Molecular Weight and Quaternary Structure of Yeast <scp>l</scp>‐Lactate Dehydrogenase (Cytochrome <i>b</i>₂)

Abstract

An X‐ray diffraction study has been performed by the powder technique on two crystalline forms of yeast cytochrome b 2 . Type II (DNA‐free) cytochrome b 2 crystallizes in the trigonal system, the parameters of the unit cell being a = 168.2 Å and c = 112.0 Å, whereas type I (DNA‐containing) cytochrome b 2 crystallizes in the tetragonal system with a = 99.1 Å and c = 87.0 Å. Measurement of the crystal density and of the partial specific volume of the protein (found to be 0.754 cm 3 /g at 25°) enables a precise determination of the molecular weight of cytochrome b 2 to be obtained. In this respect, the values 234600 ± 8000 and 235000 ± 10000 were found for type II and type I crystals respectively. These results, although in complete concordance, are quite different from the previously accepted value (approx. 180000 from sedimentation studies based on v = 0.71 cm 3 /g). Space‐groups and protein contents of the crystals show that this protein is composed of at least two, or even four, identical protomers.