Metal contact, flavin

The o-character of flavin-metal contacts is more easily accepted a priori, since kinetic stability of metal-N,0-ligation need not be apprehended. Metal chelates (bidentate or polydentate structures) may, however, exhibit enough thermodynamic advantages to preclude dissociation and thereby, electron transfer. Flavin, however, is established being a... 

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 question remains open whether or not there is a direct contact between flavin and metal in a le -accepting binuclear iron cluster as, for example, in plant ferredoxin. Outer sphere le -transfer through flavin position 8 remains the alternative. On the other hand, there can no longer be a question about direct and reversible electron transfer between dihydronicotinamide and metal proteins. Any such claims (49) must be taken with greatest reservation and require careful independent confirmation. 

When the protein contains a reducible prosthetic group (metal ion, flavin, quinone...) this group can be reduced with a yield strongly dependent on the protein. As an example, reduction of flavoproteins was extensively studied. In some of them the flavin is the most probable target (32). In others, the flavin is partly or totally non-reactive (33, 34). In flavoproteins, the flavin is almost always inside the protein matrix and contact with the solvent is limited. The degree of reactivity is thus not only related to its accessibility and the reduction might be due to intramolecular electron transfer (see 5). 

---