Isolation and characterization of the flavin-binding domain of flavocytochrome b2 expressed independently in Escherichia coli

Flavocytochrome b(2) consists of two distinct domains. The N-terminal domain contains protohaem IX and the larger, C-terminal domain contains flavin mononucleotide (FMN). We describe here the isolation of the flavin-binding domain expressed in Escherichia coli independent of the cytochrome domain. The isolated domain is an efficient lactate dehydrogenase with ferricyanide as electron acceptor but reduces cytochrome c, the physiological oxidant for flavocytochrome b(2), extremely poorly; electron transfer from the flavin-binding domain to the separately expressed cytochrome domain is undetectable. FMN reduction by lactate occurs as a single exponential process in the isolated flavin-binding domain, in contrast to the biphasic kinetics observed with native flavocytochrome b(2).