Studies of glutamate dehydrogenase. Analysis of quaternary structure and contact areas between the polypeptide chains

Investigation of the effects of crosslinking glutamate dehydrogenase with dimethyl pimelimidate

Chemical crosslinking with dimethyl pimelimidate has been used to examine the quaternary structure and conformational mobility of bovine liver glutamate dehydrogenase. Crosslinking patterns are shown to be consistent with either a stacked or staggered dimer of trimers structure of the hexamer. Crosslinking in the absence of coligands results in a small loss of activity but an almost complete loss of GTP inhibitory effects. Protection experiments show that the active site can be protected by a variety of ligand combinations, and that the loss of GTP inhibition is protected by several complexes containing either NADH or NADPH, indicating that the second coenzyme site per subunit (which preferentially binds NADH) is not involved in the protection process. A significant loss of ADP activation occurs during crosslinking which is not protected against by any combination of protecting ligands tried, including those which involve second coenzyme site binding, showing that the ADP site is functionally distinct from the GTP site and from the second coenzyme binding site. Crosslinking in the presence of protecting ligands gives similar gel patterns to those obtained in the absence of protection. Affinity chromatography experiments show that the crosslinked enzyme still binds GTP despite the loss of GTP inhibition, and hysteresis experiments show that the second coenzyme site is left functional if protected with either coenzyme. A model is presented where crosslinking affects the conformational linkage between various ligand binding sites involved in GTP inhibition rather than the sites themselves.

Specific modification of a single cysteine residue in both bovine liver glutamate dehydrogenase and yeast glyceraldehyde-3-phosphate dehydrogenase. Difference in the mode of modification by pyrene maleimide

Pyrene maleimide is shown to be a 'half of the sites' reagent for glutamate dehydrogenase and for glyceraldehyde-3-phosphate dehydrogenase. The modified residues are identified as cysteine-115 for glutamate dehydrogenase and cysteine-149 for glyceraldehyde-3-phosphate dehydrogenase. The two enzymes react differently with pyrene maleimide. Whereas the hydrophobic environment of cysteine-115 directs the modification of glutamate dehydrogenase, the high reactivity of cysteine-149 determines the specific modification of glyceraldehyde-3-phosphate dehydrogenase. Glutamate dehydrogenase activity is unaltered by the modification: glyceraldehyde-3-phosphate dehydrogenase activity in inhibited.

The quaternary structure of bovine alpha-crystallin. Chemical crosslinking with bifunctional imido esters

Reaction of calf alpha-crystallin, which consists of about 30 A and 10 B subunits, with various bisimido esters at pH 8.0-8.5 and room temperature leads to inter-subunit crosslinking. Little difference is observed in the efficiency of crosslinking by reagents of a homologous series with different lengths (from C6 to C12). At any stage of the reaction an exponential decrease is found in the amounts of crosslinked dimer, trimer, tetramer etc., with no preference for the formation of any particular oligomer, suggesting that all subunits on the surface of the spherical alpha-crystallin molecule are in quasi-equivalent positions. Molecular weight analysis by docdecylsulfate gel electrophoresis and reversible crosslinking show that both A and B subunits occur in an apparently constant ratio in the crosslinked oligomers, from which we infer that both types of subunits are randomly arranged on the surface of the alpha-crystallin molecule. Not all of the subunits can form inter-chain crosslinks, as a limiting value of about 60% of the subunits is found in oligomeric form. A simple explanation could be a core of subunits which is inaccessible to certain chemical reagents, although other alternatives are also discussed.

Molecular forms of purified human erythrocyte membrane acetylcholinesterase investigated by crosslinking with diimidates

Several molecular forms of human erythrocyte membrane acetylcholinesterase have been studied after crosslinking with bifunctional diimidates. The crosslinked products were analysed by centrifugation on linear sucrose density gradients containing Triton X-100. Molecular weights of covalently linked oligomers were estimated by sodium dodecylsulfate gel electrophoresis. It was shown that acetylcholinesterase crosslinked in absence of Triton X-100 consists of molecular forms built up by dimeric protomers. These dimers were identical with the enzymatically active species sedimenting with 6.5S in linear sucrose density gradients.

Studies of glutamate dehydrogenase. Methionine-169: the preferentially carboxymethylated residue

In phosphate buffer at pH 7.0, 5,5'-dithio-bis(2-nitrobenzoic acid), N-ethylmaleimide or iodoacetamide do not alter the activity of beef liver glutamate dehydrogenase. Iodoacetate, however, inactivities the enzyme irreversibility by alkylation. Combined addition of the coenzyme NADH and the substrate 2-oxoglutarate or the effector GTP protects against this inactivation. The alkylation reaction is independent of pH between pH 6-9 indicating that amino, imidazole or phenolic groups are probably not involved in this reaction. Titration of the thiol groups, after denaturation of the enzyme, revealed the loss of approximately one group per polypeptide chain. However, this is not due to the exclusive alkylation of a cysteine residue, since alkylation with iodo-[2-14C]acetic acid also labels a methionine residue. 50% of the label is incorporated into methionine-169 and only 7% into cysteine-115, the remaining radioactivity is distributed in minor quantities (4%) in several unidentified residues. A probable cause of the erroneous thiol groups titration is discussed.