Hydroxylation of proline residues

Rather scanty evidence exists for the participation of free radicals in Alzheimer s disease and Down s syndrome. However, more recendy, reports have appeared that suggest possible free-radical involvement in the pathogenesis of these two conditions. Zemlan et al. (1989) repotted that the activity of the free-radical scavenging enzyme, SOD, was significantly increased in fibroblast cell lines derived from familial Alzheimer s and Down s patients. They hypothesized that the elevation in SOD activity observed in the Alzheimer patients supports the theory that paired helical filaments are formed by free-radical hydroxylation of proline residues. They further su ested that SOD levels might also be increased in the brains of Alzheimer s and Down s patients, and that the increase in SOD may reflect an enhanced generation of free radicals. 

Prolyl 4-hydroxylation is the most abundant posttranslational modification of collagens. 4-Hydroxylation of proline residues increases the stability of the triple helix and is a key element in the folding of the collagen triple helix. " In vertebrates, almost all the Yaa position prolines of the Gly-Xaa-Yaa repeat are modified to 4(I( )-hydroxylproline by the enzyme P4H (EC 1.14.11.2), a member of Fe(II)- and 2-oxoglutarate-dependent dioxygenases. This enzyme is an 0 2/ b2-type heterotetramer in which the / subunit is PDI (EC 5.3.4.1), which is a ubiquitous disulfide bond catalyst. The P4H a subunit needs the 13 subunit for solubility however, the 13 subunit, PDI, is soluble by itself and is present in excess in the ER. Three isoforms of the a subunit have been identified and shown to combine with PDI to form [a(I)]2/ 2) [< (II)]2/32> or [a(III)]2/32 tetramers, called the type... 

The enzyme prolylhydroxylase catalyzes the hydroxylation of proline residues, which lie immediately before the repetitive glycine residues in the pro-a-chains ... 

The other major protein in the extracellular matrix is elastin, which is the main component of elastic fibers found in ligaments, large arteries, and lungs. After synthesis and partial hydroxylation of proline residues, a 72 kDa molecule of tropoelastin is secreted into the matrix. This protein is rich in nonpolar amino acids and contains repeating sequences, such as (Val-Pro-Gly-Val-Gly). These sections form an amorphous, random-coiled structure with frequent reverse turns. Other recurrent sequences are rich in alanine with paired lysine residues, e.g., -Ala-Ala-Ala-Ala-Lys-Ala-Ala-Lys-... 

The first evidence for PDI having a different role came from Pihla-janiemi etal. (1987) during their study of the enzyme prolyl 4-hydroxylase (P4H). This enzyme is localized in the ER and catalyzes the hydroxylation of proline residues in nascent procollagen polypeptides. The enzyme is a tetramer composed of two a and two jS subunits (Kivirikko et al., 1989). Antibodies raised against the holoenzyme were used to screen a human gtll expression library and clones were isolated that coded for the /3 subunit. After sequencing it was revealed that the human /3 subunit was 94% identical with the rat PDI sequence. Southern blot analysis identified... 

Vitamins are small biomolecules that are needed in small amounts in the diets of higher animals. The water-soluble vitamins are vitamin C (ascorbate, an antioxidant) and the vitamin B complex (components of coenzymes). Ascorbate is required for the hydroxylation of proline residues in collagen, a key protein of connective tissue. The fat-soluble vitamins are vitamin A (a precursor of retinal), D (a regulator of calcium and phosphorus metabolism), E (an antioxidant in membranes), and K (a participant in the carboxylation of glutamate). 

The most important reaction requiring ascorbate as a cofactor is the hydroxylation of proline residues in collagen. Vitamin C is, therefore, required for the maintenance of normal connective tissue as well as for wound healing since synthesis of connective tissue is the first event in wound tissue remodeling. 

Ascorbic acid is a cofactor in various hydroxylation reactions. These reactions include the hydroxylation of proline residues in a variety of proteins, such as the... 

Ascorbic acid (vitamin C) Only required by some avian species Hydroxylation of proline residues in collagen... 

Prolyl 4-hydroxylase (EC 1.14.11.2) is a key enzyme required for the posttranslational hydroxylation of proline residues in collagen. The enzyme consists of a tetramer composed of two pairs of nonidentical subimits (a.2 of 60 kDa each (Berg et al. 1979). Peptide mapping has demonstrated that the a and P subimits are products of different genes (Berg et al. 1979). The P subunit has been shown (Pihlaja-NiEMi et al. 1987) to be virtually identical with the enzyme protein disulphide isomerase (EC 5.3.4.1). 

Miller, R. L. and Udenfriend, S. (1970) Hydroxylation of proline residues in collagen nascent chains. Arch. Biochem. Biophys. 139,104-113. 

Figure 26.14 Hydroxylation of proline residues to produce collagen.

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