title: Mechanism of reconstitution of brewers' yeast pyruvate decarboxylase with thiamin diphosphate and magnesium.
abstract: Reconstitution of apo-pyruvate decarboxylase isozymes (PDC, EC 4.1.1.1) from Saccharomyces carlsbergensis was investigated by determination of the steady-state kinetics of the reaction with thiamin diphosphate (TDP) and Mg2+ in the presence and absence of substrate (pyruvate) or allosteric effector (pyruvamide). Reconstitution of the PDC isozyme mixture and alpha 4 isozyme (alpha 4-PDC) exhibits biphasic kinetics with 52 +/- 11% of the PDC reacting with k1 = (1.0 +/- 0.3) x 10(-2) s-1 and 48 +/- 12% of the PDC reacting with k2 = (1.1 +/- 0.6) x 10(-1) s-1 when TDP (KTDP = 0.5 +/- 0.2 mM) is added to apo-PDC equilibrated with saturating Mg2+. PDC reconstitution exhibits first-order kinetics with k1 = (1.6 +/- 0.5) x 10(-2) s-1 upon addition of Mg2+ (KMg2+ = 0.2 +/- 0.1 mM) to apo-PDC equilibrated with saturating TDP. Biphasic kinetics for the PDC isozymes provides evidence that apo-PDC reconstitution with TDP and Mg2+ involves two pathways, TDP binding followed by Mg2+ (k1) or Mg2+ binding followed by TDP (k2). This is supported by a change in reconstitution pathway with the order of cofactor addition and is inconsistent with a single pathway involving ordered binding of the metal ion followed by TDP. The presence of pyruvamide has no significant effect on the rate constants for apo-PDC reconstitution and favors the k2 pathway; pyruvate decreases the value of k2 < or = 3-fold and has no effect on the value of k1.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biochemistry, Johns Hopkins University, Baltimore, Maryland 21205-2179, USA.
authors.forename: J A
authors.initials: JA
authors.lastname: Vaccaro
authors.forename: E J
authors.initials: EJ
authors.lastname: Crane
authors.forename: T K
authors.initials: TK
authors.lastname: Harris
authors.forename: M W
authors.initials: MW
authors.lastname: Washabaugh
mesh_terms: D007527:Isoenzymes / Q000737:chemistry*; D007700:Kinetics; D008274:Magnesium / Q000737:chemistry*; D011767:Pyruvate Decarboxylase / Q000737:chemistry*; D012441:Saccharomyces cerevisiae / Q000201:enzymology*; D013835:Thiamine Pyrophosphate / Q000737:chemistry*
chemical_list: D007527:Isoenzymes; D011767:Pyruvate Decarboxylase; D008274:Magnesium; D013835:Thiamine Pyrophosphate