflower). This also includes products made with vegetable oil (margarine and salad dressing). Vitamin E sources also include fruits and vegetables, grains, nuts (almonds and hazelnuts), seeds (sunflower) and fortified cereals.

Vitamin E Deficiency

Vitamin E deficiency is rare. Cases of vitamin E deficiency usually only occur in premature infants and in those unable to absorb fats. Since vegetable oils are good sources of vitamin E, people who excessively reduce their total dietary fat may not get enough vitamin E.

Too much Vitamin E

The Tolerable Upper Intake Level (UL) for vitamin E is shown in Table 2. Vitamin E obtained from food usually does not pose a risk for toxicity. Supplemental vitamin E is not recommended due to lack of evidence supporting any added health benefits. Megadoses of supplemental vitamin E may pose a hazard to people taking blood-thinning medications such as Coumadin (also known as warfarin) and those on statin drugs.

Vitamin K

What is Vitamin K

Vitamin K is naturally produced by the bacteria in the intestines, and plays an essential role in normal blood clotting, promoting bone health, and helping to produce proteins for blood, bones, and kidneys.

Food Sources for Vitamin K

Good food sources of vitamin K are green, leafy-vegetables such as turnip greens, spinach, cauliflower, cabbage and broccoli, and certain vegetables oils including soybean oil, cottonseed oil, canola oil and olive oil. Animal foods, in general, contain limited amounts of vitamin K.

Vitamin K Deficiency

Without sufficient amounts of vitamin K, hemorrhaging can occur. Vitamin K deficiency may appear in infants or in people who take anticoagulants, such as Coumadin (warfarin), or antibiotic drugs. Newborn babies lack the intestinal bacteria to produce vitamin K and need a supplement for the first week. Those on anticoagulant drugs (blood thinners) may become vitamin K deficient, but should not change their vitamin K intake without consulting a physician. People taking antibiotics may lack vitamin K temporarily because intestinal bacteria are sometimes killed as a result of long-term use of antibiotics. Also, people with chronic diarrhea may have problems absorbing sufficient amounts of vitamin K through the intestine and should consult their physician to determine if supplementation is necessary.

Too much Vitamin K

Although no Tolerable Upper Intake Level (UL) has been established for vitamin K, excessive amounts can cause the breakdown of red blood cells and liver damage. People taking blood-thinning drugs or anticoagulants should moderate their intake of foods with vitamin K, because excess vitamin K can alter blood clotting times. Large doses of vitamin K are not advised.
Modul 6.Carbohydrates

Classification of carbohydrates and their most important reactions. Disaccharides and polysaccharides: lactose, maltose, sucrose, starch, glycogen, cellulose, quinine. The role of carbohydrates in a food.

CARBOHYDRATES

  The carbohydrates, or sugars, are our third group of biomolecules. They are characterized by having a carbonyl carbon (aldehyde or ketone) and multiple hydroxyl groups. The smallest sugars are thus the three carbon trioses, glyceraldehyde (aldotriose) and dihydroxyacetone (ketotriose).



Note that sugars occur in both D and L forms. As we shall see the natural sugars are generally D.

CARBOHYDRATES, cont.

  Note that sugars occur in both D and L forms. As we shall see the natural sugars are generally D. Let's look at the two families, aldoses and ketoses. The important aldoses (Figure 8.3, p 234) [overhead 9.4 P] include the five carbon aldopentose, ribose:



which commonly occurs in the cyclic furanose form.The six carbon aldohexoses, glucose, mannose, and galactose.

 

which commonly occur in the cyclic pyranose form (as shown for glucose) [glucose model], and the six carbon ketohexose, fructose.



which commonly occurs in a cyclic furanose form. The important ketoses include dihydroxyacetone, D-Xylulose, D-Ribulose, and D-Fructose [overhead 9.7 P]Note the relationship between the Fischer projections and the cyclic Haworth projections, using the example of glucose.

The ring is then sealed via a hemiacetal bond. [overhead 9.10 P] This would normally be quite unstable, however the closeness of the two reacting centers in the same chain makes them poor leaving groups, thus the hemiacetal is in fact the stable form of the six carbon aldoses. Thus the expected aldehyde chemistry for glucose is not seen (glucose is stable to oxygen etc.).Note that if drawn in the proper conformations (Figure 8.11, p 239), or if constructed as models it will be seen that the chair conformation should be more stable. In addition, the beta configuration of the hemiacetal -OH will be equatorial and should thus be preferred steriochemically as is in fact the case. Interestingly organisms can generally only use the alpha form, so isomerases are provide to interchange the two.

An important reaction is the Lobry-de-Bruyn-van Ekenstein Transformation. This base catalyzed reaction sequence interconverts three of the major hexoses, and will be used later in understanding some isomerase enzyme mechanisms. The mechanism is symmetrical. You should finish the second half on your own.

 

DISACCHARIDES

Can link sugars via acetal bonds, known as glycosidic bonds.

There are four common disaccharides (Fig 8.20, p 244) [overhead 9.24, P]:

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  maltose [-D-Glucopyranosyl-(1,4)--D-glucopyranose]
  
• 
  cellobiose [-D-Glucopyranosyl-(1,4)--D-glucopyranose]
  
• 
  lactose [beta-D-Galactopyranosyl-(1,4)-beta-D-glucopyranose], and
  
• 
  sucrose [alpha-D-Glucopyranosyl-(1,2)-beta-D-fructofuranoside]
  

The first three are reducing sugars, that is they have "free" aldehyde groups, whereas sucrose has both carbonyl groups tied up in the relatively stable glycosidic bond. Maltose and fructose are joined in alpha-glycosidic bonds. In general the alpha-glycosidic bond is easily cleaved (it is less stable chemically and organisms have enzymes to cleave it) whereas the beta-glycosidic bond is very difficult to break down.

Thus for cellobiose, and more importantly, cellulose which is also linked by beta-bonds, essentially only bacteria can digest this bond.

So animals can't digest cellulose! You may ask, What about Cows and things? Well they use bacteria. Cows for instance are basically walking fermentation tanks.

Cool biological examples of cellulose use by animals: Desert Iguana consume feces to maintain culture; Rabbits eat and reprocess first pass feces (soft) to take advantage of fermentation; Multiple stomachs in Ruminants; Ultimate symbiosis in some termites: protozoans in gut have bacteria in gut, and use spirochetes as "cilia" (rowers).