Plugging into the pump: Photosynthetic organisms use light to fix carbon dioxide in a process that requires both chemical reducing equivalents and adenosine triphosphate (ATP). Balancing the ratio of these inputs is accomplished by a short circuit in electron flow through photosynthetic complex I, a proton pump that contributes to ATP production but does not increase net reducing equivalents in the cell. Schuller et al. solved a cryo–electron microscopy structure of photosynthetic complex I (see the Perspective by Brandt) and went on to reconstitute electron transfer using the electron carrier protein ferredoxin. Science , this issue p. 257 ; see also p. 230

Chitinase A1 from Bacillus circulans WL-12 comprises an N-terminal catalytic domain, two fibronectin type III-like domains, and a C-terminal chitin-binding domain (ChBD). In order to study the biochemical properties and structure of the ChBD, ChBD _ChiA1 was produced in Escherichia coli using a pET expression system and purified by chitin affinity column chromatography. Purified ChBD _ChiA1 specifically bound to various forms of insoluble chitin but not to other polysaccharides, including chitosan, cellulose, and starch. Interaction of soluble chitinous substrates with ChBD _ChiA1 was not detected by means of nuclear magnetic resonance and isothermal titration calorimetry. In addition, the presence of soluble substrates did not interfere with the binding of ChBD _ChiA1 to regenerated chitin. These observations suggest that ChBD _ChiA1 recognizes a structure which is present in insoluble or crystalline chitin but not in chito-oligosaccharides or in soluble derivatives of chitin. ChBD _ChiA1 exhibited binding activity over a wide range of pHs, and the binding activity was enhanced at pHs near its pI and by the presence of NaCl, suggesting that the binding of ChBD _ChiA1 is mediated mainly by hydrophobic interactions. Hydrolysis of β-chitin microcrystals by intact chitinase A1 and by a deletion derivative lacking the ChBD suggested that the ChBD is not absolutely required for hydrolysis of β-chitin microcrystals but greatly enhances the efficiency of degradation.