Ascorbic acid vitamin hydroxylases

FIGURE 6.17 Hydroxylation of proUne residnes is catalyzed by prolyl hydroxylase. The reaction requires -ketoglntarate and ascorbic acid (vitamin C). 

Ascorbic acid (vitamin C fig. 10.16) is the reducing agent required to maintain the activity of a number of enzymes, most notably proline hydroxylase, which forms 4-hydroxyproline residues in collagen. Hydroxyproline (see fig. 10.16c) is not synthesized biologically as a free amino acid but rather is created by modification of proline residues already incorporated into collagen. The hydroxylation reaction occurs as the protein is synthesized in the endoplasmic reticulum. At least a third of the numerous proline residues in collagen are modified in this way, substantially increasing the resistance of the protein to thermal denaturation. 

Tyrosine is converted to dopa by the rate-limiting enzyme tyrosine hydroxylase, which requires tetrahydrobiopterin, and is inhibited by a-methyltyrosine. Dopa is decarboxylated to dopamine by L-aromatic amino acid decarboxylase, which requires pyridoxal phosphate (vitamin B6) as a coenzyme. Carbidopa, which is used with levodopa in the treatment of parkinsonism, inhibits this enzyme. Dopamine is converted to norepinephrine by dopamine P-hydroxylase, which requires ascorbic acid (vitamin C), and is inhibited by diethyldithiocarbamate. Norepinephrine is converted to epinephrine by phenylethanolamine A -methyltransferase (PNMT), requiring S-adeno-sylmethionine. The activity of PNMT is stimulated by corticosteroids. 

One-third of the amino acid residues in collagen are Gly, while another quarter are Pro. The hydroxylated amino acids 4-hydroxyproline (Hyp) and 5-hydroxylysine (Hyl) are formed post-translationally by the action of proline hydroxylase and lysine hydroxylase. These Fe2+-containing enzymes require ascorbic acid (vitamin C) for activity. In the vitamin C deficiency disease scurvy, collagen does not form correctly due to the inability to hydroxylate Pro and Lys. Hyl residues are often post-translationally modified with carbohydrate. 

Ascorbic acid (vitamin C adult RDA = 60 mg) 4g Ehlers-Danlos syndrome, type VI (ocular type) Procollagen lysyl hydroxylase... 

The next step in the catecholamine biosynthesis is side-chain hydroxylation of DA to NE. The enzyme dopamine (3-hydroxylase (DBH) catalyzes this reaction. This enzyme, like TH, is a mixed-function oxidase utilizing molecular 02, in this case to add the OH onto the (3-carbon of the phenelthylamine side chain. DBH is a Cu2+-containing enzyme that, with ascorbic acid (Vitamin C) as a cofactor, carries out the necessary electron transfers. 

Proline is incorporated into polypeptide precursors of collagen. In the procollagen polypeptide (precursor to the mature collagen), proline is converted to hydroxyproline by the enzyme procollagen proline hydroxylase (Figure 21.4). As seen in the figure, the reaction requires ascorbic acid (vitamin... 

Neurons that secrete norepinephrine synthesize it from dopamine in a hydroxylation reaction catalyzed by dopamine (3-hydroxylase (DBH). This enzyme is present only within the storage vesicles of these cells. Like tyrosine hydroxylase, it is a mixed-function oxidase that requires an electron donor. Ascorbic acid (vitamin C) serves as the electron donor and is oxidized in the reaction. Copper (Cu ) is a bound cofactor required for the electron transfer. 

Vitamin C, ascorbic acid, is a coenzyme for the enzyme prolyl hydroxylase. The action of this enzyme is critical for the formation of normal collagen, a key component of structural and connective tissues. 

Ascorbic acid a coenzyme for prolyl hydroxylase the preventive and cure for scurvy also known as vitamin C. 

The ability to synthesise ascorbic acid from glucose is absent in a small group of animal species that include man, primates, the guinea pig and the fruit-bat this is due to the absence of the gene that codes for one of the enzymes required for ascorbate synthesis. These species are therefore dependent on an external source of the vitamin in their diet and it is needed as a cofactor for several hydroxylase enzymes, notably the iron-dependent proline and lysine hydroxylases and the copper-dependent dopamine-(3-hydroxylase the function of ascorbate in these enzymes is likely to be its ability to keep the metal in the reduced form which is necessary for hydroxylation. The ability of ascorbate to reduce Fe3+ to Fe2+ is important in promoting the gastrointestinal uptake of iron and for its release from the iron store ferritin. 

Tyrosine monooxygenase uses biopterin as a cofactor. Biopterin is made in the body and is not a vitamin. Its structure resembles that of folic acid. Dopa decarboxylase is a vitamin B -requiring enzyme. Dopamine hydroxylase is a copper metalloenzyme. The active form of the enzyme contains copper in the reduced state (cuprous, Cu+). With each catalytic event, the copper is oxidized to the cupric state (Cu ). The enzyme uses ascorbic acid as a cofactor for converting the cupric copper back to cuprous copper. Thus, each catalytic event also results in the conversion of ascorbic acid to semidehydroascorbate. The semidehydroascorbate, perhaps by disproportionation, is converted to ascorbate and dehydroascorbate. The catalytic cycle of dopamine hydroxylase is shown in Figure 9,86. Dopamine hydroxylase, as well as the stored catecholamines, are located in special vesicles... 

Figure 4. Reversal of decreased aniline hydroxylase, aminopyrine N-demethylase and p-nitroanisole O-demethytase activities in vitamin C deficient guinea pigs with ascorbic acid (15).

Vitamin C (ascorbic acid, Fig. 2) is a water-soluble vitamin that dissociates at physiological pH. It is essential as a cofactor of several enzymes, including proline hydroxylase and lysine hydroxylase. Scurvy is known as the result of malnutrition with ascorbic acid. This vitamin deficiency is characterized by instable collagen. This results from insufficient hydroxylation of collagen molecules. Besides this, ascorbic acid has a function as an antioxidant. 

Both the above hydroxylations seem to involve the recycling of tetrahydrobiopterin which may require ascorbic acid. Recently it has been suggested that dopamine-jS-hydroxylase (DBH) works in tandem with semidehydroascorbate reductase (SDR) in order to recycle the vitamin and oxidise NADHj as in Figure 5.15. 

Procollagen(I) is an example of a protein that undergoes extensive posttransla-tional modifications. Hydroxylation reactions produce hydroxyproline residues from proline residues and hydroxylysine from lysine residues. These reactions occur after the protein has been synthesized (Fig. 49.3) and require vitamin C (ascorbic acid) as a cofactor of the enzymes, for example, prolyl hydroxylases and lysyl hydroxylase. Hydroxyproline residues are involved in hydrogen bond formation that helps to stabilize the triple helix, whereas hydroxylysine residues are the sites of attachment of disaccharide moieties (galactose-glucose). 

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