Lysyl hydroxylase, collagen synthesis

The copper metalloenzymes are involved in oxygen-using reactions. These enzymes include cytochrome c oxidase (respiratory chain), lysyl oxidase (collagen synthesis), and dopamine [3-hydroxylase (neurotransmitter synthesis). Lysyl oxidase is a small protein with a molecular weight of 32 kDa. This enzyme contains an unusual modification, namely cross-linking between two different parts of its polypeptide chain. The cross-linked region consists of a structure called lysine tyrosylquinone (Klinman, 1996). Two amino acids are involved in this cross-linked structure, and these are Lys 314 and Tyr 349. Lysine tyrosylquinone is used as a cofactor and is necessary for the catalytic activity of the enzyme. Other copper metalloenzymes contain a related cofactor, namely 2,4,5-tiihydrox5q5henylalanine (topaquinone, TPQ). Serum amino oxidase is a copper metalloenzyme that contains TPQ. TPQ consists of a modified residue of phenylalanine. The copper in the active site of the enzyme occurs immediately adjacent to the TPQ cofactor. 

AA serves as an important cofactor for enzymes. Lack of AA in food causes scurvy in humans due to inefficient collagen synthesis, caused by the inactivation of the Fe(II)-activating prolyl hydroxylase and lysyl hydroxylase which catalyze the formation of hydroxyproline and hydroxylysine as essential components for collagens. Prolyl hydroxylases can also hydroxylate conserved prolyl residues in the alpha subunit of the hypoxia-inducible transcription factor, which signals for proteasomal degradation of the transcription factor . The proper action of these hydrolases requires dioxygen, thus they can act... 

The procollagen peptidase is sometimes lacking. In other cases a person synthesizes an abnormal pro-o2 chain that is resistant to cleavage by the peptidase because of deletion of the normal cleavage site. In others collagen is formed in only small amounts or is degraded rapidly. Some individuals lack lysyl hydroxylase and others have a defect in mRNA splicing which causes loss of an exon from the mRNA and synthesis of shortened pro-o2 chains. ... 

It is a cofactor of prolyl and lysyl hydroxylase enzymes in the first stages of collagen synthesis these are necessary for the inclusion of proline in collagen. 

Most signs of scurvy can be related to inadequate or abnormal collagen synthesis. Ascorbate enhances prolyl and lysyl hydroxylase activities (Chapter 25). Collagen formed in scorbutic patents is low in hydroxyproline and poorly cross-linked, resulting in skin lesions, bone fractures, and rupture of capillaries and other blood vessels. The absolute amount of collagen made in scorbutic animals may also decrease independently of the hydroxylation defect. The anemia of scurvy may result from a defect in iron absorption or folate metabolism. 

Hydroxylation of proline and lysine occurs following translation of the polypeptide chain, but probably begins before the nascent chain is released from the ribosomes (59,110,111). Hydroxylation must take place while the chain is still in the random coil configuration since native collagen is resistant to hydroxylation (see below). In fact, it appears that hydroxylation is essential for normal secretion of the pro a-chain from the cell (112,113). Present data indicate it takes ca. 6 min to synthesize a pro -chain (26) and that hydroxyprohne appears in the chain after about 2 min from start of synthesis (114). Prolyl hydroxylase is present within the cell and is distributed throughout the lumen of the cistemae (115). Lysyl hydroxylase probably follows a similar pattern. Figure 1 shows a postulated scheme for the hydroxylation of proline and lysine. 

Ascorbic acid has been shown to increase collagen synthesis by fibroblasts in vitro (99, 280) and to maintain collagen synthesis In nonmitotic fibroblEists for extended periods (99). Prolyl hydroxylase, the enzyme hydroxylating prolyl and lysyl residues of procollagen, requires ascorbate to function in vitro (181), and the addition of ascorbic acid to tissue cultures stimulates the prolyl hydroxylase activity of fibroblasts (174). 

As already mentioned, collagen contains the unique amino acids hy-droxyproline and hydroxylysine, which are necessary for the stability of the molecule and for its complete maturation. The synthesis of these amino acids occurs posttranslationally during the assembly of the polypeptidic chain (Uitto and Proc-kop, 1974), is independent of the age (Brinckmann et al., 1994), and is catalyzed by prolyl and lysyl hydroxylases in the presence of oxygen, a-ketoglutarate, ferrous ions, and ascorbic acid (Hutton et al., 1967 Kivirikko and Prockop, 1967). Ascorbic acid has been found to be specifically required for the decarboxylation of a-ketoglutarate in the proly 1-4-hydroxylase reaction, where it may act as a com-... 

Figure 3.4 Factors affecting foreign body reaction and potential points of intervention at the level of the myofibroblast (1) inhibit synthesis or release of TGF-P (2) block stimulation by TGF-P of its membrane receptors on the activated fibroblast (3) inhibit the Smad proteins, which transfer the TGF-P effect to the nucleus (4) inhibit transcription of procollagen mRNA (5) inhibit translation of the message to form procollagen (6) inhibit prolyl-4-hydroxylase, which creates hydroxyproline and facilitates helix formation (7) inhibit lysyl oxidase, which cross-links the collagen (8) enhance the function of MMPs, which degrade collagen, or inhibit TIMPs, which degrade MMPs.

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