Vitamins Are Often Precursors to Coenzymes - Biochemistry - NCBI Bookshelf
These can be organic vitamins, sugars, lipids, or inorganic metal ions. However, unlike coenzymes or cofactors, these groups bind very tightly or covalently to an. coenzyme, without which the enzyme would . Other sources quote Figure 9- 14 Relation Between Extraction Rate and Proportion of Total Vitamins of the. View Vitamin-Coenzyme Relationship from BIS at University of California, Davis. BIS Vitamin - Co-enzyme Relationships Many (but not all) co- enzymes.
Studies of the thermal stability of synthetic polypeptides have been especially informative. Hydroxyproline stabilizes the collagen triple helix by forming interstrand hydrogen bonds.
The abnormal fibers formed by insufficiently hydroxylated collagen contribute to the skin lesions and blood-vessel fragility seen in scurvy. Vitamin A retinol is the precursor of retinal, the light-sensitive group in rhodopsin and other visual pigments Section A deficiency of this vitamin leads to night blindness. In addition, young animals require vitamin A for growth. Retinoic acid, which contains a terminal carboxylate in place of the alcohol terminus of retinol, serves as a signal molecule and activates the transcription of specific genes that mediate growth and development Section A metabolite of vitamin D is a hormone that regulates the metabolism of calcium and phosphorus.
A deficiency in vitamin D impairs bone formation in growing animals. This vitamin reacts with and neutralizes reactive oxygen species such as hydroxyl, radicals before they can oxidize unsaturated membrane lipids, damaging cell structures.
Structures of Some Fat-Soluble Vitamins. Such fat is extremely harmful, because the body is not able to process it; even so, a certain amount of natural fat in the diet can be highly beneficial.
This is true in large part because fat can serve as a medium for the fat-soluble vitamins A, D, E, and K, which are deposited in the body's fat cells. But as we noted earlier, it is important not to overdose on fat-soluble vitamins, because then what is inherently healthy can become extremely unhealthy. In the Dutch explorer Willem Barents and his shipwrecked crew spent a grueling winter on the island of Novaya Zemlya in the Arctic Ocean north of Russia.
They had sailed from Holland in search of the Northeast Passagewhich, like the more famous Northwest Passage above Canadaoffered the prospect of a short, relatively direct sea route from Europe to Asia and the Americas. The problem was that the ice made sailing the northern seas virtually impossible. It would be almost three centuries before a crew managed to negotiate the Northeast Passageby which time the European powers had long since given up all hopes of using it as a viable sailing route.
The same was true of the Northwest Passagewhich was not traversed until Barents and his men knew none of that, nor would they have cared in that miserable winter of All they cared about was survival, the chances for which seemed slim—and not just because of the almost inhuman cold or the fact that their ship had been cracked to pieces by the ice.
Men were dying of scurvy, a vitamin- deficiency disease we discuss later in the context of vitamin C, as well as from the cold. Yet there were a few blessings, mainly in the form of available wood for fuel and animals for food. The men killed polar bears and ate their meat, and no doubt they were thankful just to stay alive.
They could not have guessed, however, that they were actually killing themselves with an overdose of vitamin A. In time, they began to experience the effects of vitamin A poisoning: When spring came, the men managed to make it off the island, but many of them—Barents included—never lived to see Holland again, in part because the side effects of vitamin A toxicity had weakened them. So why take vitamin A at all?
Because it is necessary for proper growth of bones and teeth, for the maintenance and functioning of skin and mucous membranes, and for the ability to see in dim light. There is some evidence that it can help prevent cataracts a clouding of the lens in the eye and cardiovascular disease, a condition of the heart and circulatory system. Furthermore, when taken at the onset of a cold, vitamin A can ward off the illness and fight its symptoms. One of the first signs of vitamin A deficiency is " night blindness ," in which the rods of the eye necessary for night vision fail to function normally.
Extreme cases of vitamin A deficiency can lead to total blindness. Other symptoms include dry and scaly skin, problems with the mucous linings of the digestive tract and urinary system, and abnormal growth of teeth and bones.
The bodies of healthy adults who have an adequate diet can store several years' supply of this vitamin, but young children, who have not had time to build up such a large reserve, suffer from deprivation much more quickly if they do not consume enough. Vitamin A is present in meats mainly liverfish oil, egg yolks, butter, and cheese. Although plants do not have vitamin A, dark green leafy vegetables and yellow fruits and vegetables e.
It is nearly impossible to ingest beta-carotene in toxic amounts, unlike vitamin A from animal sources, since the body will not convert excess amounts to toxic levels of vitamin A. Vitamin D is actually two different substances, D2 and D3.
There was no D1, since the substance designated thus at one time turned out to be a mixture of several compounds, including calciferol, or D2 Both forms of vitamin D are activated, or made effective, by sunlight, and for this reason vitamin D often is called the sunshine vitamin. It is hard to suffer a vitamin D deficiency if one gets enough sunshine in combination with consuming such foods as eggs specifically, the yolksuch fatty fish as salmon, and enriched milk.
Milk does not naturally contain vitamin D, but the vitamin is sometimes included as an additive. Vitamin D lets the body utilize calcium and phosphorus in bone and tooth formation, and a deficiency causes a bone disease called rickets.
Under the influence of this physically debilitating and disfiguring disease, legs become bowed by the weight of the body, and the wrists and ankles thicken. The teeth are badly affected and, for a young child, take much longer to mature. Infants and children are most likely to suffer the effects of rickets, but since all milk and infant formulas have vitamin D added to them, the condition is seen rarely in the industrialized world today.
In the brutal early days of the Industrial Revolutionhowever i. Whereas rickets primarily affects children, adults may suffer from a disease called osteomalacia, caused by a deficiency of vitamin D, calcium, and phosphorous.
Sometimes seen in the Middle East and other parts of Asia, osteomalacia brings with it rheumatic pain and causes the bones to become soft and deformed. As with rickets, the treatment for osteomalacia is a combination of calcium, phosphorous, and vitamin D. On the other hand, as with all fat-soluble vitamins, a person may take in excessive amounts of vitamin D, which has its own ill effects: Damage to the kidneys and blood vessels also can occur as calcium deposits build up in these tissues.
Composed of at least seven similar chemicals called the tocopherols, vitamin E is found in green leafy vegetables, wheat germ and other plant oils, egg yolks, and meat. The main function of this vitamin is to act as an antioxidant, to counteract the harmful effects oxygen can have on tissues. It may seem strange to speak of oxygen causing harm, since it is essential to life, but oxidation is an extremely powerful chemical reaction that, under various conditions, can manifest as rotting or putrefaction, rusting, or even combustion and explosion.
When an apple turns brown a few minutes after you have cut it open, it is the result of oxidation. Oxidation also may be linked to the effects of aging in humans as well as to other conditions, such as cancer, hardening of the arteries, and rheumatoid arthritis.
It appears that oxygen molecules, which draw electrons to them, extract these electrons from the membranes in human cells. Over time, this can cause a gradual breakdown in the body's immune system.
Antioxidants, such as vitamin E or beta carotene, therefore may be important in preserving human health and well-being.
Vitamin E is particularly important for counteracting oxidation in fats. When they are oxidized, fats form a highly reactive substance called peroxide, which is often very damaging to cells. Vitamin E is more reactive i. Because cell membranes are composed partly of fat molecules, vitamin E is vitally important in maintaining the nervous, circulatory, and reproductive systems and in protecting the kidneys, lungs, and liver. Because vitamin E is so common in foods, it is very difficult to suffer from a deficiency of this vitamin unless a person avoids consuming fats altogether—another example of why a no-fat diet is not a healthy one.
The effects of vitamin E deficiency, all of which are apparently linked to the loss of its antioxidant protection, include cramping in the legs, fibrocystic breast disease a condition that involves the formation of lumps and cysts in the breastsand even muscular dystrophy. The seriousness of the latter two diseases only serves to highlight the importance of vitamin E to the body. Like vitamin D, vitamin K is composed of two groups of compounds, vitamins K1 and K2. There is also a substance called K3, but this vitamin is actually menadione, a synthetic compound from which the other forms of K are derived.
You can find vitamin K in many plants, especially green leafy ones such as spinach, and in liver. Vitamin K is also made by the bacteria that live in the intestine—the "good" bacteria that help make possible the processing of food through the body.
Coenzyme - Definition, Function and Examples | Biology Dictionary
Vitamin K appears to be critical to blood clottingthanks to its role in assisting the formation of a chemical called prothrombin in the liver. Deficiencies of this vitamin rarely occur as the result of an incomplete diet; instead, it is usually a consequence of liver damage and the blood's inability to process the vitamin.
The deficiency manifests in unusual bleeding or large bruises under the skin or in the muscles. Adults in the West seldom experience vitamin K deficiencies, but newborn infants have been known to suffer from brain hemorrhage owing to a lack of this vitamin.
The two water-soluble vitamins, as we shall see, have played a major part in medical history. Actually, there are more than two water-soluble vitamins, because vitamin B is really a complex of about a dozen vitamins—hence, the name B complex. Among them are vitamin B1 thiaminevitamin B2 riboflavinvitamin B6 pyridoxineand vitamin B12 cobalamin. A few others—for example, niacin vitamin B7 and pantothenic acid vitamin B3are known better by names other than their "B names," while biotin and folate, or folic acid, are not known by "B names" at all.
Vitamin B1, present in whole grains, nuts, legumes e. More than 4, years ago, the Chinese described a disease we know today as beriberi, which affects the nervous and gastrointestinal systems and causes nausea, fatigue, and mental confusion. The cause of beriberi is a deficiency of thiamine, or B1, found in the husks or bran of rice and grains. White rice, which most people find more pleasing to the palate than brown rice, is the result of a milling and polishing process in which the husks—and along with them, this important nutrient—are removed.
Manufacturers today produce "enriched" rice, flour, and other grain products by adding thiamine back in, but until scientists discovered the importance of thiamin in grain husks, many people, especially in the Far East, suffered the effects of beriberi. Early research on beriberi will be discussed later.
Vitamin B2 helps the body release energy from fats, proteins, and carbohydrates. It can be obtained from whole grains, organ meats e. Lack of this vitamin causes severe skin problems. Vitamin B6 is important in the building of body tissue as well as in protein metabolism and the synthesis of hemoglobin an iron-containing pigment in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide.
A deficiency can cause depression, nausea, and vomiting. Vitamin B12 is necessary for the proper functioning of the nervous system and in the formation of red blood cells. It can be obtained from meat, fish, and dairy products. Anemia a lack of red blood cells, which produces a lethargic conditionnervousness, fatigue, and even brain degeneration, can result from vitamin B12 deficiency.
Niacin is also highly important to human health, as we explain later in the context of the disease pellagra. Pantothenic acid helps release energy from fats and carbohydrates and is found in large quantities in egg yolks, liver, eggs, nuts, and whole grains.
Deficiency of this vitamin causes anemia. Biotin, widely available from grains, legumes, and liver, plays a part in the release of energy from carbohydrates and in the formation of fatty acids. A lack of biotin causes dermatitis, or skin inflammation. The American chemist and peace activist Linus Paulingwinner of the Nobel Prize in chemistry and peacehelped popularize vitamin C, also known as ascorbic acid.
It was Pauling who originated the idea, now widespread in society, that massive doses of vitamin C can ward off the common cold. Pauling went further, by maintaining that vitamin C offers protection against some forms of cancer.
While scientific studies have been unable to prove this theory, they do suggest that the vitamin can at least reduce the severity of the symptoms associated with colds. Most animals can synthesize this vitamin in the liver, where glucose a type of sugar that occurs widely in nature is converted to ascorbic acid. This is not the case with at least four types of animal: Citrus fruits, berries, and some vegetables e. It is a fragile vitamin, one that is oxidized or destroyed easily.
Food storage or food processing can render it ineffective; so, too, can soaking vitamin C-containing fruits and vegetables in water for long periods. Vitamin Deficiencies and History Most of the early history in the study of vitamins centered around what are now known as the water-soluble vitamins. Although vitamins as such were not discovered until early in the twentieth century, it was common knowledge long before that time that substances in certain foods were necessary for good health.
An important turning point came in the mid-eighteenth century, with the work of the Scottish physician James Lind on a vitamin deficiency condition that jeopardized England's vast merchant and military navies. At a time when England had emerged as the world's leading sea power, even Her Majesty's sailing crews were at the mercy of a condition known as scurvy. Common among crews who had been at sea too long, scurvy could result in swollen joints, bleeding gums, loose teeth, and an inability to recover from wounds.
Scientists today recognize scurvy as resulting from a deficiency of vitamin C, available in such citrus fruits as oranges. At the time, however, the concept of vitamins was unknown, and sailors at sea continued to live on a diet that consisted primarily of salted meats and hard biscuits—items that could be stored easily without spoilage in an era before refrigeration. InLind, a ship's doctor, observed that 80 of seamen aboard his ship came down with scurvy during a week cruise.
Conducting a controlled experiment, he took 12 of the sailors in whom scurvy had developed and divided them into six groups. He gave each pair different substances, such as nutmeg, cider, seawater, and vinegar; the final pair was given lemons or oranges. The two men given the oranges and lemons both completely recovered in about a week. Not only was this a milestone in the history of vitamin research, but it also was the first example of a clinical trial, or the testing of a medication by careful and well-documented experimentation in which other variables or factors are kept unchanged.
It would be another half-century before the British navy adopted Lind's techniques. Another Scottish physician, Sir Gilbert Blanehad long fought for the adoption of Lind's methods, and finally, inhe persuaded the navy to give each sailor a daily ration of lemons. At that time, the term lime was common for both lemons and limes, and, as a result, British sailors became known as limeys.B Vitamins: function in two ways as co-enzymes
Eventually, the treatment spread to the population as a whole, but outbreaks of scurvy continued until after World War Iwhen doctors isolated vitamin C as the controlling factor in scurvy prevention.