Flavin carbon acid oxidations

Bruice, T. Carbon Acid Oxidations and Oxygen Activation by Flavins ... 

Evidence is presented in support of free-radical mechanisms for the oxidation of ionizable carbon acids by oxidized flavin. Activation of molecular oxygen by reduced flavin is shown to occur through the formation of a 4a-hydroperoxyflavin that may, dependent upon conditions and substrate, transfer one or two oxygen atoms. Examples of all reactions are provided in the text. The present state of knowledge concerning the chemiluminescent oxidations of aldehydes by 4a-hydroperoxyfla-vin is considered. 

Having shown that general-base-catalyzed carbanion formation precedes the oxidation step in flavin oxidation of the second class of carbon acids, the question arises as to how the electron pair moves from the carbanion to flavin. Covalent addition of carbanion to Flox followed by a base-catalyzed elimination reaction is one possibility (13). Addition to the 4a- (Equation 17) and 5-position (Equation 18) would appear to be feasible and, a priori, it would seem reasonable to expect that these adducts could undergo an elimination to yield oxidized substrate and reduced flavin. Nucleophilic addition of SO J to... 

Acyl CoA Dehydrogenases. Reaction (2) is catalyzed by a family of related enzymes. The lower fatty acid derivatives are oxidized by an enzyme purified from beef liver mitochondria. The isolated enzyme, butyryl dehydrogenase, has been reported to contain both riboflavin and copper.The nature of this enzyme will be discussed later in conjunction with other flavin enzjnmes. Chains of 3 to 8 carbons are oxidized in the presence of suitable electron acceptors, including 2,6-dichloro-phenolindophenol, cytochrome c, and ferricyanide. The maximum rate is obtained with butyryl CoA. It has been reported that the initial oxidation of fatty acid-CoA and the reduction of the employed electron acceptors are carried out by two different flavoproteins, which react with each... 

Nucleic acid pathways (Mg, Zn, Fe/S, B12 or Fe20) P-carbon oxidation/reduction (flavin, Fe)... 

The third reaction is the oxidation of the ring to form a carbon-carbon bond. The reducing equivalents are transferred to a flavin cofactor of the enzyme dihydroorotate dehydrogenase. The product is orotic acid. 

Iron-sulfur proteins contain non-heme iron and inorganic (acid-labile) sulfur in their active centers as 4Fe-4S or 2Fe-2S or, in the case of rubredoxin, as one iron alone. The iron is always bonded to cysteine sulfur. They catalyze redox reactions between +350 and —600 mV (hydrogen electrode = —420 mV). They are usually of low molecular weight (6000-15,000 Daltons) but can form complex enzymes with molybdenum and flavin. They occur as soluble or membrane-bound proteins and catalyze key reactions in photosynthesis, oxidative phosphorylation, nitrogen fixation, H2 metabolism, steroid hydroxylation, carbon and sulfur metabolism, etc. They occur in all organisms so far investigated and may... 

Walsh et al. (52), as well as ourselves (53), argued that if a-proton removal were an obligatory step in flavin reduction, then substitution of a good leaving group X, such as chloride, at the )8-carbon of an amino acid oxidase substrate might reveal non-oxidative a-fi elimination of HX... 

Certain CoA thioester using enzymes catalyze reactions at the fS-carbon or other carbons of the acyl group more distant from the thioester functionality. The fatty acid fi-oxidation cycle provides some examples (Fig. 3). Fatty acids 7 enter the cycle by initial conversion to the CoA ester 8, which is then oxidized to the a,P-unsaturated thioester 9 by a flavin-dependent enzyme. Addition of water to the double bond to form the fi-hydroxy thioester 10 is catalyzed by the enzyme crotonase, which is the centerpiece of the crotonase superfamily of enzymes that catalyze related reactions (37), which is followed by oxidation of the alcohol to form the fi-keto thioester 11. A retro-Claisen reaction catalyzed by thiolase forms acetyl-CoA 12 along with a new acyl-CoA 13 having a carbon chain two carbons shorter than in the initial or previous cycle. 

Introduction of a double bond. The /3-oxidation pathway begins when fatty acid forms a thiol ester with coenzyme A to give a fatty acyl CoA. Two hydrogen atoms are then removed from carbons 2 and 3 by an acyl CoA dehydrogenase enzyme to yield an ,/3-unsaturated acyl CoA. This kind of oxidation—the introduction of a conjugated double bond into a carbonyl compound—occurs frequently in biochemical pathways and is usually carried out by the coenz5nne flavin adenine dinucleotide (FAJ ). Reduced FADH is the by-product. 

Most of bacterial biosensors are based on the operon luxCDABE that codes for the bacterial luciferase founded in the marine bacteria V. fischeri and V. harveyi, and for an essential aldehyde substrate that would otherwise have to be supplied exogenously. The cluster luxAB cassette codes for the luciferase whereas luxCDE encodes a fatty acid reductase complex. The latter enzymes are responsible for the synthesis of the long-chain aldehyde that is required as substrate in the bioluminescence reaction (Meighen and Dunlap, 1993 Hakkila et al., 2002). Luciferase catalyses the oxidation reaction of flavin mononucleotide (FMNH2). A long-chain (7 to 16 carbons) aldehyde is reduced in presence of oxygen by the aldehyde reductase. The outcome of the bioluminescent reaction can be expressed as follows ... 

Metabolic Role. Riboflavin coenzymes are required for most oxidations of carbon-carbon bonds (Fig. 8.29). Examples include the oxidation of succinyl CoA to fumarate in the Krebs cycle and introduction of a,jS-unsaturation in /3-oxidation of fatty acids. Riboflavin is also required for the metabolism of other vitamins, including the reduction of 5,10-methylene tetrahydrofolate to 5-methyl tetrahydrofolate (Fig. 8.49), and interconversion of pyridoxine-pyridoxal phos-phate-pyridoxamine (Fig. 8.33). Because oxi-dation/reductions that use FAD or FMN as the coenzyme constitute a two-step process, some flavin coenzyme systems contain more than one FAD or FMN. 

A saturated acyl CoA is degraded by a recurring sequence of four reactions oxidation by flavin adenine dinucleotide (FAD), hydration, oxidation by NAI), and thiolysis by coenzyme A (Figure 22.9). "Fhe fatty acid chain is shortened by two carbon atoms as a result of these reactions, and FADII., NADH, and acetyl CoA are generated. Because oxidation takes place at the P carbon, this series of reactions is called the oxidation pathway. 

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