2qwx

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Template:STRUCTURE 2qwx

Crystal Structure of Quinone Reductase II

Overview

Melatonin exerts its biological effects through at least two transmembrane, G-coupled receptors, MT1 and MT2, and a lower affinity, cytosolic binding site, designated MT3. MT3 has recently been identified as quinone reductase 2 (QR2, E.C. 1.10.99.2) which is of significance since it links the anti-oxidant effects of melatonin to a mechanism-of-action. Initially, QR2 was believed to function analogously to QR1 in protecting cells from highly reactive quinones. However, recent studies indicate that QR2 may actually transform certain quinone substrates into more highly reactive compounds capable of causing cellular damage. Therefore, it is hypothesized that inhibition of QR2 in certain causes may lead to protection of cells against these highly reactive species. Since melatonin is known to inhibit QR2 activity, but its binding site and mode of inhibition are not known, we determined the mechanism of inhibition of QR2 by melatonin and a series of melatonin and serotonin analogs, and we determined the x-ray structures of melatonin and 2-iodomelatonin in complex with QR2 to between 1.5 and 1.8 A resolution. Finally, the thermodynamic binding constants for melatonin and 2-iodomelatonin were determined by isothermal titration calorimetry (ITC). The kinetic results indicate that melatonin is a competitive inhibitor against N-methyldihydronicotinamide (Ki = 7.2 microM) and uncompetitive against menadione (Ki = 92 microM), and the x-ray structures shows that melatonin binds in multiple orientations within the active sites of the QR2 dimer as opposed to an allosteric site. These results provide new insights into the binding mechanisms of melatonin and analogs to QR2.

About this Structure

2QWX is a Single protein structure of sequence from Homo sapiens. Full crystallographic information is available from OCA.

Reference

Kinetic, thermodynamic, and X-ray structural insights into the interaction of melatonin and analogs with quinone reductase 2., Calamini B, Santarsiero BD, Boutin JA, Mesecar AD, Biochem J. 2008 Feb 7;. PMID:18254726 Page seeded by OCA on Wed Apr 30 13:18:09 2008