Differences in Quaternary Structure and Constitutive Chains between Two Homologous Forms of Cytochrome b2 (l-Lactate: Cytochrome c Oxidoreductase)

The subunit structure of cytochrome b2 prepared from the yeast Hansenula anomala has been studied and compared to known data concerning the homologous enzyme prepared from ordinary bakers' yeast Saccharomyces cerevisiae. By high-speed equilibrium ultracentrifugation, the molecular weight has been determined as 236000 ± 10000, while subunits of 58000 ± 5000 are observed either in 6 M guanidine under reducing conditions or in 0.01 M phosphate. By electrophoresis in 6 M urea as well as in dodecylsulfate-mercaptoethanol, this subunit behaves as a single component. In crystalline enzyme preparations from bakers' yeast, the same unit is known to be made up of two chains of about 21000 and 36000 molecular weight. The hypothesis that an artificial proteolytic cleavage is responsible for the observed two-chain pattern with the Saccharomyces enzyme is proposed. Experimental evidence for this hypothesis is given in the following paper.