Comparison of catabolic and anabolic pathways

Three stages of catabolism

Glucose Synthesis of compounds e.g. lactose glycolipids Glucose-6-P Pentosephosphate Pathway Glycolysis Glycogenesis Acetyl-CoA Citric acid cycle Fatty acid synthesis

Glycolysis - phosphorylation of glucose to glucose 6-phosphate hexokinase C C ATP ADP C C C C C C C C 2 glucose 3 P 4 glucose 6-phosphatase C C 2 P glucose 6-phosphate

Glycolysis isomerization fructose 6- phosphate C phosphohexose isomerase C 2 C C C C C C C C C 2 P C 2 P glucose 6-phosphate fructose 6-phosphate

Glycolysis phosphorylation fructose 1,6- bisphosphate C C C 2 C C C 2 P fructose 6-phosphate phosphofructokinase ATP 3 P 4 ADP fructose 1,6 bisphosphatase C C C 2 C C C 2 P P fructose 1,6-bisphosphate

Glycolysis cleavage 2 x triose phosphates C 2 P C C 2 P C 2 C C C C C 2 P fructose 1,6-bisphosphate aldolase dihydroxyacetone phosphate C C C 2 P glyceraldehyde 3-phosphate triose phosphate isomerase

Glycolysis oxidation of glyceraldehyde 3-phosphate C C C 2 P glyceraldehyde 3-phosphate 3 P 4 glyceraldehyde 3-phosphate dehydrogenase NAD + NAD C 2 P C C P bisphosphoglycerate

Glycolysis substrate-level phosphorylation - 1 C 2 P C C 3-phosphoglycerate phosphoglycerate kinase ATP ADP C C 2 C P P bisphosphoglycerate

Glycolysis isomerization 2- phosphoglycerate C 2 phosphoglyceromutase C 2 P C P C 2-phosphoglycerate C C 3-phosphoglycerate

Glycolysis dehydration phosphoenolpyruvate C 2 C C P phosphoenolpyruvate enolase C 2 C P C 2-phosphoglycerate

Glycolysis substrate-level phosphorylation - 2 C 3 C C 2 C P pyruvate kinase C pyruvate ATP ADP C phosphoenolpyruvate

2,3-Bisphosphoglycerate pathway in erythrocytes

Mitochondrial oxidation of pyruvate C 3 C C pyruvate C 3 C SCoA acetyl CoA The reaction of pyruvate dehydrogenase multi-enzyme complex decarboxylation of pyruvate (3-carbons) to a 2- carbon alcohol oxidation of the alcohol to an acid (acetic acid) with reduction of NAD + to NAD esterification to coenzyme A acetyl CoA

The five reactions of the pyruvate dehydrogenase multi enzyme complex

The Coenzymes and prosthetic groups of pyruvate dehydrogenase Cofactor Location Function Thiamine Bound to E1 Decarboxylates pyrophosphate pyruvate Lipoic acid Covalently linked Accepts to a Lys on hydroxyethyl E2 (lipoamide) carbanion from TPP CoenzymeA Substrate for E2 Accepts acetyl group from lipoamide FAD (flavin) Bound to E3 reduced by lipoamide NAD Substrate for E3 reduced by FAD2

Some mechanisms for transmission of regulatory signals between cells

Regulation of pyruvate dehydrogenase complex

acetyl CoA The Citric Acid Cycle citrate oxaloacetate malate fumarate succinate succincyl CoA cis-aconitate isocitrate -ketoglutarate A 9 step process that takes the acetate from acetyl-coa and converts it to C 2

Energy & the Citric Acid Cycle acetyl CoA citrate 2 NAD oxaloacetate cis-aconitate 2 malate isocitrate 2 C 2 + fumarate -ketoglutarate NAD FAD 2 GTP succinate succincyl CoA + Coenzyme A GDP C 2 + Coenzyme A NAD

Citric Acid Cycle Enzymes Malate dehydrogenase acetyl CoA Citrate synthase oxaloacetate malate citrate Aconitase cis-aconitate isocitrate Aconitase Fumarase Succinyl dehydrogenase fumarate succinate Succinyl CoA synthetase succincyl CoA -ketoglutarate Isocitrate dehydrogenase -Ketoglutarate dehydrogenase complex

A single molecule of glucose can potentially yield ~38 molecules of ATP

Inhibitors and activators of the TCA cycle

Gluconeogenesis occurs mainly in liver. Synthesis of glucose from pyruvate utilizes many of the same enzymes as Glycolysis. Three Glycolysis reactions have such a large negative DG that they are essentially irreversible. exokinase Phosphofructokinase Pyruvate Kinase. These steps must be bypassed in Gluconeogenesis.

Two Glycolysis reactions are bypassed by simple hydrolysis reactions: exokinase (Glycolysis) catalyzes: glucose + ATP glucose-6-phosphate + ADP Glucose-6-Phosphatase (Gluconeogenesis) catalyzes: glucose-6-phosphate + 2 glucose + P i 4 6 5 C 2 P 3 2 3 2 glucose-6-phosphate Glucose-6-phosphatase 1 2 C 2 glucose + P i

Phosphofructokinase (Glycolysis) catalyzes: fructose-6-p + ATP fructose-1,6-bisp + ADP Fructose-1,6-bisphosphatase (Gluconeogenesis) catalyzes: fructose-1,6-bisp + 2 fructose-6-p + P i Fructose-1,6-bisphosphatase 6 2 C 2 P 3 5 1 C 2 P 3 2 2 2 2 C 2 P 3 C 2 + P i 4 3 fructose-1,6-bisphosphate fructose-6-phosphate

Bypass of Pyruvate Kinase Pyruvate Kinase (Glycolysis) catalyzes: phosphoenolpyruvate + ADP pyruvate + ATP For bypass of the Pyruvate Kinase reaction, cleavage of 2 ~P bonds is required. DG for cleavage of one ~P bond of ATP is insufficient to drive synthesis of phosphoenolpyruvate (PEP). PEP has a higher negative DG of phosphate hydrolysis than ATP.

Bypass of Pyruvate Kinase (2 reactions): Pyruvate Carboxylase (Gluconeogenesis) catalyzes: pyruvate + C 3 + ATP oxaloacetate + ADP + P i PEP Carboxykinase (Gluconeogenesis) catalyzes: oxaloacetate + GTP PEP + GDP + C 2 Pyruvate Carboxylase C C C 3 ATP ADP + P i C 3 C C C 2 C PEP Carboxykinase GTP GDP C 2 C C C 2 pyruvate oxaloacetate PEP P 3 2

C C C C 2 oxaloacetate PEP Carboxykinase Reaction C C C 2 C 2 GTP GDP C C C 2 PEP 2 P 3 PEP Carboxykinase catalyzes GTP-dependent oxaloacetate PEP. It is thought to proceed in 2 steps: xaloacetate is first decarboxylated to yield a pyruvate enolate anion intermediate. Phosphate transfer from GTP then yields phosphoenolpyruvate (PEP).

The source of pyruvate and oxaloacetate for gluconeogenesis during fasting or carbohydrate starvation is mainly amino acid catabolism. Some amino acids are catabolized to pyruvate, oxaloacetate, or precursors of these. Muscle proteins may break down to supply amino acids. These are transported to liver where they are deaminated and converted to gluconeogenesis inputs. Glycerol, derived from hydrolysis of triacylglycerols in fat cells, is also a significant input to gluconeogenesis.

Regulation of glucokinase activity by glucokinase regulatory protein

REGULATIN F GLUCNEGENESIS A. Glucagon 1. Changes in allosteric effectors(frc2,6bp ). 2.Covalent modification of enzyme activity (PK P,Inactive). 3. Induction of enzyme synthesis (PEPCK ). B. Substrate availability C. Allosteric activation by acetyl CoA (PC, PD ) D. Allosteric inhibition by AMP (Frc1,6BPase)

C 2 C 2 1 glycogen C 2 C 2 6C 2 5 4 3 2 C 2 C 2 1 4 Glycogen is a polymer of glucose residues linked by (1 4) glycosidic bonds, mainly (1 6) glycosidic bonds, at branch points. Glycogen chains & branches are longer than shown. Glucose is stored as glycogen predominantly in liver and muscle cells.

C 2 N Glycogen synthesis P P C 2 N UDP-glucose Uridine diphosphate glucose (UDP-glucose) is the immediate precursor for glycogen synthesis. As glucose residues are added to glycogen, UDPglucose is the substrate and UDP is released as a reaction product.

C 2 P glucose-1-phosphate UDP-Glucose Pyrophosphorylase + P P PP i P UTP N N C 2 C 2 N N P P C 2 UDP-glucose

UDP-glucose is formed from glucose-1-phosphate: glucose-1-phosphate + UTP UDP-glucose + PP i PP i + 2 2 P i verall: glucose-1-phosphate + UTP UDP-glucose + 2 P i Spontaneous hydrolysis of the ~P bond in PP i (P~P) drives the overall reaction. Cleavage of PP i is the only energy cost for glycogen synthesis (one ~P bond per glucose residue). Glycogenin initiates glycogen synthesis. Glycogenin is an enzyme that catalyzes glycosylation of one of its own tyrosine residues.

4 6 5 C 2 3 2 1 UDP-glucose P P Uridine tyrosine residue of Glycogenin C 2 C C N -linked glucose residue 4 6 5 C 2 3 2 1 C 2 C C N + UDP C A glycosidic 2 C bond is formed 2 between the anomeric C1 of the glucose moiety derived from UDP-glucose C and the hydroxyl oxygen of a tyrosine side-chain of C C Glycogenin. 2 N UDP is released as a product.

-linked glucose residue 4 UDP-glucose 6 5 C 2 3 2 1 C 2 C C N + UDP C 2 C 2 (1 4) linkage C 2 C C N + UDP Glycosylation at C4 of the -linked glucose product yields an -linked disaccharide with (1 4) glycosidic linkage. UDPglucose is again the glucose donor. This is repeated until a short linear glucose polymer with (1 4) glycosidic linkages is built up on Glycogenin.

Glycogen Synthase catalyzes transfer of the glucose moiety of UDP-glucose to the hydroxyl at C4 of the terminal residue of a glycogen chain to form an (1 4) glycosidic linkage: glycogen (n residues) + UDP-glucose UDP glycogen (n +1 residues) + A separate branching enzyme transfers a segment from the end of a glycogen chain to the C6 hydroxyl of a glucose residue of glycogen to yield a branch with an (1 6) linkage.

Glycogen catabolism (breakdown): glucose-1-phosphate Glycogen Phosphorylase catalyzes phosphorolytic cleavage of the (1 4) glycosidic linkages of glycogen, releasing glucose-1-phosphate as reaction product. glycogen (n residues) + P i glycogen (n 1 residues) + glucose-1-phosphate This phosphorolysis may be compared to hydrolysis: ydrolysis: R--R' + R- + R'- Phosphorolysis: R--R' + -P 3 2- R- + R'--P 3 2- C 2 P 3 2

A glycogen storage site on the surface of the Phosphorylase enzyme binds the glycogen particle. Given the distance between storage & active sites, Phosphorylase can cleave (1 4) linkages only to within 4 residues of an (1 6) branch point. This is called a "limit branch".

Debranching enzyme has 2 independent active sites, consisting of residues in different segments of a single polypeptide chain: The transferase of the debranching enzyme transfers 3 glucose residues from a 4-residue limit branch to the end of another branch, diminishing the limit branch to a single glucose residue. The (1 6) glucosidase moiety of the debranching enzyme then catalyzes hydrolysis of the (1 6) linkage, yielding free glucose. This is a minor fraction of glucose released from glycogen. The major product of glycogen breakdown is glucose-1-phosphate, from Phosphorylase activity.

Glycogen Glucose exokinase or Glucokinase Glucose-6-Pase Glucose-1-P Glucose-6-P Glucose + P i Glycolysis Pathway Pyruvate Glucose metabolism in liver. The product glucose-6-phosphate may enter Glycolysis or (in liver) be dephosphorylated for release to the blood. Liver Glucose-6-phosphatase catalyzes the following, essential to the liver's role in maintaining blood glucose: glucose-6-phosphate + 2 glucose + P i Most other tissues lack this enzyme.

Glycogen Synthesis UTP UDP + 2 P i glycogen (n) + glucose-1-p glycogen (n + 1) Glycogen Phosphorylase P i Both synthesis & breakdown of glycogen are spontaneous. If both pathways were active simultaneously in a cell, there would be a "futile cycle" with cleavage of one ~P bond per cycle (in forming UDP-glucose). To prevent such a futile cycle, Glycogen Synthase and Glycogen Phosphorylase are reciprocally regulated, by allosteric effectors and by phosphorylation.

Stimulation and inhibition of glycogen degradation

ormonal regulation of glycogen synthesis

Glycogen Storage Disease Type I, liver deficiency of Glucose-6-phosphatase (von Gierke's disease) Type IV, deficiency of branching enzyme in various organs, including liver (Andersen's disease) Type V, muscle deficiency of Glycogen Phosphorylase (McArdle's disease) Type VII, muscle deficiency of Phosphofructokinase. Symptoms, in addition to glycogen accumulation hypoglycemia (low blood glucose) when fasting, liver enlargement. liver dysfunction and early death. muscle cramps with exercise. inability to exercise. 4/27/2016 Metabolism of Carbohydrates 56

NADP is generated via oxidation of Glc-6-P by the pentose phosphate pathway. Up to 30% Glc oxidation in animal liver is via this pathway. Pentose phosphate pathway also produces ribose-5-p for nucleotide biosynthesis. Pentose phosphate pathway especially in tissues synthesizing fatty acids and steroids, and in growing and regenerating tissues synthesizing nucleotides at high levels.

Three stages of pentose phosphate pathway 1. xidative reactions (1-3) that form NADP and ribulose-5-phosphate (Ru-5-P) from Glc-6-P 2. Isomerization and epimerization reactions (4 and 5) that transform Ru-5-P to either ribose-5-p(r-5- P) or xylulose-5-p (Xy-5-P) 3. A series of C-C bond cleavage and formation reactions (6-8) that convert 2 Xy-5-P and one R-5- P to 2 Frc-6-P and one glyceraldehyde-3-p (GAP) verall Reaction: 3 Glc-6-P + 6 NADP + + 3 2 -> 6 NADP + 3 C 2 + 2 Frc-6-P + GAP

xidative reactions

Isomerization and epimerization

The first reaction catalyzed by transketolase thiamine pyrophosphate General reaction Transfer of a two-carbon group from a ketose donor to an aldose acceptor

The reaction catalyzed by transaldolase The second reaction catalyzed by transketolase

Summary of pentose phosphate pathway

Role of NADP and glutathione (GS) in protecting cells against highly reactive oxygen derivatives

Phosphorylation products of fructose and their cleavage

Summary of fructose metabolism

Galactose metabolism

Metabolism of galactose

Sorbitol metabolism

Lactose synthesis

Glucuronidation Reactions: (Primarily in liver, intestine) Bilirubin di-glucuronide

Glucuronic Acid pathway

FAD2

Acetyl - Coenzyme A -N pantothenate unit C-C 2 -C 2 -N-C-C-C-C 2 C 3 C 3 P - P - phosphorylate ADP C 2 N N N 2 N N C 2 -C 2 S C 3 C acetate cysteine unit - P -

PD Regulation

Synthesis of fluoroacetate to fluorocitrate

Fluorocitrate is converted to fluoro-cis-aconitate and then to 4-hydroxy-trans-aconitate (Tn) which binds tightly to aconitase and inhibits its action. It does this by displacing the double bond closest to the Fe and preventing the hydration of the double bond.