Showing papers in "Current Topics in Cellular Regulation in 1975"

Molecular adaptation to physiological requirements: the hemoglobin system of trout.

Abstract: Publisher Summary The existence of specific carriers deputed to the transport of oxygen from the outer environment to the place of utilization is widespread in nature. Under the pressure of the variable oxygen requirements, a series of adaptive mechanisms involving the different “gears” of the machine deputed to the supply of oxygen to the tissues has come into operation. Allosteric effects have become apparent in studies on hemoglobins; those on the structural and functional properties of hemoglobins have been largely carried out on the proteins extracted from mammals. Contrary to the case in mammals and birds, a multiplicity of hemoglobin components is generally found in the blood of fish, reptiles, and amphibians. This chapter discusses the structural and functional properties of the various hemoglobin components of the blood of trout ( Salmo irideus ). On the basis of the behavior of the isolated hemoglobin components from trout blood and of their distribution among the erythrocytes, it has been possible to provide a “rationale” for the existence of several hemoglobins.

Regulation and physiological functions of malic enzymes.

Abstract: Publisher Summary This chapter discusses some of the general features of the NADP-linked malic enzyme from animal tissues, especially in terms of its regulation and physiological roles. The major emphasis has been directed toward the regulation of the enzyme found in mammalian liver, as this particular activity has been the one most thoroughly studied in terms of its modification by different types of stimuli. However, the ubiquitous nature of “malic enzyme” suggests that important metabolic functions may be associated with its activity, which has been considered “minor” with considerable frequency. The participation of the cytosol and mitochondrial malic enzymes in the transport of reducing equivalents in bovine adrenal cortex indicates the physiological importance of the enzyme in mammalian systems. The factors that control the tissue levels of malic enzyme, particularly in liver, are investigated in view of the known regulatory mechanisms that operate at the level of protein synthesis and degradation.

Intracellular proteinases in microorganisms.

Abstract: Publisher Summary One of the main functions of intracellular proteinases lies in their participation in protein turnover by the hydrolytic degradation of proteins. The rates of synthesis and degradation differ for the various enzymes and groups of enzymes. Moreover, they are influenced by many variables, such as nutritional conditions, growth phase, processes of differentiation. Protein synthesis is regulated at the level of transcription and translation by positive and negative control. The degradation of proteins is generally assumed to occur through the combined action of proteinases and peptidases, yielding amino acids after complete hydrolysis. On the other hand, it is possible that “limited proteolysis” leads to an accumulation of distinct macromolecular products that are either inactive or different in their catalytic properties from their uncleaved precursors. Microbial proteinases are commonly divided into intracellular and extracellular enzymes. Extracellular enzymes usually occur in the active state in the growth medium and are quite stable. Many of them are even available in large quantities allowing their application to medicine and industry.

L-threonine dehydrase as a model of allosteric control involving ligand-induced oligomerization.

Abstract: Publisher Summary This chapter discusses L-threonine dehydrase as a model of allosteric control involving ligand-lnduced oligomerization. The threonine dehydrase system represents one of the simplest examples of allosteric behavior involving ligand-influenced oligomeric equilibria, and it can be established that this process functions in energy-linked control of threonine utilization. L-threonine dehydrase can be described as a system of three types of random binding equilibria: (1) dehydrase-substrate, (2) dehydrase-effector, and (3) dehydrase-dehydrase. These interactions can be looked upon as a series of 12 binding equilibria arranged as 6 linked functions among pairs of equilibria. The relationship among these can be depicted as a cube where M is monomer, D is dimer, A is the allosteric effector AMP, and T is threonine. For threonine dehydrase, the equilibrium constants for some of these binding processes are such that it is possible to study several of the equilibria in the virtual absence of others.

Quantitative aspects of protein induction.

Abstract: Publisher Summary This chapter discusses the formal aspects of varying two basic parameters of enzyme induction—the parameter of time and the parameter of concentration. The application of the equations obtained was tested on a wide range of systems. The induced synthesis of new proteins is a basic event in many cellular processes, including such fundamental phenomena as cellular differentiation, growth control, metabolic regulation, and cellular secretion. Induced formation of new proteins is currently studied in a great variety of systems including bacterial regulation, viral infection in prokaryotic and eukaryotic cells, enzyme adaptation in cultured cells, hormonal and neural effects in various tissues, and model differentiation systems. The complexity of the biochemical process leading to the formation of a particular protein as a result of arrival of an inducing signal becomes increasingly clear as more evidence accumulates. Most detailed studies of the induction process have been carried out with enzymes, because of the ease with which they are assayed in crude cell preparations.

Regulation of the Caulobacter Cell Cycle

Abstract: This chapter focuses on the aquatic bacterium Caulobacter crescentus, which divides asymmetrically during each cell division cycle, yielding progeny cells that differ both structurally and functionally. The initially motile swarmer cell progeny sheds its flagellum and differentiates into a nonmotile stalked cell. In addition to morphological differences, the stalked- and swarmer cell progeny inherit different competencies for chromosome replication. A central component of any cell cycle is the initiation of chromosome replication coupled with strict controls to prevent repeated rounds of DNA replication without intervening cell divisions. The Caulobacter origin of replication was identified and cloned by taking advantage of the observation that replication is always initiated in the stalked cell. Microbial cells are able to monitor changes in their environment, detect changes in cell density, and communicate with each other and with other organisms through signals. The Caulobacter DnaA protein is a likely candidate for a positive regulator of the initiation of DNA replication. The generation of dissimilar progeny cells in both prokaryotes and eukaryotes frequently depends on asymmetric localization of regulatory factors prior to division. The periodicity of DNA replication, cell division, and, in Caulobacter, cell cycle-dependent morphological and behavioral differences contrasts with the continuous nature of most metabolic reactions that produce cellular growth. Repression is relieved in time to initiate DNA replication when origin repression by CtrA is eliminated in the stalked cell.