Vitamins and Coenzymes Vitamins are compounds that are required in the diet either because the organism cannot synthesize them or because the rate of usage by the organism typically exceeds the rate of synthesis of the compound In nearly all cases only very small amounts of these compounds are required Vitamins are generally classed as either water soluble or fat soluble The watersoluble vitamins generally act as precursors to coenzymes the functions of the fatsoluble vitamins are more diverse and less easily categorized The water soluble vitamins are readily excreted in the urine toxicity as a result of overdose is therefore rare However with few exceptions the water soluble vitamins are not stored in large amounts and therefore must be continually supplied in the diet In contrast the fat soluble vitamins are less readily excreted and are deleterious and possibly lethal in high doses Many of the fat soluble vitamins are stored for example most well nourished individuals have a three month supply of vitamin D Water soluble vitamins The water soluble vitamins include the B complex vitamins the actual B vitamins biotin and folic acid and vitamin C First we will look at three classes of vitamin derived coenzymes used to carry electrons the nicotinamide coenzymes the flavin coenzymes and ascorbic acid Vitamin B3 niacin Niacin is the name for both nicotinamide and nicotinic acid either of which can act as a precursor of nicotinamide coenzymes Niacin is required for the synthesis of two coenzyme molecules NAD and NADP Note the phosphate attached to the 2 position of the lower ribose ring in NADP which is the only difference between the molecules O O NH2 NH2 N CH2 O O O OH N OH NH2 N O P O Niacin nicotinic acid Vitamin B3 Niacin nicotinamide Vitamin B3 N O N O P O CH2 O O OH N N OH Nicotinamide adenine dinucleotide NAD Copyright 2000 2011 Mark Brandt Ph D 36 OH OH NH2 O N CH2 O NH2 O O P O OH N N O N O P O CH2 O O OH N O O P O Nicotinamide adenine dinucleotide 2 phosphate NADP O Humans can synthesize nicotinamide cofactors from tryptophan However the process is somewhat inefficient synthesis of 1 mg of niacin requires 60 mg of tryptophan Niacin deficiency therefore is usually the result of a diet deficient in both niacin and tryptophan However some diets contain tryptophan or niacin in a biologically unavailable form In corn the niacin is poorly absorbed unless the corn is treated with alkali prior to ingestion In the rural south of the early 20th century this preparation step was largely ignored the symptoms of the H resulting pellegra niacin deficiency such as sun sensitivity O N NH2 and dementia led to the pejorative term red neck for individuals from this region of the US Pellegra is also observed in high sorghum diets sorghum contains niacin synthesis inhibitors or in some individuals taking isoniazid isoniazid is Isoniazid an antibiotic used to treat tuberculosis but also inhibits niacin N uptake and synthesis Nicotinic acid reduces release of free fatty acids from adipose tissue and has been used to reduce plasma cholesterol nicotinamide is inactive for this purpose However some individuals cannot tolerate the level of nicotinic acid required Niacin is required for the synthesis of two coenzyme molecules NAD and NADP Note the 2 phosphate attached to the lower ribose ring in NADP which is the only structural difference between the molecules NAD and NADP act as soluble electron carriers between proteins In effect these compounds are substrates for enzymes involved in oxidation and reduction reactions NAD is primarily involved in catabolic reactions NAD accepts electrons during the breakdown of molecules for energy In contrast NADPH the reduced form of NADP is primarily involved in biosynthetic reactions it donates electrons required for synthesizing new molecules In most cells NAD levels are much higher than NADH levels while NADPH levels are much higher than those of NADP The two possible electronic states for the nicotinamide cofactors are shown below H 2 electrons 1 proton O H H NH2 NH2 N R Oxidized NAD P O N 2 electrons 1 proton R Reduced NAD P H The oxidized forms of both nicotinamide coenzymes can only accept electrons in pairs The reduced forms of the coenzymes can only donate pairs of electrons Note the two changes in the ring during the reduction The addition of the electron Copyright 2000 2011 Mark Brandt Ph D 37 pair is accomplished by the addition of a hydride ion to the carbon para to the pyridine nitrogen and results in the loss of the positive charge on the ring Nicotinic acid was first synthesized chemically in 1867 from nicotine O HNO3 N Nicotine OH CH3 N N Nicotinic acid The name niacin was introduced to remove the association with nicotine and tobacco Alcohol Dehydrogenase An example of the role of NAD in redox chemistry is provided by the oxidoreductase enzyme liver alcohol dehydrogenase The name of the enzyme includes the tissue of origin and the substrate The word dehydrogenase is an indication of the fact that the enzyme catalyzes an oxidation reduction reaction Dehydrogenase means catalyzes hydrogen removal Alcohol dehydrogenase can catalyze the oxidation of several different alcohols In each case it uses NAD as the electron acceptor The active site is thus moderately non specific for the alcohol although it is quite specific for NAD compared to NADP In the absence of substrate the alcohol dehydrogenase active site is occupied by water molecules Note the zinc ion a metal ion cofactor that is required for catalytic activity alcohol dehydrogenase actually binds two zinc ions but the other is thought to have an exclusively structural role The zinc is bound to three enzyme side chains two cysteine residues and a histidine residue His67 HN Cys46 S Cys174 N His51 Ser48 NH OH S N Zn O H H Binding of substrate causes a conformational change that excludes water from the active site and that positions the substrates in preparation for catalysis When the substrate binds the zinc ion coordinates i e binds to the alcohol oxygen This bond Copyright 2000 2011 Mark Brandt Ph D 38 between the zinc ion and the substrate assists in stabilizing the negative charge that will develop on the substrate oxygen to put this in familiar terms in the enzyme active site the alcohol hydroxyl group pKa decreases from 18 to 6 4 The His51 indirectly removes a proton from the alcohol This process involves a chain of proton removals the histidine removes a proton from the NAD ribose the NAD