Flavoprotein functions

Flavoproteins function in virtually every area of metabolism and we have ° =... 

Redox-Equivalents Input/Output Flavoprotein Function Prototype... 

Omura, T., E. Sanders, R.W. Estabrook, D.Y. Cooper, and O. Rosenthal (1966). Isolation from adrenal cortex of a nonheme iron protein and a flavoprotein functional as a reduced triphosphopy-ridine nucleotide-cytochrome P-450 reductase. Arch. Biochem. Biophys. 117, 660-673. 

Bartley, G.E. et al.. Carotenoid desaturases from Rhodobacter capsulatus and Neu-rospora crassa are structurally and functionally conserved and contain domains homologous to flavoprotein disulfide oxidoreductases, J. Biol. Chem. 265, 16020, 1990. 

This review will not be concerned with functionally alternative structures and metabolites which appear in iron-limited growth. Thus Clostridium pasteurianum and other bacteria when grown in the presence of iron form ferredoxin grown at low iron the same organisms form flavodoxin, a flavoprotein [Knight, ., Jr., Hardy, R. W. J. Biol. Chem. 242, 1370 (1967) Mayhem, S. G., Massey, V. J. Biol. Chem. 244, 794 (1969)]. 

In addition to these more-or-less well characterized proteins, iron is known to be bound to certain flavoproteins such as succinic dehydrogenase (20), aldehyde oxidase (27), xanthine oxidase (22) and dihydrooro-tate dehydrogenase (23). Iron is present and functional in non-heme segments of the electron transport chain but again no real structural information is at hand (24). 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

The primary function of flavoprotein NADPH-cytochrome P-450 reductase is the hydro-xylation of various substrates, which occurs during electron transfer from NADPH to cytochrome P-450 [1] ... 

However, it must be stressed that not everyone is in agreement with such a scheme. For example, flavoproteins are thermodynamically capable of transferring electrons directly to O2 to form O2", and not all functional oxidase preparations contain substantial amounts of FAD. Indeed, in one series of experiments the ratio of cytochrome b to FAD decreased from about 1 1 to 19 1 as the oxidase became progressively more purified. It may be, howev-... 

Over the years, there have been numerous reports of oxidase preparations that contain polypeptide components, additional to those described above. As yet no molecular probes are available for these, and so their true association with the oxidase is unconfirmed. There are many reports in the literature describing the role of ubiquinone as an electron transfer component of the oxidase, but its involvement is controversial. Quinones (ubiquinone-10) have reportedly been detected in some neutrophil membrane preparations, but other reports have shown that neither plasma membranes, specific granules nor most oxidase preparations contain appreciable amounts of quinone, although some is found in either tertiary granules or mitochondria. Still other reports suggest that ubiquinone, flavoprotein and cytochrome b are present in active oxidase preparations. Thus, the role of ubiquinone and other quinones in oxidase activity is in doubt, but the available evidence weighs against their involvement. Indeed, the refinement of the cell-free activation system described above obviates the requirement for any other redox carriers for oxidase function. 

Table 14.2. Proposed functions of rubrerythrin, rubredoxin oxidoreductase, rubredoxin, high molecular weight rubredoxin, and type A flavoprotein in different bacteria.

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. 

Reductive sequences involving flavoproteins may be represented as the reverse reaction, where hydride is transferred from the coenzyme, and a proton is obtained from the medium. The reaction mechanism shown here is in many ways similar to that in NAD+ oxidations, i.e. a combination of hydride and a proton (see Box 11.2) it is less easy to explain adequately why it occurs, and we do not consider any detailed explanation advantageous to our studies. We should register only that the reaction involves the N=C-C=N function that spans both rings of the pteridine system. 

Adrenodoxin. Adrenodoxin is the only iron-sulfur protein which has been isolated from mammals. This protein from mitochondria of bovine adrenal cortex was purified almost simultaneously by Kimura and Suzuki (32) and Omura et al. (33). It has a molecular weight of 12,638 (34) and the oxidized form of the protein shows maximal absorbances at 415 and 453 nm. Adrenodoxin acts as an electron carrier protein in the enzyme system required for steroid hydroxylation in adrenal mitochondria. In this system, electron transfer is involved with three proteins cytochrome P. gQ, adrenodoxin and a flavoprotein. Reduced NADP gives an electron to Tne flavoprotein which passes the electron to adrenodoxin. Finally, reduced adrenodoxin transfers the electron to cytochrome Pas shown in Fig. 3. The mechanism of cytochrome P cq interaction with steroid, oxygen and adrenodoxin in mixed-function oxidase of adrenal cortex mitochondria has been reviewed by Estabrook et al. (35). 

Putidaredoxin. Cushman et al. (36) isolated a low molecular iron-sulfur protein from camphor-grown Pseudomonas putida. This protein, putidaredoxin, is similar to the plant type ferredoxins with two irons attached to two acid-labile sulfur atoms (37). It has a molecular weight of 12,000 and shows absorption maxima at 327, 425 and 455 nm. Putidaredoxin functions as an electron transfer component of a methylene hydroxylase system involved in camphor hydroxylation by P. putida. This enzyme system consists of putidaredoxin, flavoprotein and cytochrome P.cQ (38). The electron transport from flavoprotein to cytochrome P.cq is Smilar to that of the mammalian mixed-function oxidase, but requires NADH as a primary electron donor as shown in Fig. 4. In this bacterial mixed-function oxidase system, reduced putidaredoxin donates an electron to substrate-bound cytochrome P. g, and the reduced cytochrome P. g binds to molecular oxygen. One oxygen atom is then used for substrate oxidation, and the other one is reduced to water (39, 40). 

Megaredoxin. Another example of a bacterial mixed-function oxidase was found in the steroid 15 6-hydroxylase system of Bacillus megaterium (41). This enzyme system consists of three proteins FMN-containing flavoprotein (megaredoxin reductase), iron-sulfur protein... 

Flavodoxins are a group of flavoproteins which function as electron carriers at low potential in oxidation-reduction systems. The proteins of this group contain one molecule of FMN as their prosthetic group, but, in contrast to ferredoxins, do not contain metals such as iron. 

It has been well recognized that the mixed-function oxidase system of Bacillus megaterium is involved in steroid hydroxylation (, as already described above. This enzyme system is composed of a NADPH-specific FMN flavoprotein (megaredoxin reductase), an iron-sulfur protein (megaredoxin) and cytochrome P cn. The megaredoxin protein plays an important role as an intermediate component of electron transfer from reduced flavoprotein to cytochrome P en. 

It has been suggested that an FeS flavoprotein with presumed electron transporting function may be involved in the reduction of the flavodiiron protein in Methanosarcina acetivorans (Saraiva et al. 2004). Since putative FeS flavoproteins with predicted hydrogenosomal target peptides have also... 

In the following the basic chemical and physical properties of free flavin will be described in some detail, because the knowledge of these properties is the key for a detailed understanding of the function of flavoproteins. In addition, some general and common properties of the classes of flavoproteins will be presented and discussed in relation to some new concepts. It will however not be possible to cover the whole literature. For the reader interested in more details, the recent review papers by Massey and Hemmerich Bruice , Walsh Simondson and Tollin , and Hemmerich (and references therein) should be consulted. An overview of the pertinent research on flavins and flavoproteins is easily accessible by the proceedings of the international symposia... 

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