Site-directed mutagenesis of the alpha subunit of human prolyl 4-hydroxylase. Identification of three histidine residues critical for catalytic activity

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.

Cloning and characterization of a novel human lysyl hydroxylase isoform highly expressed in pancreas and muscle

We report the isolation and characterization of cDNA clones for a novel isoform of lysyl hydroxylase (lysyl hydroxylase 2), a posttranslational enzyme of collagen biosynthesis. The open reading frame predicted a protein of 737 amino acids, including an amino-terminal signal peptide. The amino acid sequence has overall similarity of over 75% to the lysyl hydroxylase (lysyl hydroxylase 1) characterized earlier. This similarity is even higher in the carboxyl-terminal end of the molecules. Lysyl hydroxylase 2 contains nine cysteine residues, which are conserved in lysyl hydroxylase 1. Furthermore, the conserved histidines and aspartate residues required for lysyl hydroxylase activity are present in the sequence. Northern analysis identified a transcript of 4.2 kilobases, which was highly expressed in pancreas and muscle tissues. Expression of cDNA in insect cells using a baculovirus vector yielded proteins with lysyl hydroxylase activity and an antiserum against a synthetic peptide of the deduced amino acid sequence recognized proteins with molecular weights of 88 and 97 kDa in homogenates of the transfected cells.

Involvement of prolyl 4-hydroxylase in the assembly of trimeric minicollagen XII. Study in a baculovirus expression system

We have shown previously that hydroxylation played a critical role in the trimer assembly and disulfide bonding of the three constituent alpha chains of a minicollagen composed of the extreme C-terminal collagenous (COL1) and noncollagenous (NC1) domains of type XII collagen in HeLa cells (Mazzorana, M., Gruffat, H., Sergeant, A., and van der Rest, M. (1993) J. Biol. Chem. 268, 3029-3032). We have further characterized the involvement of prolyl 4-hydroxylase in the assembly of the three alpha chains to form trimeric disulfide-bonded type XII minicollagen in an insect cell expression system. For this purpose, type XII minicollagen was produced in insect cells from baculovirus vectors, alone or together with wild-type human prolyl 4-hydroxylase or with the human enzyme mutated in the catalytic site of its alpha or beta subunits or with the individual alpha or beta subunits. When type XII minicollagen was produced alone, negligible amounts of disulfide-bonded trimers were found to be produced by the cells. However, coproduction of the collagen with the two subunits of the wild-type human enzyme dramatically increased the amount of disulfide-bonded trimeric type XII minicollagen molecules. In contrast, coproduction of the collagen with alpha subunits that had a mutation completely inactivating the human enzyme failed to enhance the trimer assembly. These results directly show that an active prolyl 4-hydroxylase is required for the assembly of disulfide-bonded trimers of type XII minicollagen.

Intracellular dissociation and reassembly of prolyl 4-hydroxylase:the alpha-subunits associated with the immunoglobulin-heavy-chain binding protein (BiP) allowing reassembly with the beta-subunit

Prolyl 4-hydroxylase (P4-H) consists of two distinct polypeptides; the catalytically more important alpha-subunit and the beta-subunit, which is identical to the multifunctional enzyme protein disulphide isomerase. The enzyme appears to be assembled in vivo into an alpha 2 beta 2 tetramer from newly synthesized alpha-subunits associating with an endogenous pool of beta-subunits. Using a cell-free system, we have shown previously that enzyme assembly is redox-dependent and that assembled alpha-subunits are intramolecularly disulphide-bonded [John and Bulleid (1994) Biochemistry 33, 14018-14025]. Here we have studied this assembly process within intact cells by expressing both subunits in COS-1 cells. Newly synthesized alpha-subunits were shown to assemble with the beta-subunit, to form insoluble aggregates, or to remain soluble but not associate with the beta-subunit. Treatment of cells with dithiothreitol (DTT) led to dissociation of P4-H into subunits and on removal of DTT the enzyme reassembled. This reassembly was ATP-dependent, suggesting an interaction with an ATP-dependent chaperone. This was confirmed when immunoglobulin-heavy-chain binding protein (BiP) and alpha-subunits were co-immunoprecipitated with antibodies against the alpha-subunit and BiP, respectively. These results indicate that unassembled alpha-subunits are maintained in an assembly-competent form by interacting with the molecular chaperone BiP.

Site-directed mutagenesis of human lysyl hydroxylase expressed in insect cells. Identification of histidine residues and an aspartic acid residue critical for catalytic activity

Lysyl hydroxylase (EC 1.14.11.4), an alpha 2 homodimer, catalyzes the formation of hydroxylysine in collagens. We expressed here human lysyl hydroxylase in insect cells by baculovirus vectors. About 90% of the enzyme produced was soluble 32 h after infection, whereas only 10% was soluble at 72 h. Twelve histidines, five aspartates, and all four asparagines that may act as N-glycosylation sites were converted individually to serine, alanine, or glutamine, respectively, and the mutant enzymes were expressed in insect cells. Three histidine mutations and one aspartate mutation appeared to inactivate the enzyme completely. These and other data suggest that histidines 656 and 708 and aspartate 658 provide the three ligands required for the binding of Fe2+ to a catalytic site, whereas the role of the third critical histidine (residue 706) remains to be established. Three additional histidine mutations also had a major effect, although they did not inactivate the enzyme completely, whereas six further histidine mutations and four out of five aspartate mutations had a much more minor effect. Data on the four asparagine mutations suggested that only two of the potential N-glycosylation sites may be fully glycosylated in insect cells and that one of these carbohydrate units may be needed for full enzyme activity.

Co-expression of the alpha subunit of human prolyl 4-hydroxylase with BiP polypeptide in insect cells leads to the formation of soluble and insoluble complexes. Soluble alpha-subunit-BiP complexes have no prolyl 4-hydroxylase activity

Prolyl 4-hydroxylase (EC 1.14.11.2) catalyses the post-translational formation of 4-hydroxyproline in collagens. The vertebrate enzymes are alpha2beta2 tetramers, their beta subunit being identical to protein disulphide isomerase (PDI). The function of the PDI-beta subunit in prolyl 4-hydroxylases is not fully understood, but it seems to be that of keeping the highly insoluble alpha subunits in solution. We report here that expression of the alpha subunit of human type I prolyl 4-hydroxylase in insect cells together with BiP polypeptide leads to the formation of both soluble and insoluble alpha-subunit-BiP complexes. Formation of the soluble complexes was evident from (1) a marked increase in the amount of the alpha subunit in the soluble fraction of the cell homogenates when expressed together with BiP, (2) immunoprecipitation experiments and (3) demonstration of the presence of some of the complexes by polyacrylamide gel electrophoresis under non-denaturing conditions. Formation of the insoluble complexes was suggested by an increase in the amount of BiP in the insoluble fraction when expressed together with the alpha subunit. Nevertheless the soluble alpha-subunit-BiP complexes had no prolyl 4-hydroxylase activity. This indicates that the function of the PDI-beta subunit in the prolyl 4-hydroxylase tetramer is not only that of keeping the alpha subunits in solution but appears to be more specific, probably that of keeping them in a catalytically active, non-aggregated conformation.

Site-directed mutagenesis of residues in a conserved region of bovine aspartyl (asparaginyl) beta-hydroxylase: evidence that histidine 675 has a role in binding Fe2+

The roles in catalysis of several residues in bovine aspartyl (asparaginyl) beta-hydroxylase that are located in a region of homology among alpha-ketoglutarate-dependent dioxygenases were investigated using site-directed mutagenesis. Previous studies have shown that when histidine 675, an invariant residue located in this highly conserved region, was mutated to an alanine residue, no enzymatic activity was detected. A more extensive site-directed mutagenesis study at position 675 has been undertaken to define the catalytic role of this essential residue. The partial hydroxylase activity observed with some amino acid replacements for histidine 675 correlates with the potential to coordinate metals and not with size, charge, or hydrophobic character. Furthermore, the increase in Km for Fe2+ observed with the H675D and H675E mutant enzymes can account for their partial activities relative to wild type. No significant changes in the Km for alpha-ketoglutarate (at saturating Fe2+) or Vmax were observed for these mutants. These results support the conclusion that histidine 675 is specifically involved in Fe2+ coordination. Further site-directed mutagenesis of other highly conserved residues in the vicinity of position 675 demonstrates the importance of this region of homology in catalysis for Asp (Asn) beta-hydroxylase and, by analogy, other alpha-ketoglutarate-dependent dioxygenases.

Functional analysis of conserved histidine residues in Cephalosporium acremonium isopenicillin N synthase by site-directed mutagenesis

The isopenicillin N synthase of Cephalosporium acremonium (cIPNS) involves a catalytically important non-heme iron which is coordinated credibly to histidine residues. A comparison of the IPNS genes from various microbial sources indicated that there are seven conserved histidine residues. These were individually replaced by leucine residues through site-directed mutagenesis, and the sites of mutation were confirmed by DNA sequencing. The seven mutant genes were cloned separately into the vector pET24d for expression in Escherichia coli BL21 (DE3), and the proteins were expressed as soluble enzymes. All the resulting mutant enzymes obtained have mobilities of approximately 38 kDa, identical with the wild-type enzyme on SDS-polyacrylamide gel electrophoresis, and were also reactive to cIPNS antibodies. The enzymes were purified by ammonium sulfate precipitation and DEAE-Sephadex A-50 ion exchange chromatography, and these were analyzed for enzyme activity. A group of mutant enzymes, H49L, H64L, H116L, H126L, and H137L, were found to be enzymatically active with reduced activities of 16-93.7%, indicating that they are not essential for catalysis. Two of the mutant enzymes, H216L and H272L, were found to have lost their enzymatic activity completely, indicating that both His-216 and His-272 are crucial for catalysis. It is suggested that these histidines are likely to serve as ligands for binding to the non-heme iron in the IPNS active site. Alignment of the amino acid sequence of IPNS to related non-heme Fe(2+)-requiring enzymes indicated that the two essential histidine residues correspond to two invariant residues located in highly homologous regions. The conservation of the two closely located histidine residues indicates the possible conservation of similar iron-binding sites in these enzymes.