Elementary steps in the reaction mechanism of pyruvate dehydrogenase multienzyme complex from Escherichia coli: kinetics of flavin reduction

Protein-protein interactions in assembly of lipoic acid on the 2-oxoacid dehydrogenases of aerobic metabolism

Lipoic acid is a covalently attached cofactor essential for the activity of 2-oxoacid dehydrogenases and the glycine cleavage system. In the absence of lipoic acid modification, the dehydrogenases are inactive, and aerobic metabolism is blocked. In Escherichia coli, two pathways for the attachment of lipoic acid exist, a de novo biosynthetic pathway dependent on the activities of the LipB and LipA proteins and a lipoic acid scavenging pathway catalyzed by the LplA protein. LipB is responsible for octanoylation of the E2 components of 2-oxoacid dehydrogenases to provide the substrates of LipA, an S-adenosyl-L-methionine radical enzyme that inserts two sulfur atoms into the octanoyl moiety to give the active lipoylated dehydrogenase complexes. We report that the intact pyruvate and 2-oxoglutarate dehydrogenase complexes specifically copurify with both LipB and LipA. Proteomic, genetic, and dehydrogenase activity data indicate that all of the 2-oxoacid dehydrogenase components are present. In contrast, LplA, the lipoate protein ligase enzyme of lipoate salvage, shows no interaction with the 2-oxoacid dehydrogenases. The interaction is specific to the dehydrogenases in that the third lipoic acid-requiring enzyme of Escherichia coli, the glycine cleavage system H protein, does not copurify with either LipA or LipB. Studies of LipB interaction with engineered variants of the E2 subunit of 2-oxoglutarate dehydrogenase indicate that binding sites for LipB reside both in the lipoyl domain and catalytic core sequences. We also report that LipB forms a very tight, albeit noncovalent, complex with acyl carrier protein. These results indicate that lipoic acid is not only assembled on the dehydrogenase lipoyl domains but that the enzymes that catalyze the assembly are also present "on site."

Unusual kinetic behavior predicted for alpha-keto acid dehydrogenase complexes

A novel regulation type, which may be observed as an unusual kinetic 'cooperativity', is predicted for the alpha-keto acid dehydrogenase complexes. The inter-relationship of this regulation with the well-known regulatory effect of enzyme phosphorylation is discussed.

Kinetic fluorescence measurement of fluorescein di-beta-D-galactoside hydrolysis by beta-galactosidase: intermediate channeling in stepwise catalysis by a free single enzyme

Kinetic fluorescence measurements were employed to quantitative to stepwise hydrolysis of fluorescein di-beta-D-galactoside (FDG) by beta-galactosidase and the intermediate fluorescein mono-beta-D-galactoside (FMG) channeling. The kinetic parameters, Michaelis-Menten constant Km and enzymatic catalysis rate k2, for FDG hydrolysis to FMG by beta-galactosidase were obtained as 18.0 microM and 1.9 mumol.(min-mg)-1, respectively. The FMG intermediate is hydrolyzed via two modes: (1) FMG that is in free solution binding to the enzyme substrate binding site in competition with FDG and then being hydrolyzed (binding mode); (2) FMG being directly hydrolyzed into the final products of fluorescein and galactose before the FMG can diffuse away from the enzyme active site (channeling mode). The extent of the FMG channeling mode was found to depend on the FDG hydrolysis rate but to be independent of the free enzyme concentration. A channeling factor, defined as the ratio of the real FMG hydrolysis rate with both binding and channeling modes over that which would be observed with an exclusive binding mode, was used to quantitate the effect of the intermediate channeling. The FMG channeling factor was determined to be close to 1 at low FDG concentration (about 5.1 microM), where the slow FDG hydrolysis rate gives an ineffective channeling and where the FMG is then hydrolyzed mainly via the binding mode. However, the channeling factor dramatically increases at higher FDG concentrations (greater than Km), strongly indicating that the effective FMG channeling mode, resulting from the considerable FDG hydrolysis rate at high FDG concentrations, becomes a primary pathway to channel a steady system hydrolysis with a high rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate-induced structural changes of the pyruvate dehydrogenase multienzyme complex

The time course of the overall reaction catalyzed by the pyruvate dehydrogenase multienzyme complex produces an unexpectedly high lag (tau = 8 S) even in the presence of saturating concentrations of its substrates. The preincubation of the pyruvate dehydrogenase complex with one of the substrates alone decreases the duration of this lag, and all the substrates of the pyruvate dehydrogenase component (E1) and dihydrolipoyl transacetylase component (E2) together (pyruvate, thiamine pyrophosphate, and CoA) result in the complete disappearance of the lag. The reduction of the dihydrolipoyl dehydrogenase component (E3) of the pyruvate dehydrogenase complex with the substrates of the complex in the absence of NAD+ produces significantly different quenching in the FAD fluorescence, and then the reduction with the substrates of E3 as dihydrolipoic acid and dithioerythritol. (The formation of FADH2 was not observed in the system.) The higher fluorescence quenching in the presence of substrates of pyruvate dehydrogenase complex compared to the effect caused by the substrates of the E3 component (dihydrolipoic acid and DTE) indicates conformational changes additionally manifested in the fluorescence properties of the enzyme complex. The substrate-induced quenching of the enzyme-bound FAD fluorescence shows biphasic kinetics. The rate constant of the slow phase is comparable with the rate constant calculated from the time duration of the lag phase observed in the overall reaction. The kinetic analysis of both intensity and anisotropy decrease of the FAD fluorescence suggests a consecutive transmittance of an all substrate-coordinated, induced conformational changes directed from the pyruvate dehydrogenase-via the lipoyl transacetylase--to the lipoyl dehydrogenase. Two simultaneous conformational effects caused by binding of the substrates can be distinguished; one of them results the fluorescence of the bound FAD to be more quenched, while the other makes the FAD more mobile. The first-order rate constants of both these conformational changes were determined. The present observations suggest that the pyruvate dehydrogenase complex exists in a partially inactive state in the absence of its substrates, and it becomes active due to conformational changes caused by the binding of its substrates.

Elementary steps in the reaction mechanism of chicken liver fatty acid synthase: beta-ketoacyl reductase and enoyl reductase

The following reactions catalyzed by chicken liver fatty acid synthase have been studied with the stopped-flow method in 0.1 M potassium phosphate (pH 7.0) and 1 mM ethylenediaminetetraacetic acid at 25 degrees C by monitoring the change in NADPH fluorescence: the transfer of acetoacetyl from acetoacetyl coenzyme A to the enzyme, reduction of the enzyme-bound acetoacetyl by NADPH (beta-ketoacyl reductase), and reduction of enzyme-bound D-hydroxybutyryl/crotonyl by NADPH (enoyl reductase). The first two reactions were studied by mixing enzyme-NADPH with acetoacetyl-CoA under conditions where the kinetics can be analyzed as two consecutive pseudo-first-order processes: a mechanism consistent with the aceto-acetyl-CoA dependence of the pseudo-first-order rate constant associated with formation of the aceto-acetyl-enzyme is a relatively rapid binding of substrate to the enzyme, with a dissociation constant of 650 microM, followed by formation of covalently bound acetoacetyl, with a rate constant of 10.2 s-1. The aceto-acetyl-enzyme is reduced by enzyme-bound NADPH with a rate constant of 20 s-1, and the NADPH binding is characterized by a dissociation constant of 5.3 microM. Reduction of the D-hydroxybutyryl-/crotonyl-enzyme was studied by mixing NADPH with enzyme that was equilibrated with D-hydroxybutyryl-CoA or crotonyl-CoA; the rate constant for reduction of an equilibrium mixture of D-hydroxybutyryl- and crotonyl-enzyme is 36.6 s-1. Steady-state kinetic studies of the reduction of acetoacetyl-CoA and crotonyl-CoA by NADPH also have been carried out.(ABSTRACT TRUNCATED AT 250 WORDS)