Quinone Reductase 2
Created by Nathan Damiano
Quinone reductase 2 (QR2) is a cytosolic, globular, 53.85 kD human protein (PDB ID:2QX4). It is a homodimer composed of two identical subunits, A and B. Each subunit is 231 amino acids in length.
The secondary structure of each subunit consists of seven alpha helices (red) loosely grouped around a central beta sheet (yellow), along with three short random coils (green) and four H-bonded turns (purple). Each subunit also has two associated cofactors. The first is a zinc ion, bound in place by the residues His-173, His-177, and Cys-222 (Jain et al., 2004). It has been speculated that the zinc ion might be involved in the unique catalytic activity of QR2 as compared to the highly homologous protein quinone reductase 1 (QR1). However, it is more likely that the zinc ion is required only for the proper folding of QR2, as it can be replaced by iron or manganese ions with no effect upon protein function (Buryanovskyy et al., 2004). The second cofactor is FAD, bound to QR2 by His-12, Tyr-155, Glu-193, and Arg-200 (Jain et al., 2004).
FAD is essential to the proper functioning of QR2, as it forms part of the enzyme's active site. QR2 actually has two identical catalytic sites located on opposite sides of the protein. Until quite recently, the biological function of these active sites was something of a mystery. As mentioned earlier, QR2 is very similar in both primary and secondary structure to QR1. In fact, as this superimposition shows, the two proteins are almost identical (QR1 is displayed in red, QR2 in green, in honor of the season), sharing a 47% identity according to a DALI server search (Holm and Rosenstrom, 2010). QR1 is a well known regulator of quinone toxicity. Quinones are common naturally-ocurring compounds, present in all burned organic materials (Vella, Ferry, Delagrange, & Boutin, 2005). They are highly reactive and can lead to the formation of cell-damaging reactive oxygen species. QR1 catalyzes a 2-electron reduction of para-quinones to less reactive hydroquinones, and it was initially thought that QR1 did the same. However, gene deletion of QR2 has been shown to increase, rather than decrease, para-quinone toxicity in vivo (Calamini, Santarsiero, Boutin, & Mesecar, 2008). Further studies have shown that small differences between the QR2 and QR1 active sites actually have a dramatic effect upon their function. While both enzymes can reduce para-quinones, only QR1 can do the same to catechol-quinones such as adenochrome, shown here held in the active site of QR1 (Fu, Buryanovskyy, & Zhang, 2008). Catechol-quinones are the breakdown products of many nuerotransmitter molecules, so it's not unsurprising that various neurological disorders, including Parkinson's disease and schizophrenia, have been associated with mutations of the QR2 gene. Fu et al. found that in QR1, the carbonyl group of Gly-174 is exposed and able to form a strong hydrogen bond with catechol quinone substrates via a water molecule and that, "This hydrogen bond maintains the substrate in an optimum position for its neighboring ketone group... to accept the transfer of a hydride from reduced FAD." The case of QR2 demonstrates how even small mutations in the primary sequence of a polypeptide can have profound implications for its biological function.
In recent years QR2 has become a protein of great interest to the scientific community, and its active site, shown here with functionally relevant residues labeled, has been characterized with some detail. One of its defining characteristics is its hydrophobicity, with residues Phe-178, Phe-126, Ile-128, and Trp-105, among others, contributing to this character (Calamini et al., 2008). The QR2 active site is also quite narrow, and together these factors facilitate the binding of flat, hydrophobic molecules. The various inhibitors and substrates of QR2 all fit this general profile, but each interacts with the active site in a slightly different manner. Melatonin is a good example of this, as it fits in one of three similar orientations, two of which are shown here and here. An overlap view shows that the primary difference between the two orientations is the direction in which the long side chain is pointed. Melatonin is a neurohormone involved in a host of important biological functions, including regulation of internal circadian rhythms (Calamini et al., 2008), and has also been shown to exert antidepressant effects (Oxenkrug, Bachurin, Prakhie, & Zefirov, 2010).
Resveratrol is another inhibitor of ML2, and a very potent one as compared to melatonin. Like other QR2 inhibitors and substrates, it consists of a central, flat hydrophobic region surrounded by polar groups capable of forming hydrogen bonds. However, it is unusual in that it completely fills the QR2 active site, instead of only a portion of it. All three of resveratrol's hydroxyl groups form hydrogen bonds with side chains of QR2 amino acids. Abundant in grapes and grape products, resveratrol has demonstrated cancer chemopreventative properties. Because of its strong and specific inhibition by resveratrol, QR2 might be involved in the suspected mechanism of the compound's beneficial effects: the up-regulation of antioxidant and detoxification enzyme expression (Buryanovskyy et al., 2004).