Horizons in biochemistry and biophysics最新文献

Several aspects of membrane structure and function have been treated in which the dynamic properties of membrane components are particularly significant. The establishment and maintenance of asymmetries across the membrane, and heterogeneities in the plane of the membrane, place certain restrictions on the nature and extent of membrane fluid properties. Long-range order, which may give differential restrictions to rotational versus translational diffusion, requires specific interactions between membrane components that are strong enough to overcome thermal energy. Processes such as membrane fusion are likely to involve local areas in the membrane where certain membrane proteins are sequestered. And finally, the budding of virus membranes by mechanisms that specifically exclude host cell membrane proteins will require specialized interpretations in view of the fluid membrane model. These and other membrane phenomena illustrate the importance of the dynamic properties of membranes.

Overproduction of purine nucleotides de novo is the cause of hyperuricemia in a substantial portion of the gouty population. Specific enzyme abnormalities--deficiency of hypoxanthine-guanine phosphoribosyltransferase (an enzyme of the purine "salvage" pathway) and overactivity of 5- phosphoribosyl-1-pyrophosphate (PP-ribose-P) synthetase--result in hyperuricemia, and are associated with increased de novo purine synthesis and increased intracellular concentrations of PP-ribose-P. The latter is a common substrate for the first enzyme of the de novo pathway (phosphoribosyl amidotransferase) and the purine base salvage enzymes. Studies in cultured cells from patients, and in mutant cells derived from normal cell lines in vitro, suggest that elevated intracellular PP-ribose-P concentrations may increase the rate of de novo purine biosynthesis. This regulation can be explained in terms of the normal intracellular concentration of PP-ribose-P which is lower tthan the Km for the amidotransferase, and by allosteric activation of this enzyme by PP-ribose-P. Feedback inhibition of the first step in the de novo pathway by exogenous purines can be explained either by end-product (nucleotide) inhibition of the amidotransferase, or by competition for PP-ribose-P by the salvage enzymes which have lower Km's for this substrate, or by a combination of these effects. Evidence for and against these mechanisms is discussed. Evidence is presented which suggests that exogenous purines exert a feedback effect, not only on the first step of the de novo pathway, but also at the distal branch point in the pathway. Several potential regulatory mechanisms which might lead to excessive production of uric acid are discussed.

The chloroplast represents a relatively autonomous metabolic compartment within the plant cell. It is surrounded by an envelope consisting of two membranes of which the inner membrane is the functional barrier. Utilizing the energy of light the chloroplast is able to synthesize dihydroxyacetonephosphate from carbon dioxide and water. To provide the cell with this substrate, inorganic phosphate is required. In the case of phosphate deficiency the product of CO2 fixation may be temporarily stored within the chloroplast as starch. Specific transport processes across the inner envelope membrane permit the transfer of metabolites between the chloroplast and the cytosol. The phosphate translocator facilitates the export of dihydroxyacetone phosphate in exchange for inorganic phosphate. It also catalyzes a shuttle for inorganic phosphate with 3-phosphoglycerate, permitting the indirect transfer of reducing equivalents and of ATP from the chloroplast to the cytosol. The dicarboxylate carrier transporting various dicarboxylates may be suited for the transfer of reducing equivalents from the cytosol into the chloroplast. The ATP translocator, catalyzing a transport of ATP into the chloroplast in exchange for ADP, appears to be important for providing the chloroplast with ATP during the night phase, as required for the mobilization of starch.

The hydrolytic reactions catalyzed by pancreatic lipase represent a good example of heterogeneous catalysis. The particularity of this enzyme is provided by its preferential action on emulsified substrates. The first step of catalysis resides in a reversible adsorption of the enzyme to the oil-water interface. In fact, the formation of this adsorption complex is an obligatory step for the enzyme to display its full activity. Two principal but not necessarily exclusive hypotheses have been proposed to explain the observed interfacial activation: Either the interface confers new properties on the substrate which allow its subsequent hydrolysis, or the enzyme itself is modified by adsorption at the interface. Different approaches have recently been developed to clarify this point further. The results obtained by chemical modifications of lipase are consistent with the following hypothesis. The active site preexists in solution and becomes fully functional only by interaction of the interface with an additional site on the enzyme molecule which can be tentatively called the "interfacial activation site." Finally, a protein of low molecular weight, colipase, seems necessary for lipase to express its activity under physiological conditions. This protein enters specific interactions with bile salts micelles and is responsible for the reversal of the inhibition of lipolysis brought about by these detergents.