Glucose Metabolism

C6H12O6

Complete Oxidation of Glucose

+ 6O2 6CO2 + 6H2O

Glucose Metabolism

• Central position in metabolism • A good cellular fuel (- 2,840 kJ/mol for complete oxidation) • Released from high molecular weight polymers • Production of ATP • Supplies metabolic intermediates for biosynthesis

Glucose

Fates of glucose in animals and plants

• Storage

• Glycolysis: oxidation to 3-carbon compounds to provide ATP and metabolic intermediates

• Pentose phosphate pathway - oxidation to yield ribose-5-phosphate and NADPH

Synthesis of glucose

• Gluconeogenesis

• Photosynthesis

Glycolysis - A series of enzyme-catalyzed reactions in a pathway

- Overall conversion:

- Generation of 2 ATPs (net gain)

- These reactions are the way in which carbohydrate is broken down into smaller units. - Anaerobic metabolism: pyruvate is converted to lactate or ethanol (fermentation) - Aerobic metabolism: pyruvate is fed into the TCA cycle where it is fully metabolized to carbon dioxide releasing more ATPs

Hexose stage: (Preparatory phase)

Triose stage: (Payoff phase)

The ten steps of glycolysis

Hexose stage: 1 to 4 Triose stage: 5 to 10

(1) Hexokinase: phosphorylation of glucose

Hexokinases - Hexose kinases - Four isoenzymes isolated from mammalian liver - Hexokinase I, II, III: Km for glucose = 10-6 – 10-4 M - Hexokinase IV = Glucokinase: Km for glucose = 10-2 M (more active at high blood glucose conc.)

(2) Conversion of G-6-P to F-6-P:

(aldose-ketose isomerization)

(3) Transfer of a second phosphoryl group from ATP to F-6-P

- The first committed step in glycolysis

(β-D-fructofuranose form)

PFK-1

Glucose Glucose 6-P Fructose 6-P

Fructose 1,6-bisP

PFK-1

Glycolysis only

other pathways

other pathways other pathways

(4) Cleavage of C3-C4 bond - Production of two triose phosphates

- Unfavorable under standard condition (G > 0) - Actual free energy change ~ 0, thus a near-equilibrium reaction - Rapid consumption of DHAP and G3P “pulls” the reaction forward

Aldolases - Class I: plants and animals - Class II: microorganisms - Classes I and II are not structurally related - An example of “convergent evolution”

Mechanism of F1,6-bisP cleavage by aldolases:

X – an electron withdrawing group - Lysine amino group in Class I - Zn2+ cofactor in class II

(5) Conversion of DHAP to G3P

- Enzyme mechanism: general acid-base catalysis (see lecture notes on enzyme mechanism)

- Only G3P continues in the glycolytic pathway - Which direction of the reaction is favored?

Fate of carbon atoms from the hexose stage to the triose stage:

(6) Formation of a “high-energy” compound: 1,3-Bisphosphoglycerate

Acid anhydride linkage

- Beginning of the recovery of energy from triose phosphates - Oxidation-reduction reaction catalyzed by a dehydrogenase (why is it called a

dehydrogenase?) - Phosphorylation reaction (how is it different from the earlier kinase reactions?)

(7) Generation of ATP - 1,3-BPG hydrolysis releases large amount of energy - Substrate level phosphorylation

- Stabilization of product by resonance hybrid formation:

- G3P dehydrogenase and PG kinase associate to form a complex: efficient channeling of 1,3-bisphosphoglycerate

Arsenate poisoning - AsO4

3- replaces inorganic phosphate (PO43-) in phosphoryl transfer

reactions

Glyceraldehyde 3-P +

AsO43-

+ NAD+

Glyceraldehyde 3-P dehydrogenase

(No 1,3-pisphosphoglycerate production)

(8) Intramolecular phosphoryl group transfer

Phosphoglycerate mutase

Mutase: an isomerase that catalyzes the intramolecular shifting of a chemical group

2

3

2

3

Phosphoglycerate kinase: mechanism

(9) Dehydration to an energy-rich compound, PEP

(PEP)

(10) Generation of ATP

- PEP: high-energy phosphate compound

- Tautomerization stabilizes products of hydrolysis

- Substrate level phosphorylation

- Glycolysis finally turns a profit!

tautomerization

(keto form) (enol form)

Overall process of glycolysis

Glucose + 2ATP + 2NAD+ + 4ADP + 2Pi → 2 pyruvate + 2ADP + 2NADH + 2H+ + 4ATP + 2H2O Simplifying the equation will give you: Glucose + 2NAD+ + 2ADP + 2Pi → 2 pyruvate + 2NADH + 2H+ + 2ATP + 2H2O

The fate of pyruvate

Metabolism of pyruvate to ethanol

- Regeneration of NAD+ for glycolysis

Summary of glycolysis and ethanol formation

Reduction of pyruvate to lactate

- Regeneration of NAD+ for glycolysis - Anaerobic bacteria - Mammals (skeletal muscles during vigorous exercise)

Summary of glycolysis and lactate formation

Other sugars can enter glycolysis

Entry of fructose into glycolysis through fructokinase

Conversion of mannose to fructose 6-phosphate

Conversion of galactose to glucose 6-phosphate

Regulation of glycolysis

1. Regulation of hexose transporters

- Membrane-embedded transporters (e.g. GLUT family transporters) - Stimulation of cellular glucose uptake by insulin

(Skeletal and heart muscle cells, Adipocytes)

2. Regulation at 3 irreversible steps

(a) Hexokinase

Glucose + ATP Glucose 6-phosphate (G-6-P) + ADP G-6-P - Allosteric inhibitor of hexokinases I, II, III (found in muscles)

Glucokinase - Not inhibited by G-6-P - the major hexokinase in liver - Converts glucose to G-6-P in liver after a meal - G-6-P is used for glycogen synthesis when glucose is sufficient in other tissues - High Km value for glucose - Never saturated with glucose - Activity increases with increasing concentration of available glucose

(b) Phosphofructokinase-1 (PFK-1)

Fructose 6-phosphate + ATP Fructose 1,6-bisphosphate + ADP

ATP - a substrate and an allosteric inhibitor AMP and ADP – allosteric activators

ATP reduces the affinity of PFK-1 for F 6-P

AMP relieves the inhibition by ATP

Citrate – feedback inhibitor; a TCA cycle intermediate - high conc indicates that TCA cycle is blocked

Fructose 2, 6-bisphosphate – an allosteric activator of PFK-1

Glucagon

Low blood glucose

In liver cells:

+

PFK-1 less active

(Phosphofructokinase-2)

(c) Pyruvate kinase

Phosphoenolpyruvate (PEP) + ADP Pyruvate + ATP

Fructose 1,6-bisphosphate (F1,6BP) – an allosteric activator

ATP – a strong allosteric inhibitor

PFK-1 , F1,6BP , Pyruvate kinase Thus, PFK-1 activation leads to subsequent pyruvate kinase activation - Feed-forward activation

The Entner-Doudoroff Pathway in Bacteria

- No PFK-1 enzyme in bacteria - No fructose 1,6-bisphosphate

formation from glucose - Generates fewer ATP than glycolysis,

why? - Earliest pathway for glucose

degradation

PFK-1 (not in some bacteria)

fructose 1,6-bisphosphate

fructose 6-phosphate

Two products in this pathway: Pyruvate (end product of glycolysis) Glyceraldehyde 3-phosphate

Glycolysis (triose phase)

Pyruvate