Oxygen K-edge shift at the Verwey transition of magnetite

The temp. dependence of the O K-edge threshold has been investigated in a temp. range encompassing the Verwey transition. Both above and below TV the O K-edge threshold changes linear with temp. whereas there is discontinuous energy shift of 13 meV at TV. A comparison of our results to published low and high energy photoemission expts. suggests the presence of an asym. gap with respect to the Fermi level. This explains present fundamental differences between photoemission and IR derived gap energies, and supports the polaronic image of the elec. cond. of Fe3O4 above the Verwey transition temp. [on SciFinder (R)

Energy transduction in protein transport and the ATP hydrolytic cycle of SecA

The motor protein SecA drives the transport of polypeptides through the ubiquitous protein channel SecYEG. Changes in protein-nucleotide binding energy during the hydrolytic cycle of SecA must be harnessed to drive large conformational changes resulting in channel opening and vectorial substrate polypeptide transport. Here, we elucidate the ATP hydrolysis cycle of SecA from Escherichia coli by transient and steady-state methods. The basal ATPase activity of SecA is very slow with the release of ADP being some 600-fold slower than hydrolysis. Upon binding to SecYEG the release of ADP is stimulated but remains rate-limiting. ADP release is fastest in the fully coupled system when a substrate protein is being translocated; in this case hydrolysis and ADP release occur at approximately the same rate. The data imply that ADP dissociation from SecA is accompanied by a structural rearrangement that is strongly coupled to the protein interface and protein translocation through SecYEG