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Cytochrome b2 from bakers' yeast (L-lactate dehydrogenase). A double-headed enzyme.

Bakers' yeast cytochrome b2 [lL-(+)-lactate dehydrogenase or l-(+)-lactate cytochrome c oxidoreductase], usually purified by crystallisation according to Appleby and Morton, was shown to be a degraded form of the enzyme, arising by a proteolytic modification of the species formed in vivo. We present here an improved purification procedure and some properties of the intact enzyme. 
 
 
 
This intact cytochrome b2 is shown, by cross-linking and by gel filtration studies, to be in a tetrameric form with a molecular weight of about 230 000. The four identical subunits are composed of a single polypeptide chain associated with one haem and one flavin (FMN). The amino-acid composition is presented. Alanine and glutamic acid are shown to be respectively C- and N-terminal residues. 
 
 
 
A comparison of these results with those concerning the degraded form suggests that there is little loss of peptide material and that the gross quaternary structure of the protein is not modified by the proteolytic process. However circular dichroism studies show that the haem environment is altered. 
 
 
 
The evidence leading to propose a biglobular structure for the enzyme is discussed.