Lehninger Ch. 14BIO 322 Recitation 1 / Spring 2013GLYCOLYSIS, GLUCONEOGENESIS,AND THE PENTOSE PHOSPHATEPATHWAY
OUTLINE*GlycolysisFates of PyruvateRegulation of Glycolysis (Chapter 15)GluconeogenesisPentose Phosphate Pathway
Fates of Glucose*
Glycolysis*D-Glucose (6C) 2 molecules of Pyruvate (3C)ATP, NADH conserve some released energyIn some mammalian tissues/cell types sole source of energyTen steps: 5 in preparatory phase & 5 in payoff phase
First 5 steps Prepatory Phase 2 ATP invested , raising the free energy content of intermediates Metobolized hexoses converted to common product G3P. One molecule of glucose yields to two molecules of G3P.
Last 5 steps Payoff Phase4 ATP produced, Net yield 2 ATPEnergy Conserved via 2 NADH per glucose molecule.
Three types of chemical transformation:Degradation of glucose carbon skeleton to yield pyruvate.ADP into ATPNAD+ - NADH*Lysis stepTriose PhosphatesPhosphorylation without ATP
Under STD conditions, glycolysis is irreversible, completed by a large net decrease in free energy
At cellular conditions, energy recovered as ATP with an efficiency of more than %60
Most of the energy is still in pyruvate and can be extracted by oxidative reactions inCitric Acid Cycle (Ch. 16)Oxidative Phosphorylation (Ch. 19)Fates of Pyruvate*
STEP 1: Phosphorylation of Glucose
Glucose Phosphorylation at C-6Irreversible step
HexokinaseRequires Mg (True Substrate Mg+ATP)
Mg shields negative charges of phosphoryl groups of ATP
Mg makes terminal phosphorus easier target for nucleophilic attack
Soluble, cytosolic Protein
Hepatocytes an extra hexokinase called hexokinase IV or glucokinase different from other hexokinase in kinetic and regulatory properties
STEP 3: Phosphorylation of Fructose 6-Phosphate to Fructose 1,6-Bisphosphate
Irreversible under celluar conditions
Commited Step, since G6P and F6P has other fates, but F16BP is targeted for glycolysis.
Major regulatory point in glycolysis.
PFK-1 activity increased, when ATP is low or ADP and AMP are in excess
PFK-1 inhibited by ATP, activated by F26BP (product of PFK-2) Next week in regulation
STEP 10: Transfer of the Phosphoryl Group from Phosphoenolpyruvate to ADP
PEP + ADP > Pyruvate + ATPPyruvate kinaseMg, K, MnSubstrate Level Phosphorylation
At pH 7, keto form dominates (non-enzymatic process)
Due to keto form, large negative standard free energy change (-31,4) large driving force pushing reaction forward to ATP synthesis
PEP hydrolysis (-61.9), ATP formation (-30,5)
Irreversible and important site of regulation
Glycogen phosphorylase (alpha 1-4) at non-reducing end until alpha 1-6 branch point. (debranching enzyme)
Starch by alpha-amylase (mouth), by pancreatic alpha amylase maltose (1,4), dextrin (1,6)
Lactic Acid Fermentation
Hypoxic conditions;NADH generated by glycolysis cannot be reoxidixed by oxygen
Glycolysis would stop due to lack NAD+ no electron acceptor for the oxidation of G3P.Regeneration required in an other way.
NAD+ regenerated from NADH by reduction of pyruvate to lactate via lactate dehydrogenase. (Exergonic)No net change in NAD+ or NADH (erythrocytes-no mitochondria- produce lactate even under aerobic conditions)
Lactate from muscle or erythrocytes to blood, targeted to liver, converted back to glucose back to muscle (Cori cycle)Fermentation*
Pyruvate carboxylase (Mg, Thiamine pyrophosphate (coenzyme))IrreversiblePresent in brewers`and baker`s yeastCarbon dioxideAbsent in vertebrate tissues and in organisms that carry lactic acid fermentation.
2. Alcohol dehydrogenase (Zn)-is present in many organisms that metabolize ethanol, including humans.-human liver, oxidation of ethanol with reduction in NAD+/NADH ratio
TPP : Derived from vitamin B1Deficiency beriberi
*Cleavage of bonds adjacent to carbonyl group.
Brain 120 g of glucose, more than half of the glucose stored as glycogen in muscle and liver.Source: Lactate, pyruvate, glycerol, certain AA (3C)In mammals - takes place in the liver and renal cortex7/10 identical rxns to glycolysisHexokinase, PFK-1 and Pyruvate KinaseAll have large negative free energy change IrreversibleWhereas others have free energy change near to zero.In gluconeogenesis 3 separate set of enyzmes catalyze exergonic reactions. Both irreversible processes.
Conversion of Pyruvate to PEPPyruvate Carboxylase:Pyruvate to oxaloacetate by pyruvate carboxylase (mitoch enzyme, coenzyme biotin -carries HCO3-)First regulatory enzyme, requires Acetyl-CoA as positive regulator (accumulation is a sign of FA availability as fuel)No mitoch transporter for oxaloacetate, malate dehydrogenase reduces it to malate for export to cytosol. (malate leaves mitoch via specific transporter, back to oxaloacetate in cytosol)PEP Carboxykinase:Oxaloacetate to PEP by PEP carboxykinase (Mg dependent, GTP as the phophoryl donor)
Pyruvate cytosol to mitochondria for conversion Or can be generated from alanine within mitochondria by transaminationOverall actual free energy change -25 kj/mol due to high PEP consumption in other rxns The reaction must be effectively irreversible.Bypass of Irreversible Steps*
Produced and consumed NADH in balance
Lactate as glucogenic precursorLactate to pyruvate in cytosol by lactate dehydrogenasePyruvate to oxaloacetate by pyruvate carboxylaseMitochondrial isozyme of PEP carboxykinase PEP transported out.*
G6P into pentose phosphatesOxidative pathway NADP elecctron acceptor NADPH Rapidly dividing cells such as bone marrow, skin, intestinal mucosa use pentoses to make RNA, DNA, ATP, NADH, FADH2, CoA
Other tissues product is just NADPH Needed for reductive biosynthesisTo prevent oxidative damage by maintaining high NADPH/NADP+
Glucose 6-Phosphate dehydrogenrase oxidizes glucose 6-phosphate6-phosphogluconate oxidized and decarboxylated NADP+ electron acceptorPentose Phosphate Pathway of Glucose Oxidation*
Nonoxidative PathwayIn tissues require primarly NADPH, the pentose phophates produced in oxidative phase are recyled into G6P.*
Glycolysis PPPHigh [NADP+] PPPLow [NADP+] Glycolysis*