1. Additional Pathways in Carbohydrate Metabolism

2. Following the carbons through the TCA cycle

3. 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

5. 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

7. 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

8. 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

9. 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.

11. 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

12. 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

13. Hormonal Regulation of Glycogen Metabolism

14. Effect of glucagon and epinephrine on glycogen phosphorylase glycogen synthase activities

15. Effect of insulin on glycogen phosphorylase glycogen synthase activities

16. 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 g of glucose
The body must still be able to make its own glucose

17. 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!

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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.

24. 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

25. 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!

27. 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.

28. Oxidative Stage
Non-oxidative
Stage