The structures of cytochrome b 562 from E. coli and of cytochrome b2 (L-(plus)-lactate dehydrogenase) from bakers yeast will be studied by x-ray diffraction methods. Both the oxidized and reduced forms of each protein will be examined. The major goal will be to understand the mechanism of electron transport at the molecular level. Cytochrome b 562 appears to be different from other cytochromes of known structure, resembling myoglobin in sequence rather than cytochrome b5 or cytochrome c. It crystallizes in a triclinic cell with two molecules per asymmetric unit. The position of the iron atoms and relative orientation of the proteins in the cell are known. One heavy atom derivative has been prepared and a low-resolution electron density map has been calculated. Cytochrome b2 contains both a flavin and a heme prosthetic group. It catalyzes the dehydrogenation of L-lactate, transfering electrons to cytochrome c in the mitochondria. It contains a trypsin resistant heme peptide core which resembles microsomal cytochrome b5 in sequence spectrum. One goal of this research is to learn the relative orientation and position of the heme and flavin groups in b2. A second is to learn the structural basis for catalysis of the dehydrogenation reaction and subsequent electron transfer. The third is to learn the structural and evolutionary relationship between b2 and b5. Cytochrome b2 crystallizes in a trigonal cell with one tetramer of 230,000 Daltons per asymmetric unit. Large crystals have been grown which diffract to at least 3.0 A resolution. One potential heavy atom derivative has been found. The isomorphous replacement method of structure investigation will be coupled with the molecular replacement method to resolve these structures. The structure-function relationships and comparison with other proteins, as well as electron density fitting, will be carried out on the MMS-X molecular graphics system.