Nature Structural Biology pp 191 - 195

Nitrous oxide (N₂O) is a 'greenhouse gas', the third most significant contributor to global warming. It originates from activities such as fuel burning and agriculture.

The conversion of N₂O to N₂ cuts down on the amount of this gas in the biosphere. This reaction is performed by denitrifying bacteria, which obtain metabolic energy by using nitrogen-oxidized compounds instead of oxygen as terminal electron acceptors in anaerobic respiration. Four denitrifying enzymes have been identified: nitrate reductase, nitrite reductase, nitric oxide reductase and nitrous oxide reductase, named after the substrate they transform.

Nitrous oxide reductase (N2OR) catalyzes the final step of denitrification, that is, the two-electron reduction of N₂O to N₂. The structure of the N2OR enzyme has now been determined by Christian Cambillau, of the CNRS in France, and his coworkers. It was known that N2OR requires copper ions to catalyze its reaction. But, the structure reveals that N2OR contains a type of catalytic copper cluster has never been seen before.

Catalytic copper centers with one, two, or three copper ions are common, but N2OR has four copper ions coordinated by seven histidine amino acids in its catalytic center. The identification of this novel structure allows the researchers to propose a catalytic mechanism for the reduction of N₂O to N₂.

Amy Rosenzweig discusses these results in an accompanying News and Views report.

Nature Structural Biology pp 224 - 229

Eukaryotic cell division is a complex and highly coordinated process. Because it is absolutely essential that, at the end of this process, each daughter cell inherit one copy of the genetic material, 'checkpoint' mechanisms are in place to ensure that DNA duplication and chromosome alignment are complete before the subsequent segregation and division events can occur.

In human cells Mad2 is one of several proteins involved in the mechanism that controls chromosome alignment and the subsequent chromosome separation. From genetic and biochemical evidence, Mad2 is known to interact with another protein, Cdc20, to inhibit the onset of chromosome segregation until proper alignment is achieved. In an effort to define the interactions between these two proteins and possible roles of such interactions in the checkpoint mechanism, Gerhard Wagner of the Harvard Medical School and Hongtao Yu of the University of Texas Southwestern Medical Center, both in the USA, and their coworkers have determined the solution structure of Mad2 and mapped the sites of interactions between these two proteins.

Many proteins other than Mad2 and Cdc20 are also involved in controlling the alignment and segregation of chromosomes, including those identified in biochemical and genetic experiments (for example, Mad1). Thus, the results reported in this paper describe part of the checkpoint mechanisms. Importantly, they represent a starting point from which a molecular structural framework of the cell division process could be constructed.