Carbonic anhydrase (CA) is a zinc-containing enzyme that catalyzes the reversible hydration of CO 2 . Based on the amino acid sequences, CAs are classified into three evolutionarily distinct families, designated -, -and -CAs. CA from Chlamydomonas reinhardtii belong to the type enzyme and, homologous to CA from animal and eubacteria source. To reveal the structure, we have been engaged on X-ray study of Chlamydomonas CA. The enzyme was extracted from Cultured Chlamydomonas cells and highly purified by column chromatography [1]. The enzyme is composed of two subunits, small and large. Crystals were obtained in a solution containing ammonium sulfate. X-ray diffraction data were collected at 100K with synchrotron radiation at NW12 of PF-AR (Tsukuba). Diffraction patterns were processed with the program HKL2000. The crystal diffracted to a maximum resolution of 2.4 . The unit-cell dimensions were a=b=134.6 and c=120.0 with a space group of P6 1 or P6 5 . Initial phases were derived by the molecular replacement method using the atomic coordinates of CA from Neisseria gonorrhoeae, which has a sequence identity of 37 % with the present CA.Elongation factor G (EF-G) catalyzes translocation in protein synthesis on the ribosome. EF-G is inhibited by Fusidic acid (FA), a commonly used antibiotic. Structural information on FA binding to EF-G has not yet been available.We present three crystal structures of two mutant EF-G factors; G16V, highly FA sensitive, and T84A, highly FA resistant. The crystal structures provide a first insight into the conformational changes induced by GTP binding and how this affects FA binding. These structural conformations show the general importance of the interface of domain G, III and V as a key component of the FA binding site and the specific role of Phe90 as a gatekeeper and conformational regulator. We provide an explanation on how EF-G is able to discriminate between GDP and GTP.
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