On the molecular structure of crystalline yeast cytochrome b₂

The structure of the two crystalline forms of cytochrome b₂, the L-lactate: ferricytochrome c oxidoreductase of baker's yeast (EC 1.1.2.3), has been studied by electron microscopy. From the results, the approximate dimensions of a single enzyme molecule of molecular weight 170,000 were determined as 92 × 82 × 26 Å. Sections of the hexagonal bipyramid crystals (type II) of the DNA-free enzyme at right angles to the c axis showed a regular hexagonal network with one protein molecule forming the side of each hexagon. Sections parallel to the c axis showed that the empty, hexagonal tubes of protein ran right through the crystal. The flat, square plate crystals (type I) of the DNA-containing enzyme were seen as parallel rows of protein molecules arranged as layers stacked on top of each other to form the crystal. It appeared that alternate layers of protein were arranged at right angles to the ones in between. It has not been possible to obtain satisfactory side views of the crystal nor to locate visually the position of the DNA. However, a structure of these nucleoprotein crystals consistent with available data has been proposed.