Ascorbate is consumed stoichiometrically in the uncoupled reactions catalyzed by prolyl 4-hydroxylase and lysyl hydroxylase

The hydroxylation of proline and lysine residues by the collagen hydroxylases is coupled with a stoichiometric decarboxylation of 2-oxoglutarate. Ascorbate is virtually a specific requirement for these enzymes, but previous studies have demonstrated that it is not consumed during most catalytic cycles. Prolyl 4-hydroxylase and lysyl hydroxylase are known also to catalyze an uncoupled decarboxylation of 2-oxoglutarate in the absence of the peptide substrate. It is shown here that, unlike the complete hydroxylation reaction, the uncoupled decarboxylation reaction involves stoichiometric ascorbate consumption. This stoichiometric ascorbate consumption was also seen when the rate of the uncoupled prolyl 4-hydroxylase reaction was enhanced by the addition of poly(L-proline). Since collagen hydroxylases may catalyze occasional uncoupled reaction cycles even in the presence of the peptide substrates, the main function of ascorbate in these reactions in vivo is suggested to be that of reactivating the enzymes after such uncoupled cycles.     

Molecular cloning of the beta-subunit of human prolyl 4-hydroxylase. This subunit and protein disulphide isomerase are products of the same gene.

Prolyl 4-hydroxylase (EC 1.14.11.2), an alpha 2 beta 2 tetramer, catalyses the formation of 4-hydroxyproline in collagens by the hydroxylation of proline residues in peptide linkages. We report here the isolation of cDNA clones coding for the beta-subunit of prolyl 4-hydroxylase from a human hepatoma lambda gt11 library and a corresponding human placenta library. Five overlapping clones covering all the coding sequences and almost all the non-coding sequences were characterized. The size of the mRNA hybridizing with these clones in Northern blotting is approximately 2.5 kb. The clones encode a polypeptide of 508 amino acid residues, including a signal peptide of 17 amino acids. These human sequences were found to be very similar to those recently reported for rat protein disulphide isomerase (EC 5.3.4.1). The degree of homology between these two proteins was 84% at the level of nucleotide sequences or 94% at the level of amino acid sequences. Southern blot analyses of human genomic DNA with a cDNA probe for the beta-subunit indicated the presence of only one gene containing these sequences. The product of a single gene thus appears to possess two different enzymatic functions depending on whether it is present in cells in monomer form or in the prolyl 4-hydroxylase tetramer.     

Involvement of superoxide in the prolyl and lysyl hydroxylase reactions

Four superoxide dismutase active copper chelates, Cu(acetylsalicylate)₂, Cu(salicylate)₂, Cu(lysine)₂ and Cu(tyrosine)₂, proved to be inhibitors of prolyl and lysyl hydroxylase. The kinetics of the inhibition are consistent with the proposal that these compounds dismutated ${}^{\mathrm{?}}\text{O}{}_2{}^{\mathrm{staggered?}}$ at the active site of the enzymes. The data strongly suggest that ${}^{\mathrm{?}}\text{O}{}_2{}^{\mathrm{staggered?}}$ is the active form of O₂ in the prolyl and lysyl hydroxylase reactions.     

The primary structure of the alpha 4 subunit of VLA-4: homology to other integrins and a possible cell-cell adhesion function.

VLA-4 is a cell surface heterodimer in the integrin superfamily of adhesion receptors. Anti-VLA-4 antibodies inhibited cytolytic T cell activity, with inhibitory activity directed against the effector T cells rather than their targets. Thus, whereas other VLA receptors appear to mediate cell--matrix interactions, VLA-4 may have a cell--cell adhesion function. To facilitate comparative studies of VLA-4 and other integrins, cDNA clones for the human alpha 4 subunit of VLA-4 were selected and then sequenced. The 3805 bp sequence encoded for 999 amino acids, with an N-terminus identical to that previously obtained from direct sequencing of purified alpha 4 protein. The alpha 4 amino acid sequence was 17-24% similar to other integrin alpha chains with known sequences. Parts of the alpha 4 sequence most conserved in other alpha chains include (i) the positions of 19/24 cysteine residues, (ii) three potential divalent cation binding sites of the general structure DXDXDGXXD and (iii) the transmembrane region. However, alpha 4 stands apart from all other known integrin alpha subunit sequences because (i) alpha 4 has neither an inserted I-domain, nor a disulfide-linked C-terminal fragment, (ii) its sequence is the most unique and (iii) only alpha 4 has a potential protease cleavage site, near the middle of the coding region, which appears responsible for the characteristic 80,000 and 70,000 Mr fragments of alpha 4.     

Activities of prolyl hydroxylase, lysyl hydroxylase, collagen galactosyltransferase and collagen glucosyltransferase in the liver of rats with hepatic injury

The activities of four enzymes catalysing post-translational modifications of the collagen polypeptide chains were assayed in the livers of rats with experimental hepatic injury. The liver injury was induced by injecting carbon tetrachloride twice weekly, and assays of the enzymic activities were carried out 2 and 4 weeks after commencement of administration of carbon tetrachloride. The liver homogenates were preincubated with Triton X-100 before the assays, because such treatment was found to increase the activities of all four enzymes in the supernatants of liver homogenates. The activities of all four enzymes had increased by 2 weeks after commencement of carbon tetrachloride administration. No increase was found in the collagen content of the livers at this stage and thus an increase in all four enzyme activities preceded an increase in the collagen content of the liver. A further slight increase was found in three of the enzyme activities during the subsequent 2 weeks of the experiment, whereas no further increase was found in the collagen galactosyltransferase activity. A statistically significant correlation was found between all four enzyme activities, but the magnitude of the increases varied considerably. The largest increase was found in lysyl hydroxylase activity, and at 4 weeks the magnitude of this was about three times that of the collagen galactosyltransferase activity. The results thus indicate that the increased enzyme activities cannot be explained simply by an increase in the number of collagen-producing cells having similar enzyme activity patterns to those of the cells initially present in the liver.     

Identification of a novel proline-rich peptide-binding domain in prolyl 4-hydroxylase.

Prolyl 4-hydroxylase (EC 1.14.11.2) catalyzes the hydroxylation of -X-Pro-Gly- sequences and plays a central role in the synthesis of all collagens. The [alpha(I)]2beta2 type I enzyme is effectively inhibited by poly(L-proline), whereas the [alpha(II)]2beta2 type II enzyme is not. We report here that the poly(L-proline) and (Pro-Pro-Gly)10 peptide substrate-binding domain of prolyl 4-hydroxylase is distinct from the catalytic domain and consists of approximately 100 amino acids. Peptides of 10-19 kDa beginning around residue 140 in the 517 residue alpha(I) subunit remained bound to poly(L-proline) agarose after limited proteolysis of the human type I enzyme tetramer. A recombinant polypeptide corresponding to the alpha(I) subunit residues 138-244 and expressed in Escherichia coli was soluble, became effectively bound to poly(L-proline) agarose and could be eluted with (Pro-Pro-Gly)10. This polypeptide is distinct from the SH3 and WW domains, and from profilin, and thus represents a new type of proline-rich peptide-binding module. Studies with enzyme tetramers containing mutated alpha subunits demonstrated that the presence of a glutamate and a glutamine in the alpha(II) subunit in the positions corresponding to Ile182 and Tyr233 in the alpha(I) subunit explains most of the lack of poly(L-proline) binding of the type II prolyl 4-hydroxylase. Keywords: collagen/dioxygenases/peptide-binding domain/ proline-rich/prolyl hydroxylase     

Processive action of the two peptide binding sites of prolyl 4-hydroxylase in the hydroxylation of procollagen

The number of peptide binding sites of prolyl 4-hydroxylase was manipulated with the peptide photoaffinity label N-(4-azido-2-nitrophenyl)glycyl-(Pro-Pro-Gly)5, and the effect on hydroxylation of the relatively short peptide substrate (Pro-Pro-Gly)5 and of the long natural substrate procollagen was studied. With (Pro-Pro-Gly)5 as a substrate, a linear relation was found between enzyme activity and the amount of covalently bound photoaffinity label, approximately 50% inactivation being reached at 1 mol of label/mol of enzyme. No difference in Km value for (Pro-Pro-Gly)5 was detected between unlabeled and partially labeled enzyme preparations. These results indicate that enzyme molecules with only one free active site hydroxylated the synthetic substrate (Pro-Pro-Gly)5 with the same Km and at half the rate of native enzyme. In contrast, with procollagen as a substrate a 5-10-fold increase in Km was found with the fraction of enzyme containing only one free active site, as compared to the Km for procollagen with nonlabeled enzyme. This finding is explained by an enzyme-kinetic model based on a processive action of the two peptide substrate binding sites of prolyl 4-hydroxylase, preventing dissociation of the enzyme-substrate complex between successive hydroxylations of a long peptide with multiple substrate sites. Such a mechanism leads to a low Km for a long peptide by overcoming the diffusional constraints on the rate of association between the enzyme and the individual substrate sites.     

High-level production of human collagen prolyl 4-hydroxylase in Escherichia coli.

The collagen prolyl 4-hydroxylases (C-P4Hs), enzymes residing within the lumen of the endoplasmic reticulum, play a central role in the synthesis of all collagens. The vertebrate enzymes are alpha(2)beta(2) tetramers in which the two catalytic sites are located in the alpha subunits, and protein disulfide isomerase serves as the beta subunit. All attempts to assemble an active C-P4H tetramer from its subunits in in vitro cell-free systems have been unsuccessful, but assembly of a recombinant enzyme has been reported in several cell types by coexpression of the two types of subunit. An active type I C-P4H tetramer was obtained here by periplasmic expression in Escherichia coli strains BL21 and RB791. Further optimization for production by stepwise regulated coexpression of its subunits in the cytoplasm of a thioredoxin reductase and glutathione reductase mutant E. coli strain resulted in large amounts of human type I C-P4H tetramer. The specific activity of the C-P4H tetramer purified from the cytoplasmic expression was within the range of values reported for human type I C-P4H isolated as a nonrecombinant enzyme or produced in the endoplasmic reticulum of insect cells, but the expression level, about 25 mg/l in a fermenter, is about 5-10 times that obtained in insect cells. The enzyme expressed in E. coli differed from those present in vivo and those produced in other hosts in that it lacked the N glycosylation of its alpha subunits, which may be advantageous in crystallization experiments.     

A single polypeptide acts both as the beta subunit of prolyl 4-hydroxylase and as a protein disulfide-isomerase

A single polypeptide is shown to act both as the beta subunit of the proline hydroxylase (EC 1.14.11.2) and as a protein disulfide-isomerase (EC 5.3.4.1). When isolated from chick embryos or rat liver, the beta subunit of prolyl 4-hydroxylase and the enzyme protein disulfide-isomerase have identical molecular weights and peptide maps as produced by digestion with Staphylococcus aureus V8 protease. The apparent molecular weights of both proteins isolated from human placental tissue are slightly higher, and the human beta subunit and one of its peptides have molecular weights about Mr 500 higher than the protein disulfide-isomerase and its corresponding peptide. Experiments with polyclonal and monoclonal antibodies also suggest a structural identity between the two proteins. The beta subunit isolated from the prolyl 4-hydroxylase tetramer has protein disulfide-isomerase activity similar to protein disulfide-isomerase itself, and even the beta subunit when present in the prolyl 4-hydroxylase tetramer has one-half of this activity.     

Regulation of proline 3-hydroxylation and prolyl 3-hydroxylase and 4-hydroxylase activities in transformed cells.

Prolyl 3-hydroxylase activity and the extent of collagen proline 3-hydroxylation were studied in six transformed and three control human cell lines. In the transformed cell lines, the enzyme activity was markedly high in two, similar to that in control cells in two and significantly low in two. The extent of proline 3-hydroxylation was markedly high in cell lines with high enzyme activity, but it was also significantly high in some transformed cell lines with enzyme activities similar to those in the controls. The results thus suggest that, in addition to the amount of enzyme activity present, the rate of collagen synthesis also affects the extent of proline 3-hydroxylation in the newly synthesized collagen. The effect of acute cell transformation on prolyl 3-hydroxylase and 4-hydroxylase activities was studied by infecting chick-embryo fibroblasts with Rous sarcoma virus mutant NY68, temperature-sensitive for transformation. At the permissive temperature prolyl 3-hydroxylase activity showed a more rapid increase and decrease than did prolyl 4-hydroxylase activity, the maximal activity for both enzymes being about 2.5 times that in the control chick fibroblasts. When the transformed cells were shifted to the non-permissive temperature the decays in the elevated enzyme activities were similar, suggesting identical half-lives.     

Evidence for the association of prolyl hydroxylase from chick embryos with membranes through membrane-bound procollagen.

Proline hydroxylase of membrane pellets from tissues of chick embryo was solubilized by several extraction mediums. Three kinds of enzyme linkage with membranes were pointed out: a labile and non specific adsorption, a specific linkage on endogenous procollagen which is linked with the membranes, and a part of membranes themselves. Our results suggest that, in the liver, these three kinds of linkage exist. Whereas, in tibia bones only labile linkages appear by non specific adsorption or through the link with membrane procollagen.     

Failure of highly purified lysyl hydroxylase to hydroxylate lysyl residues in the non-helical regions of collagen.

The activity of highly purified lysyl hydroxylase towards lysyl residues within both the helical and the N-terminal non-helical telopeptide regions of chick type I collagen has been examined. The peptides alpha 1(I)-CB1 and alpha 2(I)-CB1, isolated from protocollagen following CNBr digestion and containing the N-terminal telopeptidyl lysyl residues, failed themselves to act as substrates. With protocollagen as substrate, analysis of products obtained following bacterial collagenase digestion of the reaction mixture showed that overall 37% hydroxylation of lysyl residues within the helical region of collagen had been obtained, which may be maximal. No hydroxylation, however, of the single lysyl residue in either alpha 1(I)-CB1 or alpha 2(I)-CB1, isolated following CNBr digestion of the reaction mixture, was observed, despite the known susceptibility of these residues to hydroxylation. These findings provide strong circumstantial evidence for the suggestion that a lysyl hydroxylase specific for the telopeptidyl residues and distinct from that active towards lysyl residues in the helical portion of the molecule may exist [Barnes, Constable, Morton & Royce (1974) Biochem. J. 139, 461-468].     

Molecular cloning and primary structure of the Escherichia coli methionyl-tRNA synthetase gene.

The intact metG gene was cloned in plasmid pBR322 from an F32 episomal gene library by complementation of a structural mutant, metG83. The Escherichia coli strain transformed with this plasmid (pX1) overproduced methionyl-tRNA synthetase 40-fold. Maxicell analysis showed that three major polypeptides with MrS of 76,000, 37,000, and 29,000 were expressed from pX1. The polypeptide with an Mr of 76,000 was identified as the product of metG on the basis of immunological studies and was indistinguishable from purified methionyl-tRNA synthetase. In addition, DNA-DNA hybridization studies demonstrated that the metG regions were homologous on the E. coli chromosome and on the F32 episome. DNA sequencing of 642 nucleotides was performed. It completes the partial metG sequence already published (D. G. Barker, J. P. Ebel, R. Jakes, and C. J. Bruton, Eur. J. Biochem. 127:449-451, 1982). Examination of the deduced primary structure of methionyl-tRNA synthetase excludes the occurrence of any significant repeated sequences. Finally, mapping of mutation metG83 by complementation experiments strongly suggests that the central part of methionyl-tRNA synthetase is involved in methionine recognition. This observation is discussed in the light of the known three-dimensional crystallographic structure.     

Characterization of the iron- and 2-oxoglutarate-binding sites of human prolyl 4-hydroxylase.

Prolyl 4-hydroxylase (EC 1.14.11.2), an alpha2beta2 tetramer, catalyzes the formation of 4-hydroxyproline in collagens. We converted 16 residues in the human alpha subunit individually to other amino acids, and expressed the mutant polypeptides together with the wild-type beta subunit in insect cells. Asp414Ala and Asp414Asn inactivated the enzyme completely, whereas Asp414Glu increased the K(m) for Fe2+ 15-fold and that for 2-oxoglutarate 5-fold. His412Glu, His483Glu and His483Arg inactivated the tetramer completely, as did Lys493Ala and Lys493His, whereas Lys493Arg increased the K(m) for 2-oxoglutarate 15-fold. His501Arg, His501Lys, His501Asn and His501Gln reduced the enzyme activity by 85-95%; all these mutations increased the K(m) for 2-oxoglutarate 2- to 3-fold and enhanced the rate of uncoupled decarboxylation of 2-oxoglutarate as a percentage of the rate of the complete reaction up to 12-fold. These and other data indicate that His412, Asp414 and His483 provide the three ligands required for the binding of Fe2+ to a catalytic site, while Lys493 provides the residue required for binding of the C-5 carboxyl group of 2-oxoglutarate. His501 is an additional critical residue at the catalytic site, probably being involved in both the binding of the C-1 carboxyl group of 2-oxoglutarate and the decarboxylation of this cosubstrate.     

Characterization of a low-relative-molecular-mass prolyl 4-hydroxylase from the green alga Chlamydomonas reinhardii.

Prolyl 4-hydroxylase was partially purified and characterized from the unicellular green alga, Chlamydomonas reinhardii. This enzyme differed from all the animal and plant prolyl 4-hydroxylases studied so far in that its Mr was only about 40,000 by gel filtration, being thus less than one-sixth of those determined for the vertebrate and higher-plant enzymes. The algal enzyme did not hydroxylate to any significant extent chick-embryo protocollagen or triple-helical (Pro-Pro-Gly)10, whereas a low hydroxylation rate was found with denatured (Pro-Pro-Gly)10. Poly(L-proline), which is an effective inhibitor of the vertebrate enzymes but acts as a substrate for some higher-plant enzymes, was a good substrate. In the absence of poly(L-proline) the enzyme catalysed an uncoupled decarboxylation of 2-oxoglutarate. Studies of the Km values for the co-substrates and cofactors and the specificity of the 2-oxoglutarate requirement, as well as inhibition studies with selected 2-oxoglutarate analogues, suggested that the catalytic site of the algal enzyme is similar to, but not identical with, those of the vertebrate enzymes. The existence of distinct similarities was further demonstrated by an inhibition of the algal enzyme activity with a monoclonal antibody to the beta-subunit of human prolyl 4-hydroxylase. The amount of prolyl 4-hydroxylase activity in the algal cells was not altered by signals which recognize the presence or absence of the cell wall, as determined in studies on experimental cell-wall regeneration and wall-less mutants.     

Molecular cloning of two genes encoding cinnamate 4-hydroxylase (C4H) from oilseed rape (Brassica napus)

Cinnamate 4-hydroxylase (C4H) is a key enzyme of phenylpropanoid pathway, which synthesizes numerous secondary metabolites to participate in development and adaption. Two C4H isoforms, the 2192-bp BnC4H-1 and 2108-bp BnC4H-2, were cloned from oilseed rape (Brassica napus). They both have two introns and a 1518-bp open reading frame encoding a 505-amino-acid polypeptide. BnC4H-1 is 57.73 kDa with an isoelectric point of 9.11, while 57.75 kDa and 9.13 for BnC4H-2. They share only 80.6% identities on nucleotide level but 96.6% identities and 98.4% positives on protein level. Showing highest homologies to Arabidopsis thaliana C4H, they possess a conserved p450 domain and all P450-featured motifs, and are identical to typical C4Hs at substrate-recognition sites and active site residues. They are most probably associated with endoplasmic reticulum by one or both of the N- and C-terminal transmembrane helices. Phosphorylation may be a necessary post-translational modification. Their secondary structures are dominated by alpha helices and random coils. Most helices locate in the central region, while extended strands mainly distribute before and after this region. Southern blot indicated about 9 or more C4H paralogs in B. napus. In hypocotyl, cotyledon, stem, flower, bud, young- and middle-stage seed, they are co-dominantly expressed. In root and old seed, BnC4H-2 is dominant over BnC4H-1, with a reverse trend in leaf and pericarp. Paralogous C4H numbers in Brassicaceae genomes and possible roles of conserved motifs in 5' UTR and the 2nd intron are discussed.     

The Skp1 prolyl hydroxylase from Dictyostelium is related to the hypoxia-inducible factor-alpha class of animal prolyl 4-hydroxylases

Skp1 is a cytoplasmic and nuclear protein of eukaryotes best known as an adaptor in SCF ubiquitin-protein isopeptide ligases. In Dictyostelium, Skp1 is subject to 4-hydroxylation at Pro(143) and subsequent O-glycosylation by alpha-linked GlcNAc and other sugars. Soluble cytosolic extracts have Skp1 prolyl 4-hydroxylase (P4H) activity, which can be measured based on hydroxylation-dependent transfer of [(3)H]GlcNAc to recombinant Skp1 by recombinant (Skp1-protein)-hydroxyproline alpha-N-acetyl-d-glucosaminyltransferase. The Dictyostelium Skp1 P4H gene (phyA) was predicted using a bioinformatics approach, and the expected enzyme activity was confirmed by expression of phyA cDNA in Escherichia coli. The purified recombinant enzyme (P4H1) was dependent on physiological concentrations of O(2), alpha-ketoglutarate, and ascorbate and was inhibited by CoCl(2), 3,4-dihydroxybenzoate, and 3,4-dihydroxyphenyl acetate, as observed for known animal cytoplasmic P4Hs of the hypoxia-inducible factor-alpha (HIFalpha) class. Overexpression of phyA cDNA in Dictyostelium yielded increased enzyme activity in a soluble cytosolic extract. Disruption of the phyA locus by homologous recombination resulted in loss of detectable activity in extracts and blocked hydroxylation-dependent glycosylation of Skp1 based on molecular weight analysis by SDS-PAGE, demonstrating a requirement for P4H1 in vivo. The sequence and functional similarities of P4H1 to animal HIFalpha-type P4Hs suggest that hydroxylation of Skp1 may, like that of animal HIFalpha, be regulated by availability of O(2), alpha-ketoglutarate, and ascorbate, which might exert novel control over Skp1 glycosylation.