Active subunits in hybrid-modified malate dehydrogenase

Engineering the quaternary structure of an enzyme: construction and analysis of a monomeric form of malate dehydrogenase from Escherichia coli

The citric acid cycle enzyme, malate dehydrogenase (MDH), is a dimer of identical subunits. In the crystal structures of 2 prokaryotic and 2 eukaryotic forms, the subunit interface is conformationally homologous. To determine whether or not the quaternary structure of MDH is linked to the catalytic activity, mutant forms of the enzyme from Escherichia coli have been constructed. Utilizing the high-resolution structure of E. coli MDH, the dimer interface was analyzed critically for side chains that were spatially constricted and needed for electrostatic interactions. Two such residues were found, D45 and S226. At their nearest point in the homodimer, they are in different subunits, hydrogen bond across the interface, and do not interact with any catalytic residues. Each residue was mutated to a tyrosine, which should disrupt the interface because of its large size. All mutants were cloned and purified to homogeneity from an mdh- E. coli strain (BHB111). Gel filtration of the mutants show that D45Y and D45Y/S226Y are both monomers, whereas the S226Y mutant remains a dimer. The monomeric D45Y and D45Y/S226Y mutants have 14,000- and 17,500-fold less specific activity, respectively, than the native enzyme. The dimeric S226Y has only 1.4-fold less specific activity. All forms crystallized, indicating they were not random coils. Data have been collected to 2.8 A resolution for the D45Y mutant. The mutant is not isomorphous with the native protein and work is underway to solve the structure by molecular replacement.

Dissociation of enzyme oligomers: a mechanism for allosteric regulation

Most enzymes exist as oligomers or polymers, and a significant subset of these (perhaps 15% of all enzymes) can reversibly dissociate and reassociate in response to an effector ligand. Such a change in subunit assembly usually is accompanied by a change in enzyme activity, providing a mechanism for regulation. Two models are described for a physical mechanism, leading to a change in activity: (1) catalytic activity depends on subunit conformation, which is modulated by subunit dissociation; and (2) catalytic or regulatory sites are located at subunit interfaces and are disrupted by subunit dissociation. Examples of such enzymes show that both catalytic sites and regulatory sites occur at the junction of 2 subunits. In addition, for 9 enzymes, kinetic studies supported the existence of a separate regulatory site with significantly different affinity for the binding of either a substrate or a product of that enzyme. Over 40 dissociating enzymes are described from 3 major metabolic areas: carbohydrate metabolism, nucleotide metabolism, and amino acid metabolism. Important variables that influence enzyme dissociation include: enzyme concentration, ligand concentration, other cellular proteins, pH, and temperature. All these variables can be readily manipulated in vitro, but normally only the first two are physiological variables. Seven of these enzymes are most active as the dissociated monomer, the others as oligomers, emphasizing the importance of a regulated equilibrium between 2 or more conformational states. Experiments to test whether enzyme dissociation occurs in vivo showed this to be the case in 6 out of 7 studies, with 4 different enzymes.

Aspartate aminotransferase isoenzymes in Leptosphaeria michotii. Properties and intracellular location

Two forms of aspartate aminotransferase were obtained from the fungus Leptosphaeria michotii and purified to a state of apparent homogeneity by a five-step purification procedure ending with blue Ultrogel chromatography. Holoenzyme specific activities were 13430 and 9110 nkat oxalacetate/mg protein-1 and isoelectric points were 7.1 and 7.0 for forms A and B, respectively. Both isoenzymes were isologous dimers of Mr 92,000. They differed mainly in their Km for 2-oxoglutarate and aspartate, their ability to use cysteine sulfinate as a substrate and their ability in vitro to be specifically tightly associated as follows: form A with a malate dehydrogenase monomer of Mr 25,000; form B with an unidentified protein of Mr 40,000-44,000. Rabbit antiserum raised against the form A holoenzyme was not reactive against the form B holoenzyme and vice versa. Association of the holoenzyme with the complex essentially provoked a shift of the isoelectric point to 5.8 for form B [corrected] and to 5.2 for form B, without affecting kinetic parameters. In order to localize in situ the two transaminase forms, ultrastructural detection was carried out by immunogold staining of thin sections of Lowicryl-K4M-embedded colonies. Antiserum against form A essentially labelled cytoplasm and cell wall and, to some extent, mitochondria, while antiserum against form B heavily labelled mitochondria and cell wall and to a lesser extent cytoplasm. Moreover, mitochondria were isolated and purified by Percoll-density-gradient centrifugation. Only form A was identified in this subcellular fraction using ELISA.

Regulation of mitochondrial malate dehydrogenase: kinetic modulation independent of subunit interaction

Porcine heart mitochondrial malate dehydrogenase (EC 1.1.1.37), a dimeric enzyme of Mr = 70,000, is both allosterically activated and inhibited by citrate. Using an affinity elution procedure based upon citrate binding to malate dehydrogenase, the isolation of pure heterodimer (a dimeric species with one active subunit and one iodoacetamide-inactivated subunit) has been achieved. Investigations utilizing this heterodimer in conjunction with resin-bound monomers of malate dehydrogenase have allowed the formulation of a definite conclusion concerning the role of subunit interactions in catalysis and regulation of this enzyme. The citrate kinetic effects, oxaloacetate inhibition, malate activation, and the effects of 2-thenoyl-trifluoroacetone (TTFA) are shown to be independent of interaction between catalytically active subunits. Previous kinetic data thought to support a reciprocating catalytic mechanism for this enzyme may be reinterpreted upon closer analysis in relation to an allosteric, conformationally specific binding model for malate dehydrogenase.

A facile method for the isolation of porcine heart mitochondrial malate dehydrogenase by affinity elution chromatography on Procion Red HE3B

A quick, simple method has been devised for isolating pig heart mitochondrial malate dehydrogenase in apparently homogeneous state and good yield. It entails the adsorption of the enzyme to agarose-linked Procion Red HE3B and specific elution of a ternary complex consisting of the malate dehydrogenase, NAD+, and L-malate.