Publications

• Structure and function of S-adenosylhomocysteine hydrolase.

  Turner M.A., Yang X., Yin D., Kuczera K., Borchardt R.T., Howell P.L.

  In mammals, S-adenosylhomocysteine hydrolase (AdoHcyase) is the only known enzyme to catalyze the breakdown of S-adenosylhomocysteine (AdoHcy) to homocysteine and adenosine. AdoHcy is the product of all adenosylmethionine (AdoMet)-dependent biological transmethylations. These reactions have a wide ... >> More

  In mammals, S-adenosylhomocysteine hydrolase (AdoHcyase) is the only known enzyme to catalyze the breakdown of S-adenosylhomocysteine (AdoHcy) to homocysteine and adenosine. AdoHcy is the product of all adenosylmethionine (AdoMet)-dependent biological transmethylations. These reactions have a wide range of products, and are common in all facets of biometabolism. As a product inhibitor, elevated levels of AdoHcy suppress AdoMet-dependent transmethylations. Thus, AdoHcyase is a regulator of biological transmethylation in general. The three-dimensional structure of AdoHcyase complexed with reduced nicotinamide adenine dinucleotide phosphate (NADH) and the inhibitor (1'R, 2'S, 3'R)-9-(2',3'-dihyroxycyclopenten-1-yl)adenine (DHCeA) was solved by a combination of the crystallographic direct methods program, SnB, to determine the selenium atom substructure and by treating the multiwavelength anomalous diffraction data as a special case of multiple isomorphous replacement. The enzyme architecture resembles that observed for NAD-dependent dehydrogenases, with the catalytic domain and the cofactor-binding domain each containing a modified Rossmann fold. The two domains form a deep active site cleft containing the cofactor and bound inhibitor molecule. A comparison of the inhibitor complex of the human enzyme and the structure of the rat enzyme, solved without inhibitor, suggests that a 17 degrees rigid body movement of the catalytic domain occurs upon inhibitor/substrate binding. << Less

  Cell Biochem Biophys 33:101-125(2000) [PubMed] [EuropePMC]

• Catalytic strategy of S-adenosyl-L-homocysteine hydrolase: transition-state stabilization and the avoidance of abortive reactions.

  Yang X., Hu Y., Yin D.H., Turner M.A., Wang M., Borchardt R.T., Howell P.L., Kuczera K., Schowen R.L.

  S-Adenosylhomocysteine hydrolase (AdoHcy hydrolase) crystallizes from solutions containing the intermediate analogue neplanocin A with the analogue bound in its 3'-keto form at the active sites of all of its four subunits and the four tightly bound cofactors in their reduced (NADH) state. The enzy ... >> More

  S-Adenosylhomocysteine hydrolase (AdoHcy hydrolase) crystallizes from solutions containing the intermediate analogue neplanocin A with the analogue bound in its 3'-keto form at the active sites of all of its four subunits and the four tightly bound cofactors in their reduced (NADH) state. The enzyme is in the closed conformation, which corresponds to the structure in which the catalytic chemistry occurs. Examination of the structure in the light of available, very detailed kinetic studies [Porter, D. J., Boyd, F. L. (1991) J. Biol. Chem. 266, 21616-21625. Porter, D. J., Boyd, F. L. (1992) J. Biol. Chem. 267, 3205-3213. Porter, D. J. (1998) J. Biol. Chem. 268, 66-73] suggests elements of the catalytic strategy of AdoHcy hydrolase for acceleration of the reversible conversion of AdoHcy to adenosine (Ado) and homocysteine (Hcy). The enzyme, each subunit of which possesses a substrate-binding domain that in the absence of substrate is in rapid motion relative to the tetrameric core of the enzyme, first binds substrate and ceases motion. Probably concurrently with oxidation of the substrate to its 3'-keto form, the closed active site is "sealed off" from the environment, as indicated by a large (10(8)(-)(9)-fold) reduction in the rate of departure of ligands, a feature that prevents exposure of the labile 3'-keto intermediates to the aqueous environment. Elimination of the 5'-substituent (Hcy in the hydrolytic direction, water in the synthetic direction) generates the central intermediate 4',5'-didehydro-5'-deoxy-3'-ketoadenosine. Abortive 3'-reduction of the central intermediate is prevented by a temporary suspension of all or part of the redox catalytic power of the enzyme during the existence of the central intermediate. The abortive reduction is 10(4)-fold slower than the productive reductions at the ends of the catalytic cycle and has a rate constant similar to those of nonenzymic intramolecular model reactions. The mechanism for suspending the redox catalytic power appears to be a conformationally induced increase in the distance across which hydride transfer must occur between cofactor and substrate, the responsible conformational change again being that which "seals" the active site. The crystal structure reveals a well-defined chain of three water molecules leading from the active site to the subunit surface, which may serve as a relay for proton exchange between solvent and active site in the closed form of the enzyme, permitting maintenance of active-site functional groups in catalytically suitable protonation states. << Less

  Biochemistry 42:1900-1909(2003) [PubMed] [EuropePMC]

• The enzymatic synthesis of S-adenosyl-L-homocysteine from adenosine and homocysteine.

  DE LA HABA G., CANTONI G.L.

  J Biol Chem 234:603-608(1959) [PubMed] [EuropePMC]

• The mechanism of action of S-adenosylhomocysteinase.

  Palmer J.L., Abeles R.H.

  S-Adenosylhomocysteinase catalyzes the reversible hydrolysis of S-adenosyl-L-homocysteine (AdoHcy) to adenosine and L-homocysteine without added cofactors. A mechanism is proposed which involves oxidation of the 3'-hydroxyl group of AdoHcy by enzyme-bound NAD+. Following oxidation, L-homocysteine ... >> More

  S-Adenosylhomocysteinase catalyzes the reversible hydrolysis of S-adenosyl-L-homocysteine (AdoHcy) to adenosine and L-homocysteine without added cofactors. A mechanism is proposed which involves oxidation of the 3'-hydroxyl group of AdoHcy by enzyme-bound NAD+. Following oxidation, L-homocysteine is eliminated, alpha-beta, to give 3'-keto-4'-5'-dehydroadenosine. This compound reacts with water in a Michael type addition to form 3'-ketoadenosine which is then reduced to adenosine. This mechanism is supported by these facts. 1) The enzyme contains 1 tightly bound NAD+ per subunit. Upon addition of substrate, this NAD is converted to NADH. 2) The enzyme catalyzes the exchange of the 4'-proton of substrate with solvent. This exchange is an integral part of the catalytic mechanism. 3) The hydrolysis of [4'-2H]S-adenosyl-L-homocysteine has a Vmax isotope effect of 1.44. This provides additional evidence that cleavage of the C-4' C-H bond is a step on the reaction pathway. 4) 4',5'-Dehydroadenosine is oxidized by the enzyme, then converted into adenosine or into AdoHcy in the presence of L-homocysteine. 5) An adenosine analog, 5'-deoxyadenosine, is oxidized by the enzyme to yield 3'-keto-5'-deoxyadenosine, and an analog of the proposed intermediate, 3'-ketoadenosine. 6) The enzyme catalyzes the exchange of the C-4' proton of 5'-deoxyadenosine. Since the enzyme catalyzes proton abstraction without OH elimination, it was concluded that the elimination of H2O from adenosine proceeds by a carbanion mechanism and not by a concerted elimination. Substrate analogs in which the 5'-OH group of adenosine is replaced by -F, -Cl, or -SMe are not substrates for the enzyme. << Less

  J Biol Chem 254:1217-1226(1979) [PubMed] [EuropePMC]

• Human S-adenosylhomocysteine hydrolase: common gene sequence variation and functional genomic characterization.

  Feng Q., Keshtgarpour M., Pelleymounter L.L., Moon I., Kalari K.R., Eckloff B.W., Wieben E.D., Weinshilboum R.M.

  S-Adenosylhomocysteine hydrolase (AHCY) is the only mammalian enzyme known to catalyze the hydrolysis of S-adenosylhomocysteine. We have used a genotype-to-phenotype strategy to study this important enzyme by resequencing AHCY in 240 DNA samples from four ethnic groups. Thirty-nine polymorphisms w ... >> More

  S-Adenosylhomocysteine hydrolase (AHCY) is the only mammalian enzyme known to catalyze the hydrolysis of S-adenosylhomocysteine. We have used a genotype-to-phenotype strategy to study this important enzyme by resequencing AHCY in 240 DNA samples from four ethnic groups. Thirty-nine polymorphisms were identified - 28 of which were novel. Functional genomic studies for wild type AHCY and the three variant allozymes identified showed that two variant allozymes had slight, but significant decreases in enzyme activity, but with no significant differences in levels of immunoreactive protein. Luciferase reporter gene assays for common 5'-flanking region haplotypes revealed that one haplotype with a frequency of approximately 2% in Caucasian-American subjects displayed a decreased ability to drive transcription. The variant nucleotide at 5'-flanking region single nucleotide polymorphism (SNP) (-34) in that haplotype altered the DNA-protein binding pattern during electrophoresis mobility shift assay. Finally, an AHCY genotype-phenotype association study for expression in lymphoblastoid cells identified four SNPs that were associated with decreased expression. For the IVS6 (intervening sequence 6, i.e., intron 6) G56 > C SNP among those four, electrophoresis mobility shift assay showed that a C > G nucleotide change resulted in an additional shifted band. These results represent a step toward understanding the functional consequences of common genetic variation in AHCY for the regulation of neurotransmitter, drug and macromolecule methylation. << Less

  J Neurochem 110:1806-1817(2009) [PubMed] [EuropePMC]