Jan P. Kraus

Most mitochondrial proteins are encoded in the nucleus and are translated on free cytoplasmic ribosomes as larger precursors containing amino-terminal "leader" sequences, which are removed after the precursors are taken up by mitochondria. We have deduced the complete primary structure of the precursor of a human mitochondrial matrix enzyme, ornithine transcarbamylase (OTC), from the nucleotide sequence of cloned complementary DNA. The amino-terminal leader peptide of OTC is 32 amino acids in length and contains four arginines but no acidic residues. Cleavage of the leader peptide from the "mature" protein occurs between glutamine and asparagine residues. The sequence of mature human OTC resembles that of the subunits of both OTC and aspartate transcarbamylase from Escherichia coli. The biological activity of the cloned OTC complementary DNA was tested by joining it with SV40 (an animal virus) regulatory elements and transfecting cultured HeLa cells, which do not normally express OTC. Both the precursor and mature forms of the OTC subunit were identified; in stable transformants, enzymatic activity was also detected.

The major cause of homocystinuria is mutation of the gene encoding the enzyme cystathionine beta-synthase (CBS). Deficiency of CBS activity results in elevated levels of homocysteine as well as methionine in plasma and urine and decreased levels of cystathionine and cysteine. Ninety-two different disease-associated mutations have been identified in the CBS gene in 310 examined homocystinuric alleles in more than a dozen laboratories around the world. Most of these mutations are missense, and the vast majority of these are private mutations. The two most frequently encountered of these mutations are the pyridoxine-responsive I278T and the pyridoxine-nonresponsive G307S. Mutations due to deaminations of methylcytosines represent 53% of all point substitutions in the coding region of the CBS gene.

Tissue factor is a membrane-bound procoagulant protein that activates the extrinsic pathway of blood coagulation in the presence of factor VII and calcium. lambda Phage containing the tissue factor gene were isolated from a human placental cDNA library. The amino acid sequence deduced from the nucleotide sequence of the cDNAs indicates that tissue factor is synthesized as a higher molecular weight precursor with a leader sequence of 32 amino acids, while the mature protein is a single polypeptide chain composed of 263 residues. The derived primary structure of tissue factor has been confirmed by comparison to protein and peptide sequence data. The sequence of the mature protein suggests that there are three distinct domains: extracellular, residues 1-219; hydrophobic, residues 220-242; and cytoplasmic, residues 243-263. Three potential N-linked carbohydrate attachment sites occur in the extracellular domain. The amino acid sequence of tissue factor shows no significant homology with the vitamin K-dependent serine proteases, coagulation cofactors, or any other protein in the National Biomedical Research Foundation sequence data bank (Washington, DC).

Cystathionine β-synthase (CBS) 1The abbreviations used are: CBS, cystathionine β-synthase; PLP, pyridoxal 5′-phosphate; AdoMet, S-adenosyl-l-methionine. (EC 4.2.1.22) catalyzes the pyridoxal 5′-phosphate (PLP)-dependent β-replacement reaction in which the thiolate of l-homocysteine replaces the hydroxyl group of l-serine (Equation 1). Human CBS is an especially interesting PLP enzyme because it has a complex domain structure (Fig. 1) and regulatory mechanism. The allosteric activator, S-adenosyl-l-methionine (AdoMet), increases CBS activity about 3-fold (1Finkelstein J.D. Kyle W.E. Martin J.L. Pick A.M. Biochem. Biophys. Res. Commun. 1975; 66: 81-87Crossref PubMed Scopus (236) Google Scholar) and likely binds to the C-terminal regulatory domain (2Kery V. Poneleit L. Kraus J.P. Arch. Biochem. Biophys. 1998; 355: 222-232Crossref PubMed Scopus (139) Google Scholar). CBS from higher eukaryotes contains a unique heme moiety of unknown function (3Kery V. Bukovska G. Kraus J.P. J. Biol. Chem. 1994; 269: 25283-25288Abstract Full Text PDF PubMed Google Scholar, 4Ojha S. Hwang J. Kabil O. Penner-Hahn J.E. Banerjee R. Biochemistry. 2000; 39: 10542-10547Crossref PubMed Scopus (44) Google Scholar, 5Taoka S. Ohja S. Shan X. Kruger W.D. Banerjee R. J. Biol. Chem. 1998; 273: 25179-25184Abstract Full Text Full Text PDF PubMed Scopus (226) Google Scholar), which is not found in CBS from yeast (Saccharomyces cerevisiae) (6Jhee K.-H. McPhie P. Miles E.W. J. Biol. Chem. 2000; 75: 11541-11544Abstract Full Text Full Text PDF Scopus (74) Google Scholar, 7Jhee K.-H. McPhie P. Miles E.W. Biochemistry. 2000; 39: 10548-10556Crossref PubMed Scopus (82) Google Scholar, 8Maclean K.N. Janosik M. Oliveriusova J. Kery V. Kraus J.P. J. Inorg. Biochem. 2000; 81: 161-171Crossref PubMed Scopus (41) Google Scholar) or from the protozoan hemoflagellate, Trypanosoma cruzi (9Nozaki T. Shigeta Y. Saito-Nakano Y. Imada M. Kruger W.D. J. Biol. Chem. 2001; 276: 6516-6523Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar). A large number of mutations in different regions of the human CBS have been found in patients with homocystinuria, a human hereditary disease that is characterized by very high plasma levels of the toxic amino acid l-homocysteine (10Mudd S.H. Levy H.L. Kraus J.P. Scriver C.R. Beaudet A.L. Valle D. Sly W.S. Chiles D. Kinsler K.W. Vogelstein B. 8th Ed. The Metabolic and Molecular Bases of Inherited Disease. 2. McGraw-Hill, Inc., New York2001: 2007-2056Google Scholar, 11Kraus J.P. Janosik M. Kozich V. Mandell R. Shih V. Sperandeo M.P. Sebastio G. de Franchis R. Andria G. Kluijtmans L.A. Blom H. Boers G.H. Gordon R.B. Kamoun P. Tsai M.Y. Kruger W.D. Koch H.G. Ohura T. Gaustadnes M. Hum. Mutat. 1999; 13: 362-375Crossref PubMed Scopus (269) Google Scholar). The clinical phenotype of patients with homocystinuria includes mental retardation, lens dislocation, skeletal abnormalities, and vascular disease (10Mudd S.H. Levy H.L. Kraus J.P. Scriver C.R. Beaudet A.L. Valle D. Sly W.S. Chiles D. Kinsler K.W. Vogelstein B. 8th Ed. The Metabolic and Molecular Bases of Inherited Disease. 2. McGraw-Hill, Inc., New York2001: 2007-2056Google Scholar). Mutations in the CBS gene can alter either mRNA or enzyme stability, activity, binding of PLP and heme, or impair allosteric regulation. Crystal structures of truncated forms of the human enzyme have revealed the structure of the catalytic domain and of the N-terminal heme-binding site (12Meier M. Janosik M. Kery V. Kraus J.P. Burkhard P. EMBO J. 2001; 20: 3910-3916Crossref PubMed Scopus (275) Google Scholar, 13Taoka S. Lepore B.W. Kabil O. Ojha S. Ringe D. Banerjee R. Biochemistry. 2002; 41: 10454-10461Crossref PubMed Scopus (128) Google Scholar). The location of homocystinuria-causing mutations in the three-dimensional structure of human CBS is of interest, although the roles of the mutated residues are not fully understood (12Meier M. Janosik M. Kery V. Kraus J.P. Burkhard P. EMBO J. 2001; 20: 3910-3916Crossref PubMed Scopus (275) Google Scholar, 14Meier M. Oliveriusova J. Kraus J.P. Burkhard P. Biochim. Biophys. Acta. 2003; 1647: 206-213Crossref PubMed Scopus (44) Google Scholar). This minireview focuses on relationships between CBS and other PLP enzymes, the structure, function, and regulation of CBS, and the relation of the structure and function of CBS to homocystinuria. Enzymes that have a PLP coenzyme catalyze a wide variety of reactions in amino acid metabolism (15Mehta P.K. Christen P. Adv. Enzymol. Relat. Areas Mol. Biol. 2000; 74: 129-184PubMed Google Scholar). PLP enzymes are divided into four families on the basis of similarities in three-dimensional structure, sequence, secondary structure, and hydrophobicity profiles (15Mehta P.K. Christen P. Adv. Enzymol. Relat. Areas Mol. Biol. 2000; 74: 129-184PubMed Google Scholar, 16Grishin N.V. Phillips M.A. Goldsmith E.J. Protein Sci. 1995; 4: 1291-1304Crossref PubMed Scopus (348) Google Scholar). Aspartate aminotransferase is the prototype of the largest family, the α family (15Mehta P.K. Christen P. Adv. Enzymol. Relat. Areas Mol. Biol. 2000; 74: 129-184PubMed Google Scholar) or Fold type I (16Grishin N.V. Phillips M.A. Goldsmith E.J. Protein Sci. 1995; 4: 1291-1304Crossref PubMed Scopus (348) Google Scholar). The tryptophan synthase β subunit is the prototype of the second largest family, the β family or Fold type II, which also contains CBS. Fig. 2 shows members of the β family and their evolutionary pedigree (15Mehta P.K. Christen P. Adv. Enzymol. Relat. Areas Mol. Biol. 2000; 74: 129-184PubMed Google Scholar). CBS is most closely related to O-acetylserine sulfhydrylase (cysteine synthase). The close structural relationship between the catalytic domains of these two enzymes has been demonstrated by x-ray crystallography (12Meier M. Janosik M. Kery V. Kraus J.P. Burkhard P. EMBO J. 2001; 20: 3910-3916Crossref PubMed Scopus (275) Google Scholar, 13Taoka S. Lepore B.W. Kabil O. Ojha S. Ringe D. Banerjee R. Biochemistry. 2002; 41: 10454-10461Crossref PubMed Scopus (128) Google Scholar). The alignment of several β family chains at the active site lysine that binds PLP (Fig. 3) shows that O-acetylserine sulfhydrylase (322 residues) represents the simplest catalytic core of enzymes in the β family and that most of the other enzymes have N- or C-terminal extensions that may serve regulatory roles. Biosynthetic threonine deaminase from Escherichia coli has a C-terminal extension that binds a feedback inhibitor (17Gallagher D.T. Gilliland G.L. Xiao G. Zondlo J. Fisher K.E. Chinchilla D. Eisenstein E. Structure. 1998; 6: 465-475Abstract Full Text Full Text PDF PubMed Scopus (165) Google Scholar). The structure of threonine deaminase reveals that the C-terminal regulatory domain projects out from a core of the catalytic PLP-containing N-terminal domain (17Gallagher D.T. Gilliland G.L. Xiao G. Zondlo J. Fisher K.E. Chinchilla D. Eisenstein E. Structure. 1998; 6: 465-475Abstract Full Text Full Text PDF PubMed Scopus (165) Google Scholar). Protein sequence alignments show that members of the β family exhibit significant homology in the core, catalytic region and little similarity in the N- and C-terminal extensions. A regulatory role for the C-terminal extension of human and yeast CBS is supported by the finding that removal of the C-terminal domain of the human enzyme (2Kery V. Poneleit L. Kraus J.P. Arch. Biochem. Biophys. 1998; 355: 222-232Crossref PubMed Scopus (139) Google Scholar, 18Shan X. Kruger W.D. Nat. Genet. 1998; 19: 91-93Crossref PubMed Scopus (109) Google Scholar, 19Taoka S. Widjaja L. Banerjee R. Biochemistry. 1999; 38: 13155-13161Crossref PubMed Scopus (84) Google Scholar, 20Oliveriusova J. Kery V. Maclean K.N. Kraus J.P. J. Biol. Chem. 2002; 277: 48386-48394Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar) or of the yeast enzyme (7Jhee K.-H. McPhie P. Miles E.W. Biochemistry. 2000; 39: 10548-10556Crossref PubMed Scopus (82) Google Scholar) increases the specific activity and alters the steady-state kinetic parameters. AdoMet does not activate the truncated human enzyme. T. cruzi CBS lacks the C-terminal domain and is not activated by AdoMet (9Nozaki T. Shigeta Y. Saito-Nakano Y. Imada M. Kruger W.D. J. Biol. Chem. 2001; 276: 6516-6523Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar). Removal of the C-terminal domain causes human CBS (2Kery V. Poneleit L. Kraus J.P. Arch. Biochem. Biophys. 1998; 355: 222-232Crossref PubMed Scopus (139) Google Scholar) and yeast CBS (7Jhee K.-H. McPhie P. Miles E.W. Biochemistry. 2000; 39: 10548-10556Crossref PubMed Scopus (82) Google Scholar) to dissociate from tetramers or higher multimers to dimers. Human CBS contains an N-terminal region of ∼70 amino acids (Figs. 1 and 3) that binds heme (12Meier M. Janosik M. Kery V. Kraus J.P. Burkhard P. EMBO J. 2001; 20: 3910-3916Crossref PubMed Scopus (275) Google Scholar) and is absent in yeast CBS (6Jhee K.-H. McPhie P. Miles E.W. J. Biol. Chem. 2000; 75: 11541-11544Abstract Full Text Full Text PDF Scopus (74) Google Scholar) and in T. cruzi CBS (9Nozaki T. Shigeta Y. Saito-Nakano Y. Imada M. Kruger W.D. J. Biol. Chem. 2001; 276: 6516-6523Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar), which do not contain heme. Analysis of the products of deletion mutagenesis of human CBS reveals that the N-terminal amino acids 1–39 do not play a significant role in structure or function (2Kery V. Poneleit L. Kraus J.P. Arch. Biochem. Biophys. 1998; 355: 222-232Crossref PubMed Scopus (139) Google Scholar, 20Oliveriusova J. Kery V. Maclean K.N. Kraus J.P. J. Biol. Chem. 2002; 277: 48386-48394Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar) (Fig. 3). Deletion of residues 1–70 yielded enzyme with reduced activity that did not bind heme; C-terminal truncation did not affect heme binding. Deletion of residues 1–70 and 401–551 yielded the catalytic core that had low activity and bound PLP but not heme (20Oliveriusova J. Kery V. Maclean K.N. Kraus J.P. J. Biol. Chem. 2002; 277: 48386-48394Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar). Sensitivity of CBS to AdoMet can be abolished by deleting eight residues from the C terminus but not just one residue (20Oliveriusova J. Kery V. Maclean K.N. Kraus J.P. J. Biol. Chem. 2002; 277: 48386-48394Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar). The C-terminal regulatory region also encompasses the previously defined "CBS domain" (21Bateman A. Trends Biochem. Sci. 1997; 22: 12-13Abstract Full Text PDF PubMed Scopus (452) Google Scholar). This hydrophobic sequence (CBS1), spanning amino acid residues 415–468 (Fig. 1), is conserved in a wide range of otherwise unrelated proteins (21Bateman A. Trends Biochem. Sci. 1997; 22: 12-13Abstract Full Text PDF PubMed Scopus (452) Google Scholar) (www.sanger.ac.uk/Users/agb/CBS/CBS.html). Based on sequence similarity with another CBS domain containing protein, inosine-monophosphate dehydrogenase from Streptomyces pyogenes, a second, less conserved CBS domain (CBS2) was identified (22Shan X. Dunbrack Jr., R.L. Christopher S.A. Kruger W.D. Hum. Mol. Genet. 2001; 10: 635-643Crossref PubMed Google Scholar) between amino acid residues 486 and 543 in the C-terminal regulatory region of human CBS (Fig. 1). The function of these domains in human CBS remains unknown, although the sharp transition of thermally induced CBS activation and the observation that mutations in these domains can constitutively activate the enzyme indicate that they play a role in the autoinhibitory function of the C-terminal region (23Janosik M. Kery V. Gaustadnes M. Maclean K.N. Kraus J.P. Biochemistry. 2001; 40: 10625-10633Crossref PubMed Scopus (140) Google Scholar). Recent work demonstrates that the tandem pairs of CBS domains (CBS residues 416–551) bind AdoMet (24Scott J.W. Hawley S.A. Green K.A. Anis M. Stewart G. Scullion G.A. Norman D.G. Hardie D.G. Pan D.A. Hudson E.R. Kontogiannis L. J. Clin. Invest. 2004; 113: 274-284Crossref PubMed Scopus (615) Google Scholar). Two groups have solved crystal structures of truncated forms of human CBS (12Meier M. Janosik M. Kery V. Kraus J.P. Burkhard P. EMBO J. 2001; 20: 3910-3916Crossref PubMed Scopus (275) Google Scholar, 13Taoka S. Lepore B.W. Kabil O. Ojha S. Ringe D. Banerjee R. Biochemistry. 2002; 41: 10454-10461Crossref PubMed Scopus (128) Google Scholar). The structure of enzyme containing residues 1–413 (12Meier M. Janosik M. Kery V. Kraus J.P. Burkhard P. EMBO J. 2001; 20: 3910-3916Crossref PubMed Scopus (275) Google Scholar) demonstrates that the fold of the catalytic domain closely resembles the catalytic domain of other β family structures: O-acetylserine sulfhydrylase, tryptophan synthase, threonine deaminase, aminocyclopropane deaminase, and threonine synthase. Heme binds to the N-terminal region at distal ends of the dimer (Fig. 4). His-65 and Cys-52 are the ligands to the heme iron. Both structures reveal important details about the PLP-binding site and residues in the catalytic site. Although the spectroscopic properties of PLP provide a sensitive probe for detecting intermediates in the CBS reaction, the presence of heme in human CBS largely masks the spectrum of PLP (25Kery V. Poneleit L. Meyer J.D. Manning M.C. Kraus J.P. Biochemistry. 1999; 38: 2716-2724Crossref PubMed Scopus (59) Google Scholar). Thus, the heme-independent yeast CBS is a useful substitute for studies of catalytic mechanism and kinetics (6Jhee K.-H. McPhie P. Miles E.W. J. Biol. Chem. 2000; 75: 11541-11544Abstract Full Text Full Text PDF Scopus (74) Google Scholar, 7Jhee K.-H. McPhie P. Miles E.W. Biochemistry. 2000; 39: 10548-10556Crossref PubMed Scopus (82) Google Scholar, 8Maclean K.N. Janosik M. Oliveriusova J. Kery V. Kraus J.P. J. Inorg. Biochem. 2000; 81: 161-171Crossref PubMed Scopus (41) Google Scholar, 26Jhee K.-H. Niks D. McPhie P. Dunn M.F. Miles E.W. Biochemistry. 2001; 40: 10873-10880Crossref PubMed Scopus (31) Google Scholar, 27Jhee K.-H. Niks D. McPhie P. Dunn M.F. Miles E.W. Biochemistry. 2002; 41: 1828-1835Crossref PubMed Scopus (10) Google Scholar, 28Taoka S. Banerjee R. J. Biol. Chem. 2002; 277: 22421-22425Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar, 29Aitken S.M. Kirsch J.F. Biochemistry. 2003; 42: 571-578Crossref PubMed Scopus (37) Google Scholar). Addition of l-serine to yeast CBS results in the disappearance of the 412 nm band of CBS and the appearance of a new species (λmax = 460 nm), which is attributed to a PLP-aminoacrylate intermediate (6Jhee K.-H. McPhie P. Miles E.W. J. Biol. Chem. 2000; 75: 11541-11544Abstract Full Text Full Text PDF Scopus (74) Google Scholar). This intermediate was first observed in the closely related enzyme, O-acetylserine sulfhydrylase (30Cook P.F. Wedding R.T. J. Biol. Chem. 1976; 251: 2023-2029Abstract Full Text PDF PubMed Google Scholar). Addition of l-cystathionine also yields an aminoacrylate intermediate, demonstrating the partial reversibility of the reaction. Heme-free crystals of human CBS also convert l-serine to an aminoacrylate intermediate as demonstrated by single crystal microspectrophotometry (31Bruno S. Schiarettti F. Burkhard P. Kraus J.P. Mozzarelli A. J. Biol. Chem. 2001; 276: 16-19Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar). Investigations of the steady-state kinetics of yeast CBS (7Jhee K.-H. McPhie P. Miles E.W. Biochemistry. 2000; 39: 10548-10556Crossref PubMed Scopus (82) Google Scholar) and human CBS (2Kery V. Poneleit L. Kraus J.P. Arch. Biochem. Biophys. 1998; 355: 222-232Crossref PubMed Scopus (139) Google Scholar, 32Taoka S. West M. Banerjee R. Biochemistry. 1999; 38: 2738-2744Crossref PubMed Scopus (99) Google Scholar) have utilized a sensitive but tedious 14C-labeled l-Ser assay. Kinetic data for truncated yeast CBS are consistent with a ping-pong mechanism in which aminoacrylate is a key intermediate (7Jhee K.-H. McPhie P. Miles E.W. Biochemistry. 2000; 39: 10548-10556Crossref PubMed Scopus (82) Google Scholar). Aitken and Kirsch (29Aitken S.M. Kirsch J.F. Biochemistry. 2003; 42: 571-578Crossref PubMed Scopus (37) Google Scholar) have used continuous assays for the forward and reverse reactions to study the kinetics of the truncated yeast CBS and to determine the equilibrium constant and the pH dependence of the kinetic parameters. The rate of the forward reaction is 38-fold greater than the reverse reaction. Thus, the CBS reaction strongly favors l-cystathionine formation in vivo. Recent kinetic studies of mutants of yeast CBS have characterized the roles of putative active site residues Thr-81, Thr-82, Gln-157, and Tyr-158 (33Aitken S.M. Kirsch J.F. Biochemistry. 2004; 43: 1963-1971Crossref PubMed Scopus (19) Google Scholar). Rapid kinetic studies of the truncated yeast CBS have characterized the reaction intermediates under pre-steady-state conditions (26Jhee K.-H. Niks D. McPhie P. Dunn M.F. Miles E.W. Biochemistry. 2001; 40: 10873-10880Crossref PubMed Scopus (31) Google Scholar). Binding of l-serine as the external aldimine is faster than formation of the aminoacrylate intermediate; the rate-limiting step is the reaction of aminoacrylate with l-homocysteine to form l-cystathionine. Full-length yeast CBS is reported to show some differences in kinetic behavior (28Taoka S. Banerjee R. J. Biol. Chem. 2002; 277: 22421-22425Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar). CBS catalyzes PLP-dependent β-replacement reactions (Equation 2) in which the electronegative substituent (X) in the β-position of the amino acid substrate is replaced by a nucleophile YH (reviewed in Refs. 34Miles E.W. Dolphin D. Poulson D. Avramovic O. Pyridoxal Phosphate: Chemical, Biochemical and Medical Aspects, Part B. 1B. John Wiley & Sons, Inc., New York1986: 253-310Google Scholar and 35Braunstein A.E. Goryachenkova E.V. Adv. Enzymol. Relat. Areas Mol. Biol. 1984; 56: 1-89Crossref PubMed Google Scholar). β-Replacement reactions (Equation 2) are also catalyzed by tryptophan synthase, O-acetylserine sulfhydrylase, and several other PLP enzymes.XCH2CH(NH2)COOH+YH↔XH+YCH2CH(NH2)COOH(Eq. 2) where X is OH or SH and Y is S or S-alkyl. Amino acid substrates for CBS include l-serine (X is OH), l-cysteine (X is SH), 3-chloroalanine (X is Cl), and serine O-sulfate (X is SO4); nucleophile substrates (YH) include l-homocysteine, 2-mercaptoethanol, and H2S (34Miles E.W. Dolphin D. Poulson D. Avramovic O. Pyridoxal Phosphate: Chemical, Biochemical and Medical Aspects, Part B. 1B. John Wiley & Sons, Inc., New York1986: 253-310Google Scholar, 35Braunstein A.E. Goryachenkova E.V. Adv. Enzymol. Relat. Areas Mol. Biol. 1984; 56: 1-89Crossref PubMed Google Scholar). The reaction of l-cysteine and 2-mercaptoethanol to form S-hydroxyethyl-l-cysteine and H2S is the basis of useful assay methods (7Jhee K.-H. McPhie P. Miles E.W. Biochemistry. 2000; 39: 10548-10556Crossref PubMed Scopus (82) Google Scholar, 36Willhardt I. Wiederanders B. Anal. Biochem. 1975; 63: 263-266Crossref PubMed Scopus (17) Google Scholar). Recent studies provide evidence that H2S is a gaseous neuromodulator and smooth muscle relaxant and that H2S is produced by CBS (37Kimura H. Mol. Neurobiol. 2002; 26: 13-19Crossref PubMed Google Scholar). Although the author suggests that H2S is produced by a β-elimination reaction with l-cysteine, H2S may be a product of the β-replacement reaction of l-cysteine with another thiol (38Maclean K.N. Kraus J.P. Wang R. Signal Transduction and the Gasotransmitters, NO, CO, and H2S in Biology and Medicine. Humana Press, Totowa, NJ2004Google Scholar). CBS will also very efficiently catalyze the formation of l-cysteine from l-serine and H2S. This serine sulfhydrylase reaction may be an alternative method of cysteine synthesis and H2S detoxification. 2J. P. Kraus, unpublished data. l-Allothreonine, but not l-threonine, serves as a primary substrate for yeast CBS and reacts with l-homocysteine to form a new amino acid, 3-methyl-l-cystathionine (27Jhee K.-H. Niks D. McPhie P. Dunn M.F. Miles E.W. Biochemistry. 2002; 41: 1828-1835Crossref PubMed Scopus (10) Google Scholar). The reaction has been characterized by spectroscopic measurements under pre-steady-state and steady-state conditions. The human CBS gene is transcriptionally regulated by two promoter regions designated –1a and –1b (39Bao L. Vlcek C. Paces V. Kraus J.P. Arch. Biochem. Biophys. 1998; 350: 95-103Crossref PubMed Scopus (90) Google Scholar). The major promoter (–1b) is serum and fibroblast growth factor-responsive and is down-regulated by insulin, growth arrest due to contact inhibition, nutrient depletion, or the induction of differentiation (40Maclean K.N. Janosik M. Kraus E. Kozich V. Allen R.H. Raab B.K. Kraus J.P. J. Cell. Physiol. 2002; 192: PubMed Scopus Google Scholar). The CBS –1b promoter is regulated in a by between and and and and are the 1 and is the a (reviewed in Refs. R. G. M.C. M. Valle E. G. M. de A. C. C. M. G. R. A. I. G. M. C. M. A. G. C. M. F. M. A. A. H. J. M. C. A. S. G. E. R. A. 1999; PubMed Scopus Google Scholar and G. 1999; PubMed Scopus Google Scholar). The and role of in CBS may by Y. M.A. J.W. Biochem. J. 2001; PubMed Scopus Google Scholar, K.N. Kraus E. Kraus J.P. J. Biol. Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar, J.P. Oliveriusova J. J. Kraus E. Vlcek C. de Franchis R. Maclean K.N. L. D. Paces V. Kozich V. 1998; PubMed Scopus Google Scholar). proteins and are related proteins that are important of the to the AdoMet can activate human CBS. removal of the C-terminal region also human the of activation is to that observed with AdoMet (2Kery V. Poneleit L. Kraus J.P. Arch. Biochem. Biophys. 1998; 355: 222-232Crossref PubMed Scopus (139) Google Scholar, 20Oliveriusova J. Kery V. Maclean K.N. Kraus J.P. J. Biol. Chem. 2002; 277: 48386-48394Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar, Banerjee R. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). Although the yeast enzyme is also activated by removal of the C-terminal AdoMet does not activate the yeast enzyme and other has been (7Jhee K.-H. McPhie P. Miles E.W. Biochemistry. 2000; 39: 10548-10556Crossref PubMed Scopus (82) Google Scholar). The role of heme in human CBS is not Heme is not for because it is absent in yeast CBS (6Jhee K.-H. McPhie P. Miles E.W. J. Biol. Chem. 2000; 75: 11541-11544Abstract Full Text Full Text PDF Scopus (74) Google Scholar, 7Jhee K.-H. McPhie P. Miles E.W. Biochemistry. 2000; 39: 10548-10556Crossref PubMed Scopus (82) Google Scholar, 8Maclean K.N. Janosik M. Oliveriusova J. Kery V. Kraus J.P. J. Inorg. Biochem. 2000; 81: 161-171Crossref PubMed Scopus (41) Google Scholar) and T. cruzi CBS (9Nozaki T. Shigeta Y. Saito-Nakano Y. Imada M. Kruger W.D. J. Biol. Chem. 2001; 276: 6516-6523Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar) and because human CBS has activity (20Oliveriusova J. Kery V. Maclean K.N. Kraus J.P. J. Biol. Chem. 2002; 277: 48386-48394Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar, S. Schiarettti F. Burkhard P. Kraus J.P. Mozzarelli A. J. Biol. Chem. 2001; 276: 16-19Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar). Human CBS may also be regulated by the of the heme. one group observed an in CBS activity of heme (reviewed in R. R. Kabil O. Ojha S. S. Biochim. Biophys. Acta. 2003; 1647: PubMed Scopus (99) Google Scholar), another group did not 2J. P. Kraus, unpublished data. Mutations found in patients with homocystinuria are in the catalytic and regulatory domains of human CBS a of than J.P. Janosik M. Kozich V. Mandell R. Shih V. Sperandeo M.P. Sebastio G. de Franchis R. Andria G. Kluijtmans L.A. Blom H. Boers G.H. Gordon R.B. Kamoun P. Tsai M.Y. Kruger W.D. Koch H.G. Ohura T. Gaustadnes M. Hum. Mutat. 1999; 13: 362-375Crossref PubMed Scopus (269) Google Scholar). of these mutations in a clinical with high of the PLP or is than it was that CBS activity can be by in with patients enzyme had reduced for PLP but not with patients enzyme had a reduced for the coenzyme Kraus J. J. Clin. Invest. 66: PubMed Scopus Google Scholar). have been a enzyme O. Banerjee R. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). The enzyme, a in bound PLP and a in activity O. Banerjee R. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). and a number of other mutations in the catalytic domain are by deletion of the C-terminal regulatory domain X. Kruger W.D. Nat. Genet. 1998; 19: 91-93Crossref PubMed Scopus (109) Google Scholar, O. Banerjee R. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar) or by specific mutations in region (22Shan X. Dunbrack Jr., R.L. Christopher S.A. Kruger W.D. Hum. Mol. Genet. 2001; 10: 635-643Crossref PubMed Google Scholar, M. Kery V. Gaustadnes M. Maclean K.N. Kraus J.P. Biochemistry. 2001; 40: 10625-10633Crossref PubMed Scopus (140) Google Scholar, R. Blom H. Boers G.H. Banerjee R. Biochemistry. 2002; 41: PubMed Scopus (46) Google Scholar). of these mutations are in the domain (21Bateman A. Trends Biochem. Sci. 1997; 22: 12-13Abstract Full Text PDF PubMed Scopus (452) Google 1) and or impair activation by The enzyme is constitutively although enzyme is not activated by AdoMet, it does bind AdoMet (23Janosik M. Kery V. Gaustadnes M. Maclean K.N. Kraus J.P. Biochemistry. 2001; 40: 10625-10633Crossref PubMed Scopus (140) Google Scholar). The observation that C-terminal or partial and AdoMet levels of activation of the type enzyme suggests that these different forms of activation are a by the domain from the active site (23Janosik M. Kery V. Gaustadnes M. Maclean K.N. Kraus J.P. Biochemistry. 2001; 40: 10625-10633Crossref PubMed Scopus (140) Google Scholar) Refs. X. Dunbrack Jr., R.L. Christopher S.A. Kruger W.D. Hum. Mol. Genet. 2001; 10: 635-643Crossref PubMed Google Scholar, R. R. Kabil O. Ojha S. S. Biochim. Biophys. Acta. 2003; 1647: PubMed Scopus (99) Google Scholar, and R. Blom H. Boers G.H. Banerjee R. Biochemistry. 2002; 41: PubMed Scopus (46) Google Scholar for related Analysis of the crystal structure of the truncated human CBS (12Meier M. Janosik M. Kery V. Kraus J.P. Burkhard P. EMBO J. 2001; 20: 3910-3916Crossref PubMed Scopus (275) Google Scholar, 14Meier M. Oliveriusova J. Kraus J.P. Burkhard P. Biochim. Biophys. Acta. 2003; 1647: 206-213Crossref PubMed Scopus (44) Google Scholar) shows that mutations are in several the dimer the active the heme-binding and the region between the catalytic domain and the regulatory domain (Fig. 4). The two most mutations in patients are which is to and which is not J.P. Janosik M. Kozich V. Mandell R. Shih V. Sperandeo M.P. Sebastio G. de Franchis R. Andria G. Kluijtmans L.A. Blom H. Boers G.H. Gordon R.B. Kamoun P. Tsai M.Y. Kruger W.D. Koch H.G. Ohura T. Gaustadnes M. Hum. Mutat. 1999; 13: 362-375Crossref PubMed Scopus (269) Google Scholar). the to the active may be in the and likely an role M. Oliveriusova J. Kraus J.P. Burkhard P. Biochim. Biophys. Acta. 2003; 1647: 206-213Crossref PubMed Scopus (44) Google Scholar). The residue with the of PLP (Fig. 4). The is and PLP binding. is in the dimer the enzyme is (9Nozaki T. Shigeta Y. Saito-Nakano Y. Imada M. Kruger W.D. J. Biol. Chem. 2001; 276: 6516-6523Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar). The is by mutations in the C-terminal domain or by deletion of domain X. Kruger W.D. Nat. Genet. 1998; 19: 91-93Crossref PubMed Scopus (109) Google Scholar, X. Dunbrack Jr., R.L. Christopher S.A. Kruger W.D. Hum. Mol. Genet. 2001; 10: 635-643Crossref PubMed Google Scholar). The and of the less mutations affect the the or the of CBS. work will likely on the structure of the enzyme to the regulatory region with the catalytic core and the human mutations