Heme groups flavin proteins

The success of Chapman and co-workers in expression of flavocytochrome 2 in E. coli [23] is encouraging in its impUcations for future expression of flavoproteins in this host because, in their experience both the flavin and heme groups are incorporated into the recombinant protein. Moreover, the bacterial expression system produces the protein 500-1000 fold more efficiently than the yeast from which it was cloned. The enzyme produced in E. coli, however, lacks the first five amino acid residues at its amino terminus, a result which presumably reflects subtle differences in protein synthesis between the two organisms. 

All of the complexes in the respiratory chain are made up of numerous polypeptides and contain a series of different protein bound redox coenzymes (see pp. 104, 106). These include flavins (FMN or FAD in complexes I and II), iron-sulfur clusters (in I, II, and III), and heme groups (in II, III, and IV). Of the more than 80 polypeptides in the respiratory chain, only 13 are coded by the mitochondrial genome (see p. 210). The remainder are encoded by nuclear genes, and have to be imported into the mitochondria after being synthesized in the cytoplasm (see... 

Cytochrome c552 from Euglena gracilis (also known as cytochrome / or c6) contains 87 amino acid residues, two hemes and one flavin per molecule.693 NMR studies706 indicate that the chirality of the axial methionine is similar to that of cytochrome c but different from cytochrome c5Sl. Rapid intramolecular transport has been demonstrated by the use of pulsed laser excitation, and the measurement of reduction kinetics. Both flavin and heme groups are reduced simultaneously on a multisecond time scale, with the transient formation of a protein-bound flavin anion radical.707... 

The electron carriers in the respiratory assembly of the inner mitochondrial membrane are quinones, flavins, iron-sulfur complexes, heme groups of cytochromes, and copper ions. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers. The electrons emerge in QH2, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate dehydrogenase is a component of the succinate-Q reductase complex (Complex II), which donates electrons from FADH2 to Q to form QH2.This highly mobile hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes h and c j and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a 3 and three copper ions. A heme iron ion and a copper ion in this oxidase transfer electrons to O2, the ultimate acceptor, to form H2O. 

The reduction potentials for the heme and FMN prosthetic groups of flavocytochromes 62 from S. cerevisiae and H. anomala are listed in Table I. Values for various modified forms of the enzyme, such as the flavin-free (deflavo) derivative, and the isolated cytochrome domain (the cytochrome 62 core) are also reported in Table I (64-69). The reduction potentials for the heme group are as expected for a 65-type cytochrome (70), with little difference in the values for different forms of protein, e.g., the deflavo-derivative of the holoenzyme and the isolated cytochrome 62. The reduction potentials for the FMN group are not too different from those of the heme (about 50 mV difference), consistent with reversible electron transfer between the two prosthetic groups (10). 

Four structural classes of c-type cytochromes thus are known today eukaryotic c, c, C3, and flavin-c. Other classes probably will be called for as structural information becomes available on more c proteins. These four classes probably represent independent evolutionary convergence on the use of a C. . . CH mode of attaching a heme group rather than divergence from a common ancestral prototype. Within each class, one... 

Therefore, the aim of this study is to gain further insight on the understanding of the mechanism of electron transfer between hemes and flavinic groups and of the particular role of each of the several hemes of such multi-hemic cytochromes, as well as for the general understanding of the relevant rules of protein recognition and specific complex formation. 

Formate dehydrogenases are a diverse group of enzymes found in both prokaryotes and eukaryotes, capable of converting formate to CO2. Formate dehydrogenases from anaerobic microorganisms are, in most cases, Mo- or W- containing iron-sulfur proteins and additionally flavin or hemes. Selenium cysteine is a Mo- ligand. 

The cases of myoglobin and hemoglobin are not rare. Many enzymes are dependent for their function on the presence of a nonprotein group. For example, cytochrome c also contains a prosthetic group similar, but not identical, to heme, as do a number of other proteins. These are known generically as heme proteins. There is a family of enzymes that contain a flavin group, the flavoproteins. Another family contains pyridoxal phosphate, a derivative of vitamin Be. There are a number of other examples. 

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