Investigation of the Symmetry of Oligomeric Enzymes with Bifunctional Reagents

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Abstract
The symmetry of proteins composed of identical polypeptide chains has been investigated by means of cross-linking with bifunctional reagents and subsequent sodium dodecylsulfate-polyacrylamide gel electrophoresis. The majority of the investigations were performed with diimidates of different chain lengths (C3-C12), which react exclusively with amino groups. Aldolase, catalase, fumarase, pyruvate kinase, tetrameric proteins with identical polypeptide chains, reveal a D2 symmetry, i.e. they appear to be composed of two pairs of polypeptide chains. The validity of this conclusion is demonstrated with lactate dehydrogenase. This enzyme, shown by X-ray analysis to have a D2 symmetry, yields after cross-linking and subsequent polyacrylamide electrophoresis the band pattern expected for a protein with this quaternary structure and similar to the pattern obtained with the above enzymes. 2. The influence of the experimental conditions on the cross-linking reaction has been investigated. The selectivity of the bifunctional reagent for the different contact domains within the quaternary structure of a protein depends on the reaction time, the chain length and on the concentration of the reagent. In general the D2 symmetry becomes more obvious with increasing chain length and with increasing concentration of the diimidate. Diethylpyrocarbonate showed very little selectivity.