Complete amino acid sequence of the heme-binding core in bakers' yeast cytochrome b2 (L-(+)-lactate dehydrogenase).

We are reporting here an analysis of the chymotryptic peptides obtained from the tryptic heme-binding fragment of flavocytochrome b2 (cytochrome b2 core). These results completely establish the sequence of the 96 residue-long fragment, for which preliminary evidence has been published before [24]. We also report full experimental details concerning the automatic degradation, the specific cleavages at the unique arginine and methionine residues, and the analysis of the tryptic peptides. In addition, it is shown that the main heme-binding fragment resulting from cytochrome b2 proteolysis by yeast proteases has an additional glutamic acid residue at the C-terminal end relative to the main tryptic heme-binding fragment. The slight sequence modifications presented here (amide groups and insertion of a lysine residue after position 71) do not substantially modify the comparison with liver microsomal cytochrome b5. A new sequence alignment is proposed for the two proteins, and a few structural considerations are presented, based on the inspection of calf liver cytochrome b5 three-dimensional model [50,51].