CARBOHYDRATE METABOLISM 1 web 2017 József Mandl
Strategy of metabolism 1 Strategy of metabolism to extract energy ( hydrogen ) from the environment to store the energy excess to store hydrogen
CH 3 O 2 C H OH H C C O H O OH CO 2 H 2 O
Strategy of metabolism 2 At cellular level At the level of the organism
Strategy of metabolism 3 Most effective energy storage fatty acids lipids adipose tissue Prompt energy mobilisation from carbohydrates Metabolism is convergent glucose can be utilized by every cell Maintenance of blood glucose level - liver glycogen
lucose NAD FAD O 2 ADP+P i - G + G CO 2 FADH 2 NADH+H H 2 O ATP + G -membrane transport -biosynthesis -thermogenesis -contractility
carbohydrates proteins lipids glycolysis glucose aminoacids fatty acids acyl-coa synthetase e - pyruvate acyl-coa resp. chain H + H + H + O 2 ATP synth. pyruvate H 2 O ADP + P i H + ATP cytosol e - e - CO 2 acetyl-coa citrate cycle e - acyl-coa mitochondrium
GLUCOPLASTIC glycogen KETOPLASTIC glycerolipids other monosacharides glucose pyruvate PDH ketone bodies acetyl-coa fatty acids glucoplastic aminoacids oxaloacetate cholesterol citrate bile acids steroids CO 2
Strategy of metabolism 4 Compartmentation of metabolism in various cellular organelles Mitochondria the catabolic aerob compartment
glucose Daily 160 g glucose needed by the whole body 120 g glucose is required by the brain (20 g in body fluids, 190 g available from liver glycogen) Absolute necessity of glucose: brain, CNS, kidney medulla,testes, RBC, embryonic tissues
glucose 20 g glucose in body fluids, mainly blood 190 g liver glycogen Liver glycogenolysis 24 hrs Gluconeogenesis: glucose formation from non- carbohydrate precursors: lactate, glucogenic amino acids, glycerol
GLUT transporters Plasma membrane carriers of glucose. Catalyze facilitated diffusion. (passive, bi-directional trp.) 12 transmembrane helices. More than 5 isoforms with different function and characteristics.
GLUT transporters GLUT1 and GLUT3 high affinity (K M 1 mm) Expressed in every cell except hepatocytes (liver) and pancreatic β-cells. Ensures steady glucose uptake in RBC, CNS, kidney medulla, testis (glucose-dependent cells). Blood- brain, blood placenta- barrier GLUT2 low affinity (K M 15 mm) Expressed in hepatocytes and pancreatic β-cells (glucose sensor cells). Makes glucose uptake proportional with blood glucose concentration. GLUT4 Intermediate affinity (K M 5 mm) Insulin-dependent appearance in plasma membranes of skeletal muscle and adipocytes (facultative glucose consuming cells). Adjusts glucose consumption to availability. GLUT5 Expressed in intestinal epithelial cells and kidney tubular epithelial cells. Participates in glucose absorption and re-absorption.
Insulin-dependent targeting of GLUT4 High [glucose] (fed state) high [insulin] The cell is allowed to consume glucose. Low [glucose] (starvation) low [insulin] The cell is not allowed to consume glucose. insulin IR
Role of GLUT5 in glucose absorption intestinal lumen glucose Na + apical membrane intestinal epithelial cells glucose Na + K + GLUT5 ATP ADP +P i basolateral membrane glucose Na + K +
Glycolysis - gluconeogenesis
Regulation of glycolysis and gluconeogenesis Irreversible steps: Glucokinase hexokinase versus glucose-6-phosphatase
Hexokinase - glucokinase IRREVERSIBLE
Hexokinase and glucokinase every cell except liver and β-cells poorly specific (fructose, mannose) high affinity (K M 0.1 mm) activity independent of [glucose] allosteric product inhibition (G6P) liver and pancreatic β-cells highly specific (only glucose) low affinity (K M 10 mm) activity proportional with [glucose] F6P inhibits through a reg. protein systemic blood portal blood
Role of the liver in metabolism -first pass effect one way street filter function Portal circulation
insulin secretion Glucokinase 5 diabetes mellitus normal MODY [glucose] Maturity Onset Diabetes of the Young 25 yrs MODY 2 50 % mutation of glucokinase gene in pancreatic cells Defect of a single gene causes diabetes mellitus Glucokinase gene - 10 exon Difference is in the first exon between hepatocyte cell Promoters are different Glucose sensor function of glucokinase in pancreatic beta cells
Regulation of activity of glucokinase Gk glucokinase R regulator protein binds to Inactive Gk F- 6 - P binds to R F- 6 - P R complex Gk G-6-P does not bind to R Gk inactive F-1-P binds to R R F - 1 - P Gk ActiveGk Prevents binding to Gk
Glucose -6-phosphatase system Liver, kidney, pancreatic beta cells,gall bladder, testis, spleen, intestines Role in regulation of insulin secretion Induced after birth
G6P-ase catalytic subunit
Strategy of metabolism 4 Compartmentation of metabolism in various cellular organelles Mitochondria catabolic aerob compartment Endoplasmic reticulum (ER) secretion of materials/metabolites from the cells
Regulation of glycolysis and gluconeogenesis Irreversible steps: Phosphofructokinase 1 versus fructose bisphosphatase 1
Phosphofructokinase 1 IRREVERSIBLE
Phosphofructokinase 1 Allosteric inhibitors: ATP, citrate, fatty acids Activators: AMP, F-2,6-P Irreversible, main place of regulation committed step Citrate increases the inhibitory effect of ATP F-2,6-P inhibition of the inhibitory effect of ATP
PFK1 T R transition Tetramer structure (370 kd) sigmoidal saturation curve Quaternary structure M:muscle, P:platelet, L:liver isozymes
most important allosteric effector
Regulation of glycolysis and gluconeogenesis Irreversible steps: Pyruvate kinase
Pyruvate kinase IRREVERSIBLE substrate level phosphorylation
Pyruvate kinase Tissue-specific isoenzymes. PK-L (in liver) is regulated allosterically and hormonally. PK-M (in skeletal muscle) is not regulated. The last step of a pathway is not expected to be regulated. The only reason for regulating PK-L is that glucose synthesis (gluconeogenesis) occurs in liver and it starts with PEPformation, which would be undone by active PK-L (futile-cycle). i.e. when liver synthesizes glucose PK-L must be inhibited. Leak down regulation. PK is not regulated in tissues where glucose is not produced.
positive allosteric modulator: anaplerotic reaction acetyl CoA
Amphibolic role of citrate cycle pyruvate carboxylase
Pyruvate kinase and the beginning of gluconeogenesis fructose 1,6-bisphosphate No allosteric control CO 2 GDP GTP Acetyl CoA stim ADP inhib oxaloacetate ADP + P i CO 2 phosphoenolpyruvate ATP pyruvate PK-L ADP ATP F-1,6 bisp stim ATP, Ala, fatty acids inhib
PEPCK Regulation at the level of gene expression Stimulatory: glucocorticoids, glucagon, retinoids Inhibitory: glucose, insulin
plasma membrane Control of glycolysis glucose GLUT glucose HK / GK G6P Regulated steps (glucose transport and the irreversible reactions) F6P PFK1 F1,6BP PEP PK EC space cytosol pyruvate