Chapter 15 Principles of Metabolic Regulation There are four possible pathways for glucose 6 phosphate o Breakdown by glycolysis for the production of ATP o Breakdown in the pentose phosphate pathway for the production of NADPH and pentose phosphates o Use in the synthesis of complex polysaccharides of the extracellular matrix glycosylation sterol synthesis o Hydrolysis to glucose and phosphate to replenish blood glucose Glucose 6 phosphate also has other fates in hepatocytes o It may be used to synthesize other sugars for use in protein o It may be partially degraded to provide acetyl CoA for fatty acid and When any cell uses glucose 6 phosphate for one purpose all other pathways for which glucose 6 phosphate is a precursor or intermediate are affected 15 1 Regulation of Metabolic Pathways Cells and Organisms Maintain a Dynamic Steady State the rate of metabolite flow Cells and organisms exist in a dynamic steady state Flux Homeostasis constant Cellular differentiation external developmental signals with accompanying changes in metabolism when the concentration of a molecular species is held a change in a cell s composition in response to Both the Amount and the Catalytic Activity of an Enzyme Can Be Regulated The flux through an enzyme catalyzed reaction can be modulated by changes in the number of enzyme molecules or by changes in the catalytic activity of each enzyme molecule already present Rapid allosteric changes in enzyme activity are typically triggered by changes in local concentration of small molecules o A substrate of the pathway in which that reaction is a step o A product of the pathway o A key metabolite or cofactor that indicates the cell s metabolic state Second messengers generated intracellularly in response to extracellular signals also mediate allosteric regulation on a slightly slower time scale Extracellular signals may be o Hormonal insulin or epinephrine for example o Neuronal o Growth factors o Cytokines The rate of synthesis of an enzyme can be adjusted by the activation of a transcription factor o Transcription factor specific DNA regions response elements near a gene s promoter and activate or repress the transcription of that gene nuclear proteins that when activated bind o Activation of a transcription factor is sometimes the result of its binding of a specific ligand or its phosphorylation or dephosphorylation Each gene is controlled by one or more response elements that are recognized by specific transcription factors o Some genes have several response elements and are therefore controlled by several transcription factors o Groups of genes encoding proteins that act together often share common response element sequences These genes are turned off and on together The stability and rate of translation of mRNA is regulated synthesis followed by degradation Turnover o Enzymes have finite lifetimes that depend on the cellular conditions and the enzyme itself Sequestering an enzyme and its substrate in different compartments permits regulation of the effective activity of an enzyme Transcriptome type or organ Proteome the protein complement of a cell type or organ Both DNA microarrays and gel electrophoresis can be used to display the the entire complement of mRNAs present in a given cell transcriptome or the proteome Metabolome the total ensemble of low molecular weight metabolites o Changes in the metabolome are often a result of changes in the proteosome The activity of an enzyme can be further regulated by o The concentration of substrate o The presence of allosteric effectors o Covalent modification o Binding of regulatory proteins Allosteric effectors convert hyperbolic kinetics to sigmoid kinetics or vice versa o The higher the Hill coefficient the greater the cooperativity Covalent modifications occur within seconds or minutes of a regulatory signal o Most common are phosphorylation and dephosphorylation Many enzymes are regulated by association with and dissociation from another regulatory protein Metabolic regulation molecular level processes that serve to maintain homeostasis at the o To hold some cellular parameter at a steady level over time a process that leads to a change in the output of a Metabolic control metabolic pathway over time Reactions Far from Equilibrium in Cells Are Common Points of Regulation used to identify near equilibrium reactions in a cell by Mass action ratio Q comparing to K eq o When K eq and Q are within 1 to 2 orders of magnitude of each other the reaction is near equilibrium Adenine Nucleotides Play Special Roles in Metabolic Regulation an enzyme that catalyzes the hyrdrolysis of ATP to Adenylate kinase produce ADP responds to an increase in AMP by phosphorylating key proteins and thus regulating their activities AMP activated protein kinase AMPK a mediator of regulation by AMP that 15 3 Coordinated Regulation of Glycolysis and Gluconeogenesis Gluconeogenesis glycogen stores are exhausted and no dietary glucose is available provides glucose for export to other tissues when o Occurs primarily in the liver At each of the reactions in glycolysis that must be bypassed in gluconeogenesis if they were to operate at the same time ATP would be consumed without accomplishing any work o Futile cycle the simultaneous reaction of ATP and fructose 6 phosphate to form ADP and fructose 1 6 bisphophate with fructose 1 6 bisphosphate and water to form fructose 6 phosphate and phosphate o Substrate cycle regulatory advantages what to call futile cycles when they actually provide Hexokinase Isozymes of Muscle and Liver Are Affected Differently by Their Product Glucose 6 Phosphate Hexokinase is a regulatory enzyme with four isozymes o Hexokinase II the predominant hexokinase isozyme of myocytes Has a high affinity for glucose Normally acts at or near its maximal rate o Muscle hexokinase I and hexokinase II are allosterically inhibited by their product o Hexokinase IV the predominant hexokinase isozyme of liver Has a lower affinity for glucose Directly regulated by the level of glucose in the blood Not inhibited by glucose 6 phosphate Subject to inhibition by the reversible binding of a regulatory protein specific to liver Muscle consumes glucose and uses it for energy production Glucose reverses this inhibition Liver maintains blood glucose homeostasis by consuming or producing glucose GLUT2 equilibrates the glucose concentrations in cytosol and blood an efficient glucose transporter in hepatocytes that rapidly Hexokinase IV