title: Sulfhydryl groups in relation to the structure and catalytic activity of 2-oxo-4-hydroxyglutarate aldolase from bovine liver.
abstract: Bovine liver 2-oxo-4-hydroxyglutarate aldolase (suggested name: 2-oxo-4-hydroxyglutarate glyoxylate-lyase catalyzing the reaction: 2-oxo-4-hydroxyglutarate in equilibrium pyruvate + glyoxylate) contains eight to ten sulfhydryl groups as determined by titration of the enzyme with either 5,5'-dithiobis(2-nitrobenzoic acid) (Nbs2) or p-mercuribenzoate in the presence of 1% sodium dodecyl sulfate. In the absence of a denaturant, all of the cysteinyl residues react with p-mercuribenzoate whereas only four are accessible to titration with Nbs2. No differences in -SH group reactivity can be detected during titration of the aldolase with p-mercuribenzoate. In contrast, two classes of sulfhydryls can be differentiated in the disulfide exchange reaction with Nbs2 in the absence of a denaturant; one -SH group (Class I) reacts rapidly whereas three additional thiols (Class II) titrate at approx. 0.1 the rate of the Class I-SH residue. Both pyruvate and glyoxylate protect one of the three -SH residues in Class II from reaction with Nbs2. Either substrate also prevents titration of one to two thiol groups by p-mercuribenzoate and decreases the rate of reaction of aldolase -SH groups with Nbs2 in 8 M urea. These ligand-induced changes in -SH reactivity provide a sensitive indication that the enzyme exists in an altered conformational state in the presence of either of its cosubstrates. Titration of the enzyme with either Nbs2 or p-mercuribenzoate results in a progressive loss of aldolase activity which is not proportional to the number of -SH groups modified. The enzyme retains 50% of the activity of the native enzyme when Class I and Class II thiols (i.e. four -SH groups total) are modified with Nbs2; 15% residual activity is still observed following titration of all of the cysteinyl residues with p-mercuribenzoate. Pyruvate and glyoxylate provide partial protection against inactivation. It is concluded that inactivation of 2-oxo-4-hydroxyglutarate aldolase by Nbs2 or p-mercuribenzoate is a consequence of alterations in protein structure which accompany modification of -SH groups. The data argue against the direct participation of an active-site thiol group in the catalytic mechanism of 2-oxo-4-hydroxyglutarate aldolase, be that aldol cleavage and condensation or beta-decarboxylation.
mesh_terms: D000446:Aldehyde-Lyases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D004228:Dithionitrobenzoic Acid / Q000494:pharmacology; D006038:Glyoxylates / Q000494:pharmacology; D007656:Ketoglutaric Acids; D008099:Liver / Q000201:enzymology*; D008623:Mercaptoethanol / Q000494:pharmacology; D008626:Mercuribenzoates / Q000494:pharmacology; D008968:Molecular Conformation; D007652:Oxo-Acid-Lyases; D011773:Pyruvates / Q000494:pharmacology; D013329:Structure-Activity Relationship; D013438:Sulfhydryl Compounds
chemical_list: D006038:Glyoxylates; D007656:Ketoglutaric Acids; D008626:Mercuribenzoates; D011773:Pyruvates; D013438:Sulfhydryl Compounds; D008623:Mercaptoethanol; D004228:Dithionitrobenzoic Acid; D000446:Aldehyde-Lyases; D007652:Oxo-Acid-Lyases; C021415:4-hydroxy-2-oxoglutarate aldolase