Chapter 10 Introduction to Nutrition and Metabolism, 3 rd edition David A Bender Taylor & Francis Ltd, London 2002 Chapter 10: Integration and Control of Metabolism Press the space bar or click the mouse to build each slide; when each slide is complete a hand will appear in the lower right corner to indicate that the next click will take you to the next slide. You are welcome to use or adapt this presentation for use in teaching, with due acknowledgement, but you may not publish it in any form without written permission.
Mechanisms of metabolic control Mechanisms of metabolic control Intra-cellular mechanisms instantaneous availability of substrates inhibition by end-products Inter-cellular (hormonal) mechanisms changed activity of existing enzymes fast acting changed amount of enzyme slow acting tissue selectivity amplification of hormone signal mechanism for termination or reversal
rate of reaction The effect of substrate concentration on the rate of activity The effect of substrate concentration on the rate of activity - 1 Enzyme is ± saturated rate of reaction ± constant Enzyme unsaturated rate of reaction increases with increasing substrate concentration of substrate
rate of reaction The effect of substrate concentration on the rate of activity The effect of substrate concentration on the rate of activity - 2 V max the maximum rate of reaction when the enzyme is saturated ½ V max K m = concentration of substrate to give ½ V max concentration of substrate
rate of reaction The effect of substrate concentration on the rate of activity The effect of substrate concentration on the rate of activity - 3 A P high K m low K m B X concentration of substrate Q R Y Z
rate of reaction The effect of substrate concentration on the rate of activity cooperative binding (allosteric enzyme) Cooperative binding (allosteric enzyme) - 1 substrate concentration
rate of reaction The effect of substrate concentration on the rate of activity cooperative binding (allosteric enzyme) cooperative activation binding (allosteric enzyme) - 2 lower [S] required for activity inhibition higher [S] required for activity substrate concentration
glucose glucose-6-phosphate glycogen fructose-6-phosphate Inhibition of phosphofructokinase ATP by endproducts of glycolysis citrate fructose-1,6-bisphosphate phosphoenolpyruvate phospho-enolpyruvate pyruvate amino acids Inhibition of phosphofructokinase by end-products of glycolysis acetyl CoA fatty acids, ketones, amino acids oxaloacetate citrate ADP amino acids ATP
relative activity Substrate dependence of phosphofructokinase 100 Substrate dependence of phosphofructokinase 80 60 40 20 0 low [ATP] physiological [ATP] 0 0.5 1 1.5 2 2.5 [fructose 6-phosphate], mmol /L
relative activity As ADP accumulates in the cell 2 x ADP ATP + AMP AMP reverses the inhibition of phosphofructokinase by ATP hence immediate increase in glycolysis Fructokinase activity with and without 5 AMP 100 80 60 40 20 0 no AMP + AMP 0 1 2 3 4 5 [ATP], mmol /L
Regulation of phosphofructokinase by fructose 2,6-bisphosphate Regulation of phosphofructokinase by fructose 2,6- bisphosphate fructose 6-phosphate fructose 1,6-bisphosphate P CH 2 CH 2 fructose bisphosphatase P CH 2 CH 2 P phosphofructokinase kinase phosphatase glucagon P CH 2 CH 2 P fructose 2,6-bisphosphate
Substrate cycling permits faster and more sensitive regulation Substrate cycling permits faster and more sensitive regulation H HC HC CH 2 C CH CH 2 P fructose 6-phosphate phosphofructokinase ADP ATP H H 3 P H C 4 fructose 1,6 CH 2 P bisphosphatase fructose 1,6-bisphosphate H CH 2 C CH C P
Fast hormone responses phosphorylation and dephosphorylation of serine Fast hormone responses phosphorylation and dephosphorylation of serine H P HN CH 2 CH C ATP ADP HN CH 2 CH C H 2 H 3 P 4 HN CH 2 CH C serine protein kinase phosphoserine phosphoprotein phosphatase
Fast hormone responses phosphorylation and dephosphorylation of tyrosine Fast hormone responses phosphorylation and dephosphorylation of tyrosine H P HN CH 2 CH C ATP ADP HN CH 2 CH C H 2 H 3 P 4 HN CH 2 CH C tyrosine protein kinase phosphotyrosine phosphoprotein phosphatase
Modification of enzyme activity by phosphorylation and dephosphorylation Modification of enzyme activity by phosphorylation and dephosphorylation In the control of glycogen synthesis or utilization: In response to adrenaline or glucagon: The active form of glycogen synthase is inactivated by phosphorylation phosphorylated glycogen synthase is inactive but is activated by glucose 6-phosphate The inactive form of glycogen phosphorylase is activated by phosphorylation phosphorylated glycogen phosphorylase is maximally active but is inhibited by ATP, glucose and glucose 6-phosphate
Control of glycogen synthesis response to glucagon Control and adrenaline of glycogen synthesis ATP ADP protein kinase activated by camp active glycogen synthase inactive (phosphorylated) glycogen synthase phosphoprotein phosphatase but activated by glucose 6-phosphate phosphate response to insulin
Control of glycogen utilization response to glucagon Control and adrenaline of glycogen utilization ATP ADP protein kinase activated by camp inactive glycogen phosphorylase active (phosphorylated) glycogen phosphorylase phosphoprotein phosphatase but inhibited by ATP, glucose, glucose 6-phosphate phosphate response to insulin
Fast-acting hormones the rôle of G-proteins Fast-acting hormones the rôle of G-proteins b a GDP g
Fast-acting hormones the rôle of G-proteins hormone b a GDP g
Fast-acting hormones the rôle of G-proteins hormone b a GDP g
Fast-acting hormones the rôle of G-proteins hormone b a GDP GTP g GTP GDP
Fast-acting hormones the rôle of G-proteins hormone b a GDP GTP g
Fast-acting hormones the rôle of G-proteins g b a GDP GTP effector formation of intracellular second messenger
Fast-acting hormones the rôle of G-proteins H 2 H 3 P 4 slowly a GDP GTP effector formation of intracellular second messenger
Fast-acting hormones the rôle of G-proteins a GDP GTP effector
Fast-acting hormones the rôle of G-proteins b a GDP GTP g effector
Formation of cyclic AMP as a second messenger NH 2 N N N N Formation of cyclic AMP as a second messenger CH 2 P P P adenosine triphosphate (ATP) adenylate cyclase N NH 2 N N N CH 2 P pyrophosphate H 2 N NH 2 N N N CH 2 P cyclic adenosine monophosphate (camp) phosphodiesterase adenosine monophosphate (AMP)
Phosphatidyl inositol in transmembrane signalling Phosphatidyl inositol in transmembrane signalling 1 CH 2 CH phosphatidylinositol (PI) CH 2 2 x ATP PI kinase P 2 x ADP CH 2 CH phosphatidylinositol bisphosphate (PIP 2 ) CH 2 P P P
Phosphatidyl inositol in transmembrane signalling Phosphatidyl inositol in transmembrane signalling 2 CH 2 CH phosphatidylinositol bisphosphate (PIP 2 ) H 2 hormone-sensitive phospholipase C CH 2 P P P diacylglycerol + inositol trisphosphate (IP 3 ) P P CH 2 CH CH 2 P
camp and IP 3 bind to, and activate, protein kinases Activated protein kinases phosphorylate proteins camp and IP 3 bind to, and activate, protein kinases H P ADP CH 2 HN CH 2 CH C ATP HN CH C protein kinase serine phosphoserine these may be either the final target enzymes, or intermediate protein kinases that then phosphorylate the target enzyme
Amplification of the fast-acting hormone signal For as long Amplification as hormone is of bound the fast-acting to receptor hormone signal it will recruit and activate G-protein abg complexes many active G-protein complexes for 1 mol of hormone For as long as Ga has GTP bound it will activate the effector (adenylate cyclase or phospholipase) many mol of camp or IP 3 per mol active Ga For as long as camp or IP 3 is bound to protein kinase it will phosphorylate the target protein many mol of protein phosphorylated per mol camp or IP 3 For as long as enzyme is phosphorylated it will catalyse conversion of many (10 3 10 4 ) mol of substrate /second
A model of steroid (slow-acting) hormone action A model of steroid (slow-acting) hormone action Hormone enters cell, diffuses into nucleus and binds to receptor protein activated receptor dimerises and binds to DNA hormone response element Active receptor dimer on hormone response element increases transcription of mrna
Amplification of the slow-acting hormone signal For as long Amplification as hormone is of bound the slow-acting to receptor hormone signal it will enhance transcription of the target gene many mol of mrna for 2 mol of hormone For as long as mrna survives in the cytosol it will it will be translated many times over many mol of enzyme per mol of mrna For as long as enzyme molecule survives in the cell it will catalyse conversion of many (10 3 10 4 ) mol of substrate /second
Why don t all cells respond to a hormone? Why don t all cells respond to a hormone? In order to respond to a fast-acting hormone the cell must have cell-surface receptors for that hormone In order to respond to a slow-acting hormone the cell must have nuclear receptors for that hormone
How can cells respond differently to the same hormone? How can cells respond differently to the same hormone? Different cell surface receptors for the same hormone may activate different effectors (adenylate cyclase, phospholipase) Different cells have different protein kinases which phosphorylate different target enzymes Different cells contain different enzymes that are targets for phosphorylation (ie not all cells have all enzymes) In order to respond to a slow-acting hormone the gene with the hormone response element must be expressed in that cell Different nuclear receptors for the same hormone may bind to different hormone response elements, associated with different genes
Regulation of adipose tissue metabolism plasma lipoprotein fed state glucose lipoprotein lipase Regulation insulin of adipose tissue metabolism adrenaline noradrenaline glucagon glucose fatty acids camp acetyl CoA fatty acyl CoA triacylglycerol fatty acids + glycerol hormone-sensitive lipase fasting state free fatty acids bound to albumin glycerol for gluconeogenesis
Use of fuels by exercising muscle fatty acids Use of fuels by exercising muscle glycogen moderate exercise triacylglycerol glucose fatty acids vigorous exercise triacylglycerol glucose
glucose insulin Control of muscle fuel utilization Control of muscle fuel utilization phosphofructokinase AMP ATP hexokinase glucose glucose-6-p fructose-6-p fructose-bis-p glucose-1-p glycogen phosphorylase ATP phosphoenolpyruvate pyruvate kinase glycogen oxaloacetate citrate acetyl CoA pyruvate pyruvate dehydrogenase alanine camp Ca ++ NADH adrenaline nerve stimulation fatty acids and ketones alanine
Control of blood glucose by insulin and glucagon Control of blood glucose by insulin and glucagon In the fed state insulin is secreted by the b-cells of pancreatic islets of Langerhans stimulates synthesis of metabolic fuel reserves lowers circulating glucose In the fasting state glucagon is secreted by the a-cells of pancreatic islets of Langerhans stimulates mobilization of metabolic fuel reserves raises circulating glucose
Actions of insulin and glucagon increased by insulin Actions of insulin and glucagon liver fatty acid synthesis glycogen synthesis protein synthesis decreased by insulin liver ketogenesis gluconeogenesis increased by glucagon liver ketogenesis gluconeogenesis adipose tissue glucose transport fatty acid synthesis adipose tissue lipolysis adipose tissue (lipolysis) muscle glucose transport glycogen synthesis protein synthesis
Diabetes mellitus failure of glycaemic control measurement of plasma or urine glucose Diabetes mellitus failure of glycaemic control: measurement of plasma or urine glucose HC Cu ++ (red-brown precipitate) Cu 2 C H HC CH HC alkaline copper reagent H HC CH HC Alkaline copper reagent only semiquantitative detects any reducing sugar HC CH 2 glucose oxidase HC CH 2 glucose gluconate 2 H 2 2 ABTS (colourless) peroxidase Glucose oxidase can be quantitative specific for glucose false negative with vitamin C H 2 oxidised ABTS (blue)
plasma glucose, mmol /L Diabetes mellitus failure of glycaemic control glucose Glucose tolerance tolerance test response test response to 1g glucose to 1g glucose /kg body /kg weight body weight 16 14 12 10 8 6 4 2 0 0 0.5 1 1.5 2 2.5 3 diabetic control time (hours)
Diabetes mellitus failure of glycaemic control Insulin-dependent Types (type I and I) diabetes II diabetes mellitus commonly juvenile onset failure to secrete insulin may be auto-immune disease keto-acidosis with hyperglycaemia common always requires insulin injection Non-insulin-dependent (type II) diabetes commonly adult onset often associated with obesity normal or high secretion of insulin resistance of tissues to insulin action keto-acidosis rare may be treated with dietary restriction and insulin secretagogues / oral hypoglycaemic agents better glycaemic control by insulin injection
Diabetes mellitus consequences of poor glycaemic control Diabetes mellitus consequences of poor glycaemic control neuropathy (nerve damage) impotence is commonly a presenting symptom cataracts damage to the protein a-crystallin in the lens kidney damage (nephropathy) arthritis damage to collagen in joints capillary damage circulatory defects and damage to retina abnormal plasma lipoprotein metabolism increased risk of cardiovascular disease
Mechanisms of damage in hyperglycaemia HC Mechanisms of damage in hyperglycaemia CH - sorbitol 2 HC H CH HC HC CH 2 aldose reductase HC H CH HC HC CH 2 glucose sorbitol aldose reductase has a high K m so is only significantly active at high concentrations of glucose sorbitol accumulates in tissues and can disturb control of intracellular osmotic pressure
Mechanisms of damage in hyperglycaemia HC Mechanisms HC of damage in hyperglycaemia glycation of proteins H CH HC HC CH 2 non-enzymic glycation of proteins R NH C CH NH 2 amino terminal of protein non-enzymic reaction H R HC HC CH HC HC NH C CH N CH 2 Schiff base rearrangement H H 2 R C C CH HC HC NH C CH NH CH 2 amino ketone
Glycated proteins in diabetes include: Glycated proteins in diabetes haemoglobin A (forming haemoglobin A 1c ) albumin apolipoprotein B in plasma lipoproteins a-crystallin in the lens collagen in joints and connective tissue Measurement of haemoglobin A 1c provides a way of assessing glycaemic control over a period of ~ 1 3 months
Methods of measuring insulin Methods of measuring insulin Biological assay in vivo reduction in blood glucose in a rabbit after a dose of insulin this was the original definition of a unit of insulin Biological assay in vitro stimulation of glucose oxidation in muscle traditionally rat diaphragm Radio-immunoassay binding to anti-insulin antibodies
B chain A chain Insulin synthesis and secretion Insulin consists of Insulin 2 peptide synthesis chains and ( A and secretion B) - 1 21 and 30 amino acids long 21 + 30 = 51 amino acids M r ~ 6000 It is coded for by a single gene with 330 base pairs this would give 110 amino acids There is a 72 base-pair (24 amino acid) signal sequence at the 5 end of the gene 110 24 = 86 amino acids M r ~ 9000
B chain A chain B chain A chain signal peptide Insulin synthesis and secretion 24 amino acids Insulin synthesis and secretion 21 - + 2 30 = 51 amino acids C peptide endoplasmic reticulum Golgi 110 amino acids 86 amino acids 35 amino acids
End Introduction to Nutrition and Metabolism, 3 rd edition David A Bender Taylor & Francis Ltd, London 2002 Chapter 10: Integration and Control of Metabolism End of presentation The radio-immunoassay simulation on the CD accompanies this Chapter