Sugar Transport
Year: 1973
Type: article
Abstract: The reaction catalyzed by the phosphoenolpyruvate:β-galactoside phosphotransferase system of Staphylococcus aureus was studied. The four protein components of this system were required for the phosphoenolpyruvate-dependent phosphorylation of lactose and its analogues, and the over-all reaction proceeded by an ordered sequence of phosphoryl transfer steps, each of which was studied in detail. The initial phosphoryl transfer from phosphoenolpyruvate to the phosphocarrier protein HPr was catalyzed by Enzyme I; phospho-Enzyme I appears to be an intermediate in this reaction. Only one phosphoryl group was transferred to each HPr molecule; the Keq for phospho-HPr formation was about 10. The subsequent reversible phosphoryl transfer from phospho-HPr to the sugar-specific phosphocarrier protein Factor IIIlac was entirely self-catalyzed. Since this novel reaction, shown with homogeneous HPr and Factor IIIlac, did not require any additional protein component, it appears that each protein (when phosphorylated) served as a substrate for the other. The Keq for this phosphoryl transfer is approximately 0.3. The phosphoryl groups in both phospho-carrier proteins are linked to the imidazole moieties of histidine residues, to N-1 in HPr and N-3 in Factor IIIlac. The final step, formation of galactoside 6-phosphate, was catalyzed by the membrane-bound Enzyme IIlac. Kinetic studies indicated that the reaction did not proceed by a ping-pong mechanism, but that a ternary complex was formed by the two substrates, phospho-Factor IIIlac and lactose, and Enzyme IIlac. No evidence for a phospho-Enzyme IIlac intermediate was obtained. Direct binding of lactose by Enzyme IIlac (Kd ≅ 2.5 x 10-7 m) in the absence of other components was shown.
Source: Journal of Biological Chemistry
Institution: Johns Hopkins University
Language: English
Subfield: Genetics
Domain: Life Sciences
Open Access status: hybrid
Estimated APC paid: $3,430
OpenAlex ID: W1573772031