Glycolysis pt1

Resolving the Equation of Glycolysis into Two Processes

the conversion of glucose to pyruvate is exergonic:

glucose + 2NAD+ ——>2 pyruvate + 2NADH + 2H+

∆G′°1 = −146 kJ/mol

the formation of ATP from ADP and Pi is endergonic:

2ADP + 2Pi ——→ 2ATP + 2H2O

∆G′°2 = 2(30.5 kJ/mol) = 61.0 kJ/mol

Standard Free-Energy Change of Glycolysis

under standard and cellular conditions, glycolysis is essentially irreversible because payoff phase is much higher in magnitude than investment

Energy Remaining in Pyruvate

energy stored in pyruvate can be extracted by:

aerobic processes:

• oxidative reactions in the citric acid cycle

• oxidative phosphorylation

anaerobic processes

• reduction to lactate

• reduction to ethanol

What is the Importance of Phosphorylated Intermediates

all nine intermediates are phosphorylated

functions of the phosphoryl groups:

• prevent glycolytic intermediates from leaving the cell

• serve as essential components in the enzymatic conservation of metabolic energy

• lower the activation energy and increase the specificity of the enzymatic reactions

Phosphorylation of Glucose STEP 1

hexokinase activates glucose by phosphorylating at C-6 to yield glucose 6-phosphate

• Mg2+ required

• irreversible under intracellular conditions

Conversion of Glucose 6-Phosphate to Fructose 6-Phosphate STEP 2

phosphohexose isomerase (phosphoglucose isomerase) catalyzes the reversible isomerization of glucose 6-phosphate to fructose 6-phosphate

• mechanism involves an enediol intermediate

• reaction readily proceeds in either direction

• reversible

• Mg2+ required

Phosphorylation of Fructose 6-Phosphate to Fructose 1,6-Bisphosphate STEP 3

phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation of fructose 6-phosphate to yield fructose 1,6-bisphosphate

• irreversible

• Mg2+ required

Cleavage of Fructose 
1,6-Bisphosphate STEP 4

• aldolase

• yield glyceraldehyde 3-phosphate and dihydroxyacetone phosphate

• reversible because reactant concentrations are low in the cell

Interconversion of the Triose Phosphates STEP 5

triose phosphate isomerase converts dihydroxyacetone phosphate to glyceraldehyde 3-phosphate

• reversible

• final step of the preparatory phase of glycolysis

in the payoff phase of glycolysis…

each of the two molecules of glyceraldehyde 3-phosphate undergoes oxidation at C-1

some energy from the oxidation reaction is conserved in the form of one NADH and two ATP per triose phosphate oxidized

Oxidation of Glyceraldehyde 
3-Phosphate to 1,3-Bisphosphoglycerate STEP 6

glyceraldehyde 3-phosphate dehydrogenase catalyzes the oxidation glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate

energy-conserving reaction

The First Step of the Payoff Phase is an Energy-______ Reaction 

Conserving

formation of the acyl phosphate group at C-1 of 1,3-bisphosphoglycerate conserves the free energy of oxidation

acyl phosphates have a very high standard free energy of hydrolysis (∆G′° = −49.3 kJ/mol)

Phosphoryl Transfer from 1,3-Bisphosphoglycerate to ADP STEP 7

phosphoglycerate kinase transfers the high-energy phosphoryl group from the carboxyl group of 1,3-bisphosphoglycerate to ADP

forming ATP and 3-phosphoglycerate

requires Mg2+

Steps 6 and 7 of Glycolysis Constitute an ______ Process

Energy coupling. deltaG 6-18

Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate STEP 8

phosphoglycerate mutase catalyzes a reversible shift of the phosphoryl group between C-2 and C-3 of glycerate

requires Mg2+

Dehydration of 2-Phosphoglycerate to Phosphoenolpyruvate STEP 9

enolase promotes reversible removal of a molecule of water from 2-phosphoglycerate to yield phosphoenolpyruvate (PEP)

energy-conserving reaction

Mg2+-stabilized enolic intermediate

Transfer of the Phosphoryl Group from Phosphoenolpyruvate to ADP STEP 10

pyruvate kinase catalyzes the transfer of the phosphoryl group from phosphoenolpyruvate to ADP, yielding pyruvate

requires K+ and either Mg2+ or Mn2+

Pyruvate in its Enol Form Spontaneously

Tautomerizes to its Keto Form

Overall Reaction of Glycolysis

to Net Reaction

The Overall Balance Sheet Shows a Net ____

Gain of Two ATP and Two NADH Per Glucose

Endogenous Glycogen and Starch Are Degraded by Phosphorolysis

glycogen phosphorylase = mobilizes glycogen stored in animal tissues and microorganisms by a phosphorolytic reaction to yield glucose 1-phosphate

starch phosphorylase = mobilizes starch by a phosphorolytic reaction

Glycogen Breakdown Is Catalyzed by Glycogen Phosphorylase

glycogen phosphorylase = catalyzes phosphorolytic cleavage at the nonreducing ends of glycogen chains

• requires pyridoxal phosphate

• acts repetitively until it reaches a point four residues away from a (α1→6) branch point

Glycogen phosphorylase

catalyzes phosphorolytic cleavage at the nonreducing ends of glycogen chains

requires pyridoxal phosphate

acts repetitively until it reaches a point four residues away from a (α1→6) branch point

Glycogen Breakdown Mechanism

• phosphorylase releases glucose-1-phosphate at non-reducing end until 4 residues away from branch

• phosphoglucomutase isomerizes glucose-1-phosphate into glucose-6-phosphate

• Debranching enzyme transferase branch onto main chain and exhibits glucosidase activity

α-amylase location, function, and products

salivary and small intestine enzyme that hydrolyzes the internal (α1→4) glycosidic linkages of starch and glycogen, producing di- and trisaccharides

pancreatic α-amylase yields mainly ____

maltose, maltotriose, and limit dextrins mmd (fragments of amylopectin containing (α1→6) branch points, which are removed by limit dextrinases)

Hydrolysis of Disaccharides

membrane-bound hydrolases in the intestinal brush border hydrolyze disaccharides:

monosaccharides pass through intestinal cells to the bloodstream, which transports them to the liver or other tissues

Cellulase

attacks the (β1→4) glycosidic bonds of cellulose

absent in most animals

microorganisms produce cellulase

Fructose and Mannose

fructose and mannose can be phosphorylated and funneled into glycolysis

hexokinase = phosphorylates fructose in the small intestine

fructose kinase = phosphorylates fructose in the liver

Fructose 1-Phosphate Aldolase

cleaves fructose 1-phosphate to glyceraldehyde and dihydroxyacetone phosphate

Products of Fructose 1-Phosphate Hydrolysis Enter Glycolysis as

Glyceraldehyde 3-Phosphate

triose phosphate isomerase = converts dihydroxyacetone phosphate to glyceraldehyde 3-phosphate

triose kinase = uses ATP to phosphorylate glyceraldehyde to glyceraldehyde 3-phosphate

Mannose Enters Glycolysis as

Fructose 6-Phosphate

hexokinase = phosphorylates mannose at C-6

phosphohexose isomerase = converts mannose 6-phosphate to fructose 6-phosphate

Regulation of irreversible reactions 

Step 1: Hexokinase: product inhibition

Step 3: Phosphofructokinase-1 (PFK-1): rate-limiting. Inhibited by HIGH ATP

Step 10: Pyruvate kinase: Inhibited by HIGH ATP

Fructose feeder pathway

• in liver, fructose kinase phosphorylates to fructose-1-phosphate

• in muscle and kidney, hexokinase to fructose 6 phosphate

Galactose feeder pathway

UTP phosphorylates galactose to glucose 1-phosphate

glucose 1-phosphate → phosphoglucomutase → glucose-6-phosphate