Students in my biochemistry class will recognize this as a classic oxidation-reduction reaction where succinate is oxidized to fumarate and quinone (Q) is reduced to quinol (QH2). (The green carbons are the ones originally derived from the acetyl group that starts the cycle.)
The succinate dehydrogenase complex is also complex II in membrane-associated electron transport. This is the electron transport process that's coupled to ATP formation—part of what used to be called oxidative phosphorylation or respiration. The complexes are found in the inner membranes of mitochondria in eukaryotes and the inner plasma membrane in bacteria. 
Within the complex, electrons are passed from succinate to FAD and then to three different iron-sulfur ([Fe-S]) clusters and finally to a molecule of quinone (Q) bound to the active site in the membrane (Q and QH2 are only soluble in the membranes.)
The reverse reaction, where electrons flow from QH2 to fumarate is common in bacteria. It is usually catalyzed by a separate enzyme called fumarate reductase. The two complexes (succinate dehydrogenase and fumarate reductase) are very similar in structure and the various subunits of the complexes are homologous.
The structure of succinate dehydrogenase from Escherichia coli was solved at good resolution some years ago [PDB 1NEK]. It reveals the presence of a heme b group in the membrane-bound part of the complex. The binding site for QH2 is located close to the third [Fe-S] cluster near the inner side of the membrane. The role of this heme is highly controversial. It's not present in fumarate reductase, demonstrating that it plays no role in electron transport for the reverse reaction. It seems as though the heme group might not be involved in the transfer of electrons from succinate to QH2 either but it has been difficult to rule this out.
Tran et al. (2007) have investigated the role of heme b in a paper that has just appeared in the online version of PNAS. They created mutant enzymes that could not bind the heme b molecule and examined the effect of these mutations. Mutant bacteria grew normally under conditions where the activity of succinate dehydrogenase was essential. The levels of enzyme activity of two different mutant enzymes were only 6% and 30% lower than the levels of the wild-type enzymes.
These results demonstrate that the heme b group is not required for the chemical reaction. This confirms a lot of previous work that pointed to the same conclusion. However, the mutant enzymes are less stable than the wild-type enzyme; they tend to lose activity during purification. This indicates that the heme group helps to stabilize the complex although whether this is significant in vivo remains an open question.
The result is further proof that not every feature has adaptive significance—or, at least not the significance you would normally assign. When the heme b molecule was first detected it was assumed to be involved in the chemical reaction since that's what heme groups do in most other complexes. Now we know it is not required for electron transfer but may play a small role in stabilizing the enzyme. Maybe there used to be a heme in the primitive ancestor of succinate dehydrogenase (SQR) and fumarate reductase (QFR) but it has been lost in QFR and rendered almost obsolete in SQR. Perhaps it used to be an essential component of the reaction in the ancestor enzyme or perhaps a primitive heme b enzyme that bound quinone just happened to evolve a binding site for another enzyme containing [Fe-S] clusters and succinate oxidation properties.
The more we study these molecular machines, the more we are coming to realize that the evolutionary pathway leading to their formation was somewhat haphazard and accidental. These are not cases where the final product is so well designed that a very precise series of improbable events had to happen in order for them to exist at all. Instead, proteins that may have originally served another purpose are co-opted and modified in newly evolving complexes.
Modern enzyme complexes may contain fossil evidence of their evolutionary history (e.g. heme b) that has very little to do with their current function. It's like biochemical junk.
Tran, Q.M., Rothery, R.A., Maklashina, E., Cecchini, G. and Weiner, J.H. (2007) Escherichia coli succinate dehydrogenase variant lacking the heme b. Proc. Natl. Acad. Sci. (USA) published online November 7, 2007, 10.1073/pnas.0707732104 [PNAS]
Everyone's using that icon all of the sudden...
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