Biochemistry

Handily Handling Nitrite

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Science  14 Jun 2013:
Vol. 340, Issue 6138, pp. 1267
DOI: 10.1126/science.340.6138.1267-d

Nitrogen is, of course, an essential element for life; fortunately, humans have no need to worry about its various oxidation states because we acquire it in readily usable forms (such as amino acids) in our diet. Microbes do the hard work of reducing dinitrogen and handling ammonium, nitrite, and nitrate ions. Crystal structures of some of the key enzymes and transporters have provided insights into the biochemistry of these small molecules, and Zheng et al. offer the latest step forward by presenting the 2.6 Å structure of Escherichia coli nitrate/nitrite transport protein NarK. They find the familiar 12-transmembrane helices of a major facilitator superfamily transporter, and by soaking in nitrite, they established the location of the active site, which almost certainly is also where nitrate binds. The anionic substrates are locked into place by two opposing arginine residues, and phenylalanines constitute the floor and ceiling of this cage. How reciprocal conformational changes are triggered by the binding of cytoplasmic NO2 versus the binding of extracellular NO3 is as yet unclear, but these two anions differ in their shape (vee versus trigonal) and their distribution of partial charge, so there are lots of ways for NarK to do what proteins do best.

Nature 497, 647 (2013).

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