Showing papers in "Current Topics in Cellular Regulation in 1978"
TL;DR: The role of cyclic-AMP-dependent protein kinase in the regulation of glycogen metabolism in mammalian skeletal muscle is discussed, which determines the time at which dephosphorylation of the β subunit and inactivation of the enzyme can become rapid through phosphorylations of the α subunit.
Abstract: Publisher Summary This chapter discusses the role of cyclic-AMP-dependent protein kinase in the regulation of glycogen metabolism in mammalian skeletal muscle. When assayed at optimal ATP-Mg-concentrations and in the presence of saturating amounts of Ca2+, purified phosphorylase kinase has a very low activity at physiological pH (6.8) relative to the activity at pH 8.2. Phosphorylation of α subunit of phosphorylase kinase controls the rate of dephosphorylation of the β subunit catalyzed by phosphorylase kinase phosphatase. The conversion of phosphorylase kinase a to phosphorylase b correlates with dephosphorylation of the β subunit, and the rate of dephosphorylation of the enzyme in the absence of divalent cations is determined by the extent of phosphorylation of the α subunit. Phosphorylation of the α subunit alters the conformation of phosphorylase kinase in such a way that it facilitates the action of phosphorylase kinase phosphatase on the β subunit. Cyclic-AMP-dependent protein kinase plays two roles: (1) it activates the enzyme through phosphorylation of the β subunit and (2) it determines the time at which dephosphorylation of the β subunit and inactivation of the enzyme can become rapid through phosphorylation of the α subunit.
334 citations
TL;DR: In vitro experiments strongly indicate that L-type liver pyruvate kinase is an enzyme whose activity is regulated by reversible protein phosphorylation in vivo, which may be expected with regard to the known effect of cAMP on liver gluconeogenesis.
Abstract: Publisher Summary This chapter discusses the regulation of liver pyruvate kinase by phosphorylation–dephosphorylation. The rate and extent of the phosphorylation of L-type liver pyruvate kinase in vitro indicate that it is a specific reaction. This is supported by inhibition of the enzyme by the phosphorylation, which may be expected with regard to the known effect of cAMP on liver gluconeogenesis. The specificity of the phosphorylation reaction is also demonstrated by the fact that neither the A-type enzyme from pig kidney nor the M-type muscle enzyme from the rabbit or pig are substrates of cAMP-stimulated protein kinase. However, phosphorylation of the type-A enzyme from chicken liver has been preliminarily reported. The phosphorylation of liver pyruvate kinase in vitro is reversible, owing to the presence in rat liver cell sap of phosphoprotein phosphatase, which acts on phosphorylated pyruvate kinase. In vitro experiments strongly indicate that L-type liver pyruvate kinase is an enzyme whose activity is regulated by reversible protein phosphorylation in vivo .
136 citations
TL;DR: This chapter focuses on interconvertible enzyme cascades in metabolic regulation, a dynamic process in which the coupling of two opposing cascades leads to continual cyclic activation and inactivation of the enzyme.
Abstract: Publisher Summary This chapter focuses on interconvertible enzyme cascades in metabolic regulation. The cascades involved in the cyclic covalent modification of interconvertible enzymes are fundamentally different from the irreversible, unidirectional cascades of proteolytic enzymes such as are involved in blood coagulation and complement fixation. When triggered by appropriate stimuli, the latter cascades respond in an explosive manner to produce an avalanche of product needed to meet an occasional biological emergency. However, having fulfilled this function, the cascades are terminated abruptly in response to an entirely different set of autoregulatory signals that lead to self-destruction of the catalytic process. In effect, these unidirectional cascades are contingency systems that serve as biological switches that can be turned on or off intermittently to meet emergency situations. In contrast, the covalent modification of an interconvertible enzyme is a dynamic process in which the coupling of two opposing cascades leads to continual cyclic activation and inactivation of the enzyme. Such cascades are more properly designed for controlled amplification of primary stimuli, as is needed in the regulation of key enzymes in metabolism.
131 citations
TL;DR: Fructose should not be considered a substitute for glucose in the human diet and in parenteral nutrition, but it constitutes a very useful tool for the study of metabolic regulations.
Abstract: Publisher Summary This chapter explores the metabolic effects of fructose in the liver. The main mechanism of action of fructose can be traced to its very rapid metabolization, resulting in modification of the concentration of metabolites, most notably ATP, GTP, and P i , which have important regulatory functions. Fructose metabolism bypasses the regulatory phosphofructokinase step. This provides high amounts of substrates that can be readily utilized in the absence of normal metabolic control. Fructose should thus not be considered a substitute for glucose in the human diet and in parenteral nutrition. It constitutes, however, a very useful tool for the study of metabolic regulations. It also appears that fructose favors the release of hepatic triglycerides by stimulation of their biosynthetic pathway, associated with inhibition of the catabolism of fatty acids. Fructose increases the hepatic consumption of oxygen by up to 100% in vivo , in slices, in the perfused organ, and in isolated cells. Ethanol by itself has no effect on this parameter, but if fructose is added while ethanol is being metabolized, the increase in oxygen consumption is approximately doubled as compared with the effects of adding fructose alone.
102 citations
TL;DR: This chapter discusses the regulation of nitrogen fixation, which involves the biosynthesis and the activity of nitrogenase, and the inhibition by carbamyl phosphate, which resembles that of CO.
Abstract: Publisher Summary This chapter discusses the regulation of nitrogen fixation. Both the biosynthesis and the activity of nitrogenase are regulated. The product of N2 reduction, ammonia, through its effect on the biosynthesis of glutamine synthetase, represses nitrogenase synthesis but has no effect on nitrogenase activity. A by-product of nitrogenase catalysis, ADP inhibits N2 reduction by binding to a site on the Fe protein of nitrogenase and preventing its activity, probably by preventing its reduction and its complexing with ATP, the energy source required for the process. Several of the properties of the Fe protein that result from the binding of 2MgATP2− are changed in the presence of ADP. These include its EPR spectrum, its midpoint potential, its ability to give up its iron to chelators, and its ability to accept electrons. Carbamyl phosphate, although not firmly established as a regulator, inhibits nitrogenase activity to the extent of 50%. The inhibition by carbamyl phosphate mimics that of CO in that both reduction of H+ and ATP hydrolysis by nitrogenase are not inhibited.
88 citations
TL;DR: This chapter describes enzyme alterations in some selected systems, including the nematode T. aceti, the eye lens, and red blood cells, which are the systems that allow recognition of the most frequent and unequivocal postsynthetic changes in enzymes.
Abstract: Publisher Summary This chapter describes enzyme alterations in some selected systems. The isocitrate lyase from old Turbatrix aceti ( T. aceti ) consists of a mixture of active and inactive molecules. The nematode T. aceti , the eye lens, and red blood cells are the systems that allow recognition of the most frequent and unequivocal postsynthetic changes in enzymes. G6PD shows two types of modifications: (1) a nonproteolytic lowering of the isoelectric pH and (2) a limited proteolysis of the COOH-terminal end, which takes place in the red blood cells. Aldolase undergoes a specific deamidation. Changes in enzyme level and properties can be because of changes in rates of proteolysis. Salivary amylase in humans is present as multiple isozymes, which is derived from a single gene product through posttranscriptional modifications. The lens of the eye is a very peculiar organ in many respects. It is a nonvascularized tissue, receiving nutrients from aqueous and vitreous humors. It is an organ of choice in the study of some aspects of postsynthetic changes in proteins.
78 citations
TL;DR: This chapter discusses what is known about the regulation of a metabolic variant of photosynthesis known as the C 4 pathway, which operates as a complex appendage to this cycle of photosynthetic carbon metabolism.
Abstract: Publisher Summary This chapter discusses what is known about the regulation of a metabolic variant of photosynthesis known as the C 4 pathway. Photosynthetic carbon metabolism is an inherently complex process. A cyclic sequence of reactions is essential, as the primary CO 2 acceptor must be continually regenerated from assimilated carbon, and there must be at least one branch point in the cycle from which the accumulated carbon is channeled off into end products. A further degree of complexity is introduced by the dependency of this system upon ATP and NADPH generated by light-dependent processes in chloroplast membranes. Most photosynthetic organisms, including the majority of higher plants, assimilate CO 2 directly into 3-phosphoglycerate (3-PGA) via the enzyme ribulose-1,5-P 2 carboxylase. Subsequent reactions reduce 3-PGA to products that are metabolized to reform ribulose-1,5-P 2 by the pathway known as the photosynthetic carbon reduction (PCR) cycle or the Calvin cycle. The C 4 pathway does not replace the PCR cycle but rather operates as a complex appendage to this cycle.
63 citations
TL;DR: The involvement of isozymes in metabolic regulation and the role of compartmentation in metabolicregulation are discussed and a hypothetical model of metabolism and polyisozymic complexes in glucose utilization are described.
Abstract: Publisher Summary This chapter discusses the role of isozymes in metabolism. Isozymes are the multiple forms of enzymes catalyzing the same reaction in the same cell or organism. If several forms of the same enzymic activity exist, they perform a function in a distinct manner, and the difference is not the product of the reaction. The difference must then reside in the manner in which the product is formed. In fact, the components of some isozymic systems have been shown to differ, inter alia , in their affinity for substrates or cofactors, substrate or cofactor specificities, response to allosteric effectors, subcellular localization, susceptibility to dietary and/or hormonal treatments, or time of appearance during differentiation. This chapter discusses the involvement of isozymes in metabolic regulation and the role of compartmentation in metabolic regulation. A hypothetical model of metabolism and polyisozymic complexes in glucose utilization are also described in the chapter.
55 citations
TL;DR: The primary function of 2,3- P 2 -glycerate in living cells is to act as an essential cofactor in the reaction catalyzed by phosphoglyceromutase, which is the only energy source for the energy-consuming systems of the cells.
Abstract: Publisher Summary This chapter discusses the multiple functions and the metabolism of 2,3-bisphosphoglycerate (2,3- P 2 -glycerate). 2,3-P 2 -glycerate is widely present in living cells, functioning as a cofactor for the enzyme phosphoglyceromutase. The primary function of 2,3- P 2 -glycerate in living cells is to act as an essential cofactor in the reaction catalyzed by phosphoglyceromutase. Human and many other mammalian erythrocytes contain 2,3-P 2 -glycerate in much higher concentrations than are required for phosphoglyceromutase. 2,3-P 2 -glycerate serves as an important allosteric regulator of hemoglobin function. The human erythrocyte loses nonessential pathways during maturation, thus, gaining in oxygen transportation efficiency. All subcellular particles such as nuclei, mitochondria, and ribosomes disappear. The mature human erythrocyte has actively functioning pathways to metabolize glucose and nucleotides. The Embden–Meyerhof pathway produces ATP, which is the only energy source for the energy-consuming systems of the cells. The Warburg–Dickens cycle produces NADPH, essential in reducing oxidized glutathione. The mature erythrocyte is deficient in the ability to synthesize nucleotides de novo ; however, it can metabolize adenine nucleotides.
55 citations
TL;DR: This chapter presents immobilized model systems of enzyme sequences and discusses some naturally occurring enzyme sequences that present either as components of isolatable, highly organized multienzyme systems or in loosely associated aggregates.
Abstract: Publisher Summary This chapter presents immobilized model systems of enzyme sequences and discusses some naturally occurring enzyme sequences that present either as components of isolatable, highly organized multienzyme systems or in loosely associated aggregates. Organized enzyme systems are found even in cells where, because of their small size, diffusion is not a rate-limiting step in the overall reaction sequence. Protein–protein or protein–membrane interactions can be necessary to provide the control features of organized multienzyme systems or to form entities that have intrinsic catalytic properties different from those of the isolated proteins. The tryptophan synthase system from Neurospora crassa represents an example of a multienzyme complex, where a juxtaposed arrangement of two active sites supposedly provides for channeling of the intermediate. On immobilizing an enzyme, the intrinsic kinetic parameters can or cannot be altered. Usually, the specific activity is reduced to some degree. Charged matrices can also attract or repel charged substrate or product molecules, thereby generating conditions different from those in the medium.
24 citations
TL;DR: This chapter discusses the regulation of isoleucine and valine biosynthesis, a model system for the study of mechanisms by which the cell coordinates different parts of its total metabolism.
Abstract: Publisher Summary This chapter discusses the regulation of isoleucine and valine biosynthesis. Isoleucine and valine biosynthesis is a model system for the study of mechanisms by which the cell coordinates different parts of its total metabolism. Threonine deaminase is the first enzyme in isoleucine biosynthesis. A different threonine deaminase, termed catabolic, appears only in anaerobiosis and in the absence of glucose. A third threonine deaminase, present in Escherichia coli K-12 and Salmonella Typhimurium, deaminates only D-threonine, and its presence explains why ilvA mutants of these organisms grow upon addition of D-threonine to the medium. The second step in the pathway is common to isoleucine and valine biosynthesis and is catalyzed by acetolactate synthase activity. It involves the conversion of either two molecules of pyruvate to form α-acetolactate, or one molecule of pyruvate and one molecule of α-ketobutyrate to form α-aceto-α-hydroxybutyrate. The acetolactate synthase isoenzymes have some relation to lysine biosynthesis. The third step in isoleucine biosynthesis and the second in valine biosynthesis are catalyzed by the enzyme isomeroreductase. The last intermediates in the pathway for isoleucine and valine biosynthesis are the keto acids from which the corresponding amino acids are synthesized by transamination.
TL;DR: Particles of the alternative pathway and mechanisms of action—such as activation by antibody, membrane attack system, and control processes—are also described in the chapter.
Abstract: Publisher Summary This chapter describes a molecular approach to the complement system. Complement has emerged as one of the most important defense systems of higher animals. Functioning with antibodies, it destroys foreign cells, stimulates opsonization, and generates local inflammation. Similar results are produced by stimulation of the alternative pathway by bacterial substances. Substantial progress has been made toward the molecular characterization of the individual components of the complement system. The chapter presents the main features of molecular structure of most of the major components. One of the most noticeable features is that almost all the proteins are of high molecular weight and that many of them are present in plasma at relatively low concentrations. Combined with the lability of many components, this has meant that purification and characterization has been a relatively difficult task in most cases. This chapter also discusses the classical pathway and the alternative pathway for the complement system. Components of the alternative pathway and mechanisms of action—such as activation by antibody, membrane attack system, and control processes—are also described in the chapter.
TL;DR: The chapter focuses on the biochemistry and physiological function of the reactions catalyzed by aminoacyl-tRNA protein transferases, which may contribute to the heterogeneity in NH 2 -terminal residues.
Abstract: Publisher Summary This chapter presents an overview of posttranslational NH 2 -terminal aminoacylation. Aminoacyl-tRNA protein transferases catalyze the addition of certain amino acids to the NH 2 -terminus of specific proteins and peptides. These enzymes constitute one mechanism for the posttranslational modification of NH 2 -termini and may contribute to the heterogeneity in NH 2 -terminal residues, which has been observed in bulk proteins of both prokaryotes and eukaryotes. Modifications of the NH 2 -terminus can be classified into three basic types: (1) cleavage reactions, in which a formyl, methionyl, or other residue is removed; (2) alteration reactions, in which it is converted to another type of molecule, such as an α-keto acid or a cyclized derivative; and (3) addition reactions, including acetylation, phosphorylation, and aminoacylation, in which small molecules are added to it. The chapter focuses on the biochemistry and physiological function of the reactions catalyzed by aminoacyl-tRNA protein transferases.
TL;DR: In this paper, the operator-repressor complex was separated from the unreacted lac repressor by zonal centrifugation on a 5-30% glycerol gradient.
Abstract: Publisher Summary This chapter discusses the lactose operator–repressor interaction. Any method for quantitatively studying the operator–repressor interaction requires either the operator or the repressor species to be labeled. After the interaction has taken place, the method must be able to separate the unreacted labeled material from the bound, labeled material. Two different methods have been used to study the operator–repressor interaction. Gilbert and Muller-Hill used 35 S-labeled lac repressor. The operator–repressor complex was separated from the unreacted lac repressor by zonal centrifugation on a 5–30% glycerol gradient. The method requiring labeled operator DNA is the one most commonly used. A convenient and accurate method has been developed and extensively used by Riggs and Bourgeois for studying various properties of the operator–repressor complex. In this method, the DNA–protein complex is separated from the unbound labeled DNA by passing the reaction mixture through a nitrocellulose membrane filter. The DNA–protein complex is retained on the filter, whereas the unbound DNA passes through. The chapter also discusses the lac operator interaction with a wild-type repressor.