Flavin oxygen activation

Flavohydroquinone bound to apoproteins plays a very important role in flavo-protein-catalysis, either in the electron-transfer to substrates or other enzymes or in the oxygen activation reaction. The chemical reactivity of 1,5-dihydroflavin bound to apoproteins can differ drastically from that of free flavin. The reactivity is likely governed by factors such as the conformation of the bound flavohydroquinone and the ionization state (cf. below). 

By analogy with carbenes, this mechanism is known by the name oxene mechanism . The diagram shows that the oxygen molecule reacts with the reduced flavin is activated and... 

Areas of biomimetic chemistry relating to enzyme systems that function both with and without the benefit of coenzymes are included. Special emphasis has been placed on the following subjects vitamin Bi2 and flavins oxygen binding and activation bioorganic mechanisms and nitrogen and small molecule fixation. 

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

Flavin-dependent le -transfer in enzymes and chemical model systems can he differentiated from 2e -transfer activities, i.e., (de)hydrogenation and oxygen activation, by chemical structure and dynamics. For le -transfer, two types of contacts are discussed, namely outer sphere for interflavin and flavin-heme and inner sphere for flavinr-fenedoxin contacts. Flavin is the indispensable mediator between 2e - and le -transfer in all biological redox chains, and there is a minimal requirement of three cooperating redox-active sites for this activity. The switch between 2e - and le -transfer is caused by apoprotein-dependent prototropy between flavin positions N(l)/0(2a) and N(5) or by N(5)-metal contact. 

The essential difference between nicotinamide and flavin is in the nature of the acceptor site, i.e., CH for nicotinamide as compared with N for flavoquinone. This explains why 5-deazaflavins appear to be interesting flavin (anti)metabolites since they are expected to be functional analogs of nicotinamide, exhibiting at the same time the steric shape of a flavin. Indeed it turns out that incorporation of 5-deazaflavin into apo-flavoproteins blocks the electron transfer and oxygen activation properties (32,33,34,35,36). Retaining at least partially the (de) hydro-... 

Other amino acids found to be important for catalysis include Thr 268 (involved in oxygen activation Yeom et al., 1995), Pro 25 (possibly involved in an initial docking region for fatty acid substrates Maves et al., 1997) and Gly 570 and Trp 574 (flavin-binding residues in the reductase domain, with possible roles in the mediation of electron transfer Munro et al., 1996). However, many potentially important mutants remain to be generated and characterised. The pathway of electron transfer from flavin (FMN) to haem is not defined, and the recent structure of the fused FMN-haem domains of the protein did not clarify the route, since the linker region between the... 

Flavoprotein monooxygenases mainly use NAD(P)H as electron donor and insert one atom of molecular oxygen into then-substrates. Oxygen activation of flavoprotein monooxygenases involves the (transient) stabilization of a flavin C4a-(hydro) peroxide. This species performs either a nucleophilic or electrophilic attack on the substrate (Fig. 6). Oxygenation reactions catalyzed by flavoprotein monooxygenases include hydroxyla-tions, epoxidations, Baeyer-Villiger oxidations, and sulfoxida-tions (43). 

Kantz A, Chin F, Nallamothu N, Nguyen T, Gassner GT. Mechanism of flavin transfer and oxygen activation by the two-component flavoenzyme styrene monooxygenase. Arch. Bio-chem. Biophys. 2005 442 102-116. 

Flavin containing cofactors are chemically versatile and participate in oxygen activation and in 1- and 2-electron transfer reactions [14]. In its biochemically active forms, flavin occurs as flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). 

Havoenzymes catalyze a large number of diverse redox interconversions reflecting the chemical versatility of the tricyclic isoalloxazine ring system of the flavin coenzymes, which act as a two-electron/one-electron switch as well as a molecular oxygen activator (Ghisla and Massey, 1989). Therefore flavin coenzymes serve ... 

---