A ribosomal-bound aminopeptidase in Escherichia coli B: purification and properties

Two procedures have been developed for the large-scale purification from Escherichia coli B of a ribosomal-bound aminopeptidase in a stable form: one involving the digestion of the ribosome with pancreatic RNase and the other the selective removal of the aminopeptidase from the ribosomal surface with NH4Cl. The enzyme, as isolated from the ribosome, is a polymeric protein composed of monomeric units with a molecular weight of approximately 60 000 Daltons. The purified enzyme is activated by β-mercaptoethanol or dithiothreitol, although at high concentrations dithiothreitol inhibits peptidase activity. Propylene glycol also inhibits the enzyme. The enzyme is stable at 70° in the presence of free SH groups.

A possible role of phospholipid in nerve excitation

A possible role of phospholipid in regulating the ionic permeability of excitable membranes is proposed. It is assumed that each excitable transport channel in the nerve fiber is lined with two chains of phospholipid dipoles in either parallel or antiparallel directions; a molecular mechanism of nerve excitation is developed that is qualitatively consistent with most of the relevant observations reported in the literature.

Possible role of pyrophosphate linkage in the active transport of sodium ions

Based upon the observation that the pyrophosphate group has a greater affinity for sodium ion than for potassium ion, and that both of these ions catalyze transphosphorylation, a simple molecular mechanism is formulated for the active transport of sodium ions across biological membranes. It is assumed that the active transport takes place along a chain of phosphomonoester groups in the membrane. Pyrophosphate linkages are synthesized at the inner boundary of the membrane by a sodium-activated kinase and hydrolyzed at the outer boundary of the membrane by a potassium-activated pyrophosphatase. It is shown that in such a model the pyrophosphate linkages can diffuse across the membrane and carry sodium ions in the direction of increasing sodium ion concentration.

Directional character of proton transfer in enzyme catalysis

Hydrogen bonding can facilitate proton transfer in certain directions and retard proton transfer in certain other directions. By assuming that directed proton transfer along strategically oriented hydrogen bonds in the enzyme-substrate complex plays an important role in determining the efficiency and specificity of the enzyme, we present a unified interpretation for the reported observations on carbonic anhydrase and alpha-chymotrypsin.