The rate of reaction between cytochrome C peroxidase and hydrogen peroxide is not diffusion limited.

The apparent biomolecular rate constant for the cytochrome C peroxidase (EC 1.11.1.5)-hydrogen peroxide reaction has been measured as a function of temperature between 5 and 25 degree C at pH 4,5.5, and 7 and as a function of viscosity over a fifteen-fold range. From the independence of the rate constant on the viscosity, it is concluded that the reaction is not diffusion limited.     

The transmembrane domains of the human multidrug resistance P-glycoprotein are sufficient to mediate drug binding and trafficking to the cell surface.

The human multidrug resistance P-glycoprotein (P-gp) is organized in two tandem repeats with each repeat consisting of an N-terminal hydrophobic domain containing six potential transmembrane segments followed by a hydrophilic domain containing a nucleotide-binding fold. A series of deletion mutants together with an in vivo drug-binding assay were used to test whether the deletion mutants interacted with substrates or were transported to the cell surface. We found that a deletion mutant consisting of only the transmembrane domains (residues 1-379 plus 681-1025) retained the ability to interact with drug substrates. In the absence of drug substrates, the deletion mutant was sensitive to trypsin and endoglycosidase H. Expression in the presence of verapamil, vinblastine, capsaicin, or cyclosporin A, however, resulted in a mutant protein that was resistant to trypsin and endoglycosidase H. The mutant was then detected at the cell surface and was sensitive to digestion by endoglycosidase F. By contrast, the N-terminal transmembrane domain (residues 1-379) alone did not interact with drug substrates, since it was sensitive to only endoglycosidase H and was not detected at the cell surface. These results show that the nucleotide-binding domains are not required for interaction of P-gp with substrate or for trafficking of P-gp to the cell surface.