Fcc3 Mechanism

Transfer of electrons to FAD

The heme clusters function in a way similar to a wire. They receive electrons from a source and transmit them through the enzyme. When the fourth heme is reached, the electrons are transferred to the FAD and stored. Until a suitable substrate like fumarate is bound, the enzyme is reduced (Taylor et al 1999).

How Fcc3 converts fumarate to succinate

When the substrate binds to the enzyme, the goal is for it to pass on the extra electron originating from an outside source. The enzyme creates a favorable environment for this relay.

• When fumarate (blue) binds to the enzyme (red), the substrate is forced out of planar conformation. The conjugated bonds, which stabilize fumarate, weaken when the molecule is twisted.
• The active site creates a polar environment that allows charges to develop on the fumarate. The strong hydrogen-bonding environment plus the two arginines and histidine of the active site creates a positive dipole on Carbon 2 of the substrate. This positive charge allows for a nucleophilic attack by the reduced FAD group. The electrons from the FAD coenzyme are attracted to the positive charge and leap onto the substrate. Bond rearrangements promoted by hydrogen bonds between the enzyme and substrate eventually saturates the double bond of the fumarate and reduces it.

Figure 10. The flow of electrons from the FAD to the Fumarate (Taylor et al 1999).

• A conformational change is necessary for the escape of succinate. The clamp domain of the enzyme is responsible for this change (Taylor et al 1999).

Fcc3 with pathway delineated through a molecular graphic routine

Implications of Fcc3 structure to human SDH reaction mechanism

Application to Human SDH catalytic mechanism of oxidation

Fumarate reductase catalyzes the reduction of fumarate to succinate. This reaction, however, is actually reversible in the bacterial cell. Which direction of the reaction is favored in vivo depends on the environment of the cell and the need for a final electron acceptor or an electron donor. Humans use O2 as an e- acceptor, so there is no need for the acceptor fumarate reductase. Humans, however, need an electron donor in the metabolism of high-energy molecules. SDH catalyzes the opposite reaction: succinate is transformed into fumarate. By understanding the mechanism for the fumarate reductase, the mechanism for SDH may be unlocked.

Structural generalization of Fcc3 to SDH

The flavocytochrome and SDH are in the same family because they catalyze the same reaction. Within this family, essential residues in the active site are conserved: Arg 402, Arg 544, His 504, His 365, and Thr 377. Four important characteristics of the flavocytochrome can be generalized for the fumarate reductase/succinate dehydrogenase family:

• Heme-“wire” delivers electrons to the embedded FAD.
• Enzyme has a highly polarized recognition site.
• Twist of the fumarate substrate is induced.
• Domain movement required for the release of the substrate (Taylor et al 1999)

Fig 11. The active site residues