Glycolysis

Biochemistry

2017Hayder A Giha

Definition:It is a universal pathway for glucose (glycogen) metabolism, found in the cytosol of all mammalian cells, and give pyruvate and/or lactate as an end product.

Introduction

n Most sugars converted to glucose, so, it is the major carbohydrate utilized by human tissues.

n Glycolysis is the major pathway for glucose metabolism.

n There is a basal requirement for glucose in most of the tissues, e.g. the brain needs is substantial, and erythrocyte needs is total and absolute.

Characteristics of glycolysis

n Site: is the cytosol of all tissuesn Its unique in the sense that, it can use oxygen

(aerobic) when its available, however, it can operate in absence of oxygen (anaerobic).

n Aerobic utilization of glucose demands a set of mitochondrial enzymes (Kerbs cycle & respiratory chain).

The cell

Importance

n As glucose (Glc) enter the cell, it is phosphorylatedat C6, becomes negatively charged, thus Glcremain inside the cell because the cell membrane inside is negatively charged.

n The fate of G6P: a. energy production (glycolysis) b. building of CHO or storage (glycogenesis) c. other pathways ( e.g. Pentose Phosphate Pathway)

n Main source of energy for erythrocytes, and exercising muscles.

Glycolysisn Have 2 phasesI. Energy investment phase (consume energy)

(5 reactions) II. Energy generation phase (energy production)

(5 reactions)

n Three control point (irreversible reactions)

Energy investment phase

n Two phosphorylations; of Glc at C6 & Fructose at C1 I. Hexokinase (all cells) or Glucokinase (liver and B-

cells of pancreas) n Hexokinase: have low Vmax & Km (high affinity),

work at normal or low blood glucose to provide energy for tissues, and inhibited by G6P

n Glucokinase: have high Vmax & Km (low affinity), work after meal (high BG) to store Glu

n Both enzymes catalyze irreversible reactions –suitable for regulation of the pathway

comparisonHexokinase Glucokinase

site All tissues -Liver parenchymal cell-Pancreatic b-cells

Affinity for substrate

High affinity Low Km & V-max

Low affinityHigh Km & V-max

Function To ensure glucose supply for tissues

To remove glucose from blood after meals

Specificity Glucose and other hexoses (low level)

Glucose only

Control Inhibited by glucose 6-phosphate

Affected by nutritional state

Glycolysisn G6P isomerized (phosphoglucose isomerase) to F6Pn F6P phosphorylated by phosphofructokinase-1 (PFK-1)

at C1 to Fructose 1,6 bisphosphate (F1,6P2)n This is the 2nd irreversible reaction, so is a control

(regulation) pointn High energy state (ATP & citrate) inhibit PFK-1, and

the opposite is true (AMP & F2,6P2) activate PFK-1n Small a mount of F6P is converted to Fructose 2,6

bisphosphate (F2,6P2) by phosphofructokinase-2 (PFK-2).

Energy investment phase

n F1,6P2 cleaved by aldolase A into 2 triose-P; glyceraldehyde 3-P (GAP) & dihydroxyacetone-P (DHAP).

n GAP and DHAP are inter-convertible by triose phosphate isomerase

Energy generation phase

n GAP is oxidized and phosphorylated at C1 by GAP dehydrogenase to an energy-rich 1,3 bisphosphoglycerate (1,3-BPG)

Energy generation phase

n 1,3-BPG -------- 3 phosphoglycerate (3-PG) and production of energy at substrate level (ADP ---ATP); by phosphoglycerate kinase

n In RBCs, some 1,3 BPG is converted to 2,3BPG by mutase enzyme (loss of ATP), 2,3 BPG decrease Hb affinity to O2.

n 3-PG (3 phosphoglycerate) --------- 2PG by mutasen 2PG ----------- phosphoenolpyruvate (PEP) by

Enloase (with removal of H2O)This dehydration raise the energy level of P in PEP

n PEP --------- pyruvate by pyruvate kinase (PK), associated with conversion of ADP to ATP (energy at substrate level)(it is irreversible reaction, site for control)

n PK is activated by F1,6P2 and cAMP-dependent protein kinase A

Glycolysis

H - C = O H - C - OH CH2- O - P

Glyceraldehyde 3-phosphate

O II C – O ~ P H - C - OH CH2- O - P

1,3-Biphosphoglycerate

COO- H - C - OH CH2- O - P

3-phosphoglycerate

COO- H - C – O - P CH2OH

2-phosphoglycerate

COO- C – O ~ P CH2

Phosphoenolpyruvate

COO- C - OH II CH2

(Enol) Pyruvate

Glyceraldehyde 3-phosphate Dehydrogenase

P i

NADH NAD ADP ATP

Phosphoglycerate kinase

Mg2

Phosphoglycerate mutase

Enolase

Mg2

H2O

Pyruvate kinase

ADP ATP

Mg2

FL

IA

NAD NADH

n NAD is needed for ATP production at substrate level, but it is limited, so, the produced NADH+H+ need to be oxidized

n Under anaerobic conditions or when no mitochondria, H+ is accepted by pyruvate which is converted to lactate (by lactate dehydrogenase)

n When produced in large quantities, lactate diffuse from cell to blood cause lactic acidosis

Glycolysis

Energy of glycolysis

n 2 ATP are consumed during glycolysis (investment phase) in reactions catalyzed by:

1. Hexokinase (Glucokinase), 1 ATP2. Phosphofructokinase-1, 1 ATP

n Note: if glycogen is the starting point, 1 ATP will be saved, i.e. increased net production of energy at substrate level.

Energy production in glycolysis

n 4 ATP are generated in the energy generation phase;

1. Phosphoglycerate kinase (2 ATP, substrate level)

2. Pyruvate kinase (2 ATP, substrate level).THE NET is 2 ATP

n In addition, two NADH + H+ (Glyceraldehyde 3-phosphate dehydrogenase), (under aerobic conditions, in respiratory chain, each one produce 3 ATP).

Regulation of glycolysis

n Glycolysis is controlled at 3 steps (non-equilibrium reactions) catalyzed by:

1. Hexokinase (glucokinase)

2. Phosphofructokinase-1

3. Pyruvate kinase

Oxidation of pyruvate

n Pyruvate oxidation (under aerobic conditions) occurs within the mitochondria, its transported into the later via pyruvate transporter

n Its oxidatively decarboxylated to Acetyl-CoA, before it enter the citric acid cycle.

n This reaction is catalysed by a complex of enzymes (3 enzymes), designated as, pyruvate dehydrogenase complex (PDH)

n The reaction needs CoA, thiamin diphosphate, lipoic acid, FAD and NAD+

Oxidation of pyruvate

n 2 NADH + H+ are produced for each glucose molecule

n Also a high-energy thio ester group in acetyl-CoA is produced.

The End