Additional Pathways in Carbohydrate Metabolism

Following the carbons through the TCA cycle

The Glyoxylate Cycle A variant of TCA for plants and bacteria Acetate-based growth - net synthesis of carbohydrates and other intermediates from acetate - is not possible with TCA Glyoxylate cycle offers a solution for plants and some bacteria and algae The CO2-evolving steps are bypassed and an extra acetate is utilized Isocitrate lyase and malate synthase are the short-circuiting enzymes

Glyoxylate Cycle Rxns occur in specialized organelles (glycoxysomes) Plants store carbon in seeds as oil The glyoxylate cycle allows plants to use acetyl-CoA derived from B-oxidation of fatty acids for carbohydrate synthesis Animals can not do this! Acetyl-CoA is totally oxidized to CO2 Malate used in gluconeogenesis

Metabolism of Tissue Glycogen But tissue glycogen is an important energy reservoir - its breakdown is carefully controlled Glycogen consists of "granules" of high MW Glycogen phosphorylase cleaves glucose from the nonreducing ends of glycogen molecules This is a phosphorolysis, not a hydrolysis Metabolic advantage: product is a sugar-P - a "sort-of" glycolysis substrate

Glycogen phosphorylase cleaves glycogen at non-reducing end to generate glucose-1-phosphate Debranching of limit dextran occurs in two steps. 1st, 3 X 1,4 linked glucose residues are transferred to non-reducing end of glycogen 2nd, amylo-1,6-glucosidase cleaves 1,6 linked glucose residue. Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase

Glycogen Synthase Forms -(1 4) glycosidic bonds in glycogen Glycogen synthesis depends on sugar nucleotides UDP-Glucose Glycogenin (a protein!) protein scaffold on which glycogen molecule is built. Glycogen Synthase requires 4 to 8 glucose primer on Glycogenin (glycogenein catalyzes primer formation) First glucose is linked to a tyrosine -OH Glycogen synthase transfers glucosyl units from UDP-glucose to C-4 hydroxyl at a nonreducing end of a glycogen strand.

Control of Glycogen Metabolism A highly regulated process, involving reciprocal control of glycogen phosphorylase (GP) and glycogen synthase (GS) GP allosterically activated by AMP and inhibited by ATP, glucose-6-P and caffeine GS is stimulated by glucose-6-P Both enzymes are regulated by covalent modification - phosphorylation

Hormonal Regulation of Glycogen Metabolism Insulin Secreted by pancreas under high blood [glucose] Stimulates Glycogen synthesis in liver Increases glucose transport into muscles and adipose tissues Glucagon Secreted by pancreas in response to low blood [glucose] Stimulates glycogen breakdown Acts primarily in liver Ephinephrine Secrete by adrenal gland (“fight or flight” response) Stimulates glycogen breakdown. Increases rates of glycolysis in muscles and release of glucose from the liver

Hormonal Regulation of Glycogen Metabolism

Effect of glucagon and epinephrine on glycogen phosphorylase glycogen synthase activities

Effect of insulin on glycogen phosphorylase glycogen synthase activities

Gluconeogenesis Synthesis of "new glucose" from common metabolites Humans consume 160 g of glucose per day 75% of that is in the brain Body fluids contain only 20 g of glucose Glycogen stores yield 180-200 g of glucose The body must still be able to make its own glucose

Gluconeogenesis Occurs mainly in liver and kidneys Not the mere reversal of glycolysis for 2 reasons: Energetics must change to make gluconeogenesis favorable (delta G of glycolysis = -74 kJ/mol Reciprocal regulation must turn one on and the other off - this requires something new!

Seven steps of glycolysis are retained Three steps are replaced The new reactions provide for a spontaneous pathway (G negative in the direction of sugar synthesis), and they provide new mechanisms of regulation

Pyruvate Carboxylase The reaction requires ATP and bicarbonate as substrates Biotin cofactor Acetyl-CoA is an allosteric activator Regulation: when ATP or acetyl-CoA are high, pyruvate enters gluconeogenesis

PEP Carboxykinase Lots of energy needed to drive this reaction! Energy is provided in 2 ways: Decarboxylation is a favorable reaction GTP is hydrolyzed GTP used here is equivalent to an ATP

PEP Carboxykinase Not an allosteric enzyme Rxn reversible in vitro but irreversible in vivo Activity is mainly regulated by control of enzyme levels by modulation of gene expression Glucagon induces increased PEP carboxykinase gene expression

Fructose-1,6-bisphosphatase Thermodynamically favorable - G in liver is -8.6 kJ/mol Allosteric regulation: citrate stimulates fructose-2,6--bisphosphate inhibits AMP inhibits

Glucose-6-Phosphatase Presence of G-6-Pase in ER of liver and kidney cells makes gluconeogenesis possible Muscle and brain do not do gluconeogenesis G-6-P is hydrolyzed as it passes into the ER ER vesicles filled with glucose diffuse to the plasma membrane, fuse with it and open, releasing glucose into the bloodstream.

Metabolites other than pyruvate can enter gluconeogenesis Lactate (Cori Cycle) transported to liver for gluconeogenesis Glycerol from Triacylglycerol catabolism Pyruvate and OAA from amino acids (transamination rxns) Malate from glycoxylate cycle -> OAA -> gluconeogenesis

Regulation of Gluconeogenesis Reciprocal control with glycolysis When glycolysis is turned on, gluconeogenesis should be turned off When energy status of cell is high, glycolysis should be off and pyruvate, etc., should be used for synthesis and storage of glucose When energy status is low, glucose should be rapidly degraded to provide energy The regulated steps of glycolysis are the very steps that are regulated in the reverse direction!

Pentose Phosphate Pathway Provides NADPH for biosynthesis Produces ribose-5-P for RNA and DNA oxidative steps (formation of NADPH) followed by non-oxidative steps Cytosolic pathway Active in tissues that synthesis fatty acids and sterols (liver, mammary glands, adrenal glands, adipose tissue) Active in red blood cells to maintain heme in reduced form.

Oxidative Stage Non-oxidative Stage