Flavin heme contacts

In these systems, binary carrier contacts, whether of measurable lifetime or of shorter duration, implicate, therefore, two classes— interflavin and flavin—heme contacts and flavin—iron (sulfur) contacts. Figure 3 makes it clear that, even irrespective of whether one or two redox equivalents are being transferred, the geometry of these contacts (roughly spoken irora) must be asked for, and if it is o-, the site of contact must be discussed, whether inner sphere or outer sphere. 

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 ability of these enzymes to contact directly the electrode is attributed to the peripheral location of the redox center. A detailed kinetic study [11] of the peroxidase-catalyzed reduction of H2O2 revealed that 42 % of the enzyme molecules are aligned on the electrode surface in a configuration where the redox heme site is accessible for direct electron transfer. Other enzymes possess two redox sites, and electron transfer proceeds vectorially from a peripheral site to an inner component. For example, /7-cresol methyl hydroxylase [PCMH, a flavin adenine dinucleotide (FAD)- and heme-containing redox enzyme] affects the direct oxidation of p-cresol to -hydroxybenzaldehyde [12[ ... 

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