Oxalyl hydroxamates as reaction-intermediate analogues for ketol-acid reductoisomerase.
N-Hydroxy-N-isopropyloxamate (IpOHA) is an exceptionally potent inhibitor of the Escherichia coli ketol-acid reductoisomerase. In the presence of Mg2+ or Mn2+, IpOHA inhibits the enzyme in a time-dependent manner, forming a nearly irreversible complex. Nucleotide, which is essential for catalysis, greatly enhances the binding of IpOHA by the reductoisomerase, with NADPH (normally present during the enzyme's rearrangement step, i.e., conversion of a beta-keto acid into an alpha-keto acid, in either the forward or reverse physiological reactions) being more effective than NADP. In the presence of Mg2+ and NADPH, IpOHA appears to bind to the enzyme in a two-step mechanism, with an initial inhibition constant of 160 nM and a maximum rate of formation of the tight, slowly reversible complex of 0.57 min-1 (values that give an association rate of IpOHA, at low concentration, of 5.9 X 10(4) M-1 s-1). The rate of exchange of [14C]IpOHA from an enzyme-[14C]IpOHA-Mg2(+)-NADPH complex with exogenous, unlabeled IpOHA has a half-time of 6 days (150 h). This dissociation rate (1.3 X 10(-6) s-1) and the association rate determined by inactivation kinetics define an overall dissociation constant of 22 pM. By contrast, in the presence of Mn2+ and NADPH, the corresponding association and dissociation rates for IpOHA are 8.2 X 10(4) M-1 s-1 and 3.2 X 10(-6) s-1 (half-time = 2.5 days), respectively, which define an overall dissociation constant of 38 pM. In the presence of NADP or in the absence of nucleotide (both in the presence of Mg2+), the enzyme-IpOHA complex is far more labile, with dissociation half-times of 28 and 2 h, respectively. In the absence of Mg2+ or Mn2+, IpOHA does not exhibit time-dependent inhibition of the reductoisomerase.(ABSTRACT TRUNCATED AT 250 WORDS)
Aulabaugh
A
A
Central Research and Development Department, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19880-0328.
Schloss
J V
JV
D000429:Alcohol Oxidoreductases / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D055598:Chemical Phenomena; D002621:Chemistry; D004789:Enzyme Activation / Q000187:drug effects; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology*; D006877:Hydroxamic Acids / Q000493:pharmacokinetics*; D050540:Ketol-Acid Reductoisomerase; D008274:Magnesium / Q000378:metabolism; D015394:Molecular Structure; D009243:NAD / Q000378:metabolism; D010070:Oxalates / Q000493:pharmacokinetics*
D006877:Hydroxamic Acids; D010070:Oxalates; D009243:NAD; C064405:N-hydroxy-N-isopropyloxamate; D000429:Alcohol Oxidoreductases; D050540:Ketol-Acid Reductoisomerase; D008274:Magnesium