Textbook notes corresponding to weekly reading guide of Biochemistry course (code BIOSCI98) at University of California, Irvine. Comes with reading guide, color-coded notes and diagram. Week 8.
Glycolysis and Gluconeogenesis Regulations
• If irreversible steps were reversible/pathways happened at the same rate-> hydrolysis of ATP w/o any useful
work being done; generates heat -> Futile cycle!
• Hexokinase isozymes of muscle and liver are affected differently by their product: Glu-6-PO4.
• In myocytes: muscle hexokinase I/II are allosterically inhibited by glu 6-PO4.
○ Isozymes are temporarily inhibited by high [glu 6-PO4] levels.
• *High [P] can allosterically inhibit [E].*
• Carbohydrate metabolism:
○ Muscle consumes glucose for energy production.
○ Liver maintains blood glucose levels by consuming/producing glucose.
• In hepatocytes: INC [blood glucose] above 5mM -> hexokinase IV activity INCs; hexokinase IV is still not
Wk8RG saturated until it reaches Km=10mM.
Friday, February 22, 2019 10:58 AM
○ -> hexokinase I activity would stay the same; already saturated @ Km=0.2mM.
• **Remember, high [S] can saturate the [E], and won't INC activity anymore!**
• Hexokinase IV is NOT inhibited by [glu 6-PO4]. Can continue to operate in high [glu 6-PO4] levels.
• Hexokinase IV is inhibited by regulatory protein: HIGH [Glucose] -> HIGH [Fruc 6-PO4] -> regulatory protein
releases hexokinase IV from nucleus to cytosol. (COMPARTAMENTALIZATION: Yes!) Fig 15-25, pg. 591
• Hexokinase IV (glucokinase) and glu6-PO4 are transcriptionally regulated.
• PFK-1 and FBPase-1 are reciprocally regulated.
• PFK-1 has several regulatory sites @ which allosteric activators/inhibitors bind to. -> PFK site of regulation!
Week 8 • High [ATP] -> ATP allosterically inhibits PFK by binding to site and lowering substrate affinity! -> Less
glycolysis
• High [citrate] INCs inhibitory effect of ATP.
• ***Remember, glycolysis favors high [AMP/ATP] ratio!***
• High [AMP] -> AMP allosterically inhibits FBPase. -> Less gluconeogenesis
• Fructose 2,6 bisphosphate is a potent allosteric regulator of PFK-1 and FBPase-1 (formation and
breakdown of fru 6-bisphosphate). Fig 15-18, pg. 594
• Fructose 2,6-bisphosphate is an allosteric activator of PFK (INC glycolysis)
• Fructose 2,6-bisphosphate is an allosteric inhibitor of FBPase (DEC gluconeogenesis)
• PFK-2: catalyzes phosphorylation of Fru6-PO4 -> forms Fru2,6-bisPO4!
• FBPase-2: catalyzes breakdown of Fru2,6-bisPO4.
○ Both are enzymatic activities of the same bifunctional protein.
• Xylulose 5-PO4 is a key regulator of carb/fat metabolism. -> makes Fru2,6bisPO4.
• Dephosphorylation activates PFK-2 and inhibits FBPase-2.
• Dephosphorylation done by phosphatase 2A, activated by Xylulose 5-PO4.
• Also INCs synthesis of all enzymes required for fatty acid synthesis.
• The glycolytic enzyme pyruvate kinase is allosterically inhibited by ATP.
• Pyruvate -> PEP is under multiple types of regulation.
• When cell's energy levels are sufficient, oxidative phosphorylation (ETC) slows -> [NADH/NAD+] ratio INCs ->
inhibits CAC-> Acetyl-CoA builds up ->
○ Inhibits PDH complex -> inhibits CAC
○ Activates pyruvate carboxylase -> pyruvate forms OAA, converted into PEP!
• Transcriptional regulation of glycolysis and gluconeogenesis changes # of enzyme molecules.
• Ex. Insulin stimulates transcription of genes that encode hexokinases II/IV, PFK-1, pyruvate kinase, PFK-
2/FBPASE-2 (all regulatory enzymes of glycolysis).
○ Also slows gene expression of PEPCK and glu 6-phosphatase (gluconeogenic enzymes).
• Carbohydrate response element binding protein (ChREBP): TF that coordinates synthesis of anabolic enzymes
needed for carb/fat synthesis.
Citric Acid Cycle: Summary
• ETC RECAP: Compounds from the breakdown of macromolecules (except lipids) are oxidized from
CO2, w/ most of the oxidation energy temporarily held in FADH2 and NADH. (In aerobic conditions) said
e- are transferred to O2 and energy of e- flow is trapped as ATP.
○ Enzymes that catalyze carboxylation use biotin to activate CO2 and carry it to acceptors such as
pyruvate/ phosphoenolpyruvate.
○ Vertebrates cannot synthesize glucose from acetate or fatty acids that give rise to acetyl-CoA.
(Has to go thru glycolysis first to get acetyl-CoA precursor-- pyruvate).
• Acetyl-CoA's condensation w/ OAA -> citrate.
○ Catalyzed by citrate synthase.
○ Considered first and last step in CAC.
• Cyclic pathway = intermediates are not used up: citrate is converted into OAA (end of CAC) and
releases 2 CO2.
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