Prolyl hydroxylase enzymes

It was shown that hydroxylation of prolyl moieties can mark important proteins for proteasomal degradation. This structural modification, accomplished by prolyl hydroxylase enzymes, induces a conformational bias into the protein 3D-structure that is crucial for protein-protein recognition involved in oxygen sensing/ hypoxia response (Figure 11.10). 

Three different, but closely related, genes encoding mammalian HIF prolyl hydroxylase enzymes have been identified in the human genome, which are also present in Mus musculus, referred to as PHDl, PHD2, and PHDS (Table 1) (10,56). In vitro experiments have established that each fimctions as a prolyl hydroxylase for HIF-1 alpha and have demonstrated some selectivity for the two different hydroxylation sites within HIF-1 alpha (10). As yet, mammalian cells with defects in these enzymes have not been identified, and it is not clear how much redundancy there may be in the system. What is clear is that the rate of the enzymatic reaction in cell extracts is sensitive to the amount of oxygen (10). 

Mineral dust-induced ROMs contributes to pulmonary fibrosis, malignancy, hypersensitivity and emphysema (Doelman etctl., 1990 Kamp etui., 1992). The involvement of ROMs in pulmonary fibrotic reactions is indicated by the participation of PMN oxidants in the autoactivation of latent coUagenase (Weiss et al., 1985). Prolyl hydroxylase, a key enzyme in collagen fibril formation, has been shown to be dependent on the reaction of superoxide with prolyl residues (Myllyla et al., 1979). 

So what does this magical molecule do Actually, it does two things, one rather more crystalline clear than the other. The crystal clear thing that ascorbic acid does is act as coenzyme for an enzyme known as prolyl hydroxylase. This enzyme catalyzes the conversion of the amino acid proline to hydroxyproline, a major, if exotic, amino acid in the structural protein collagen ... 

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. 

Figure 13.G. Reaction sequence of prolyl hydroxylase (EC 1.14.11.2). Enz, enzyme.

The enzyme that catalyzes this reaction, lysine hydroxylase, is, like prolyl hydroxylase, a Fe(II)/2-oxoglutarate dependent dioxygenase but exhibits a distinct substrate specificity. 

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... 

Mossbauer and EPR techniques (78). The same species was later observed in a viral prolyl hydroxylase (79), which supports the hypothesis that this species is indeed the key reactive intermediate in a conserved mechanism of these enzymes. 

Figure 8.33. Formatiou of 4-Hydroxyproliue. Proline is hydroxylated at C-4 by the action of prolyl hydroxylase, an enzyme that activates molecular oxygen.

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. 

The hydroxylation reactions are catalyzed by three enzymes prolyl 4-hydroxylase (usually known as prolyl hydroxylase), prolyl 3-hydroxylase, and lysyl hydroxylase. These enzymes are located within the cistemae or rough endoplasmic reticulum as the procollagen chains enter this compartment, the hydroxylations begin. 

A representation of hypoxia-inducible factor (HIF) function. In normoxia, HIF Ia is broken down after prolyl hydroxylation and its interaction with p300 is diminished after aspargyl hydroxylation. In hypoxia, hydroxylase enzymes are inhibited, whereby the DNA binding of HIF may occur and oxygen-sensitive genes can be induced... 

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