Glycolysis

By

Amr S. Moustafa, M.D.; Ph.D.

Assistant Prof. & Consultant, Medical Biochemistry Dept.

College of Medicine, KSUamrsm@hotmail.com

Glucose Transport

• Na+-Independent Facilitated Diffusion Glucose Transporters (GLUT 1-14)With concentration gradientEnergy Independent

• Na+-Monosaccharide Cotransporter:Against concentration gradientEnergy dependentCarrier-mediated (SGLT)Coupled to Na+ transportSmall intestine, renal tubules & choroid plexus

Glucose Transport: Facilitated Diffusion

Glucose Transporters

• Tissue-specific expression pattern GLUT-1 RBCs and brainGLUT-2 Liver, kidney & pancreasGLUT-3 NeuronsGLUT-4 Adipose tissue & skeletal muscleGLUT-5 Small intestine & testesGLUT-7 Liver (ER-membrane)

• Functions:GLUT-1, 3 & 4 Glucose uptake from bloodGLUT-2 Blood & cells (either direction)GLUT-5 Fructose transport

Glucose Transport & Insulin

Metabolic Pathway

DefinitionSite:

Cellular (tissue) and SubcellularReactionsRate-limiting enzyme(s)Regulatory mechanism(s):

Rapid, short-term:Allosteric Covalent modification

Slow, long-term:Induction/repression

Glycolysis

Aerobic Vs Anaerobic Glycolysis

Aerobic Glycolysis: Total Vs Net ATP

Production

Aerobic Glycolysis-1

HexokinaseGlucokinase

Aerobic Glycolysis-2

Aerobic Glycolysis: 3-5

Aerobic Glycolysis:

6 -10

2

2

2

2

2

2

Aerobic Glycolysis-1

HexokinaseGlucokinase

Hexokinase Vs GlucokinaseHexokinase Glucokinase

Site Most tissues Hepatocytes

Islet cells (pancreas)

Kinetics Low Km

Low Vmax

High Km

High Vmax

Regulation G-6-phosphate F-6-phosphate

Insulin: Induction

Function Low glucose conc. High glucose conc.

Glucose sensor

Hexokinase Vs Glucokinase -2

Glucokinase Regulatory Protein

PFK-1 : Regulation

F – 2,6 – Bisphosphate and PFK-2

Aldolase and Triose Isomerase

Glyceraldehyde3-

Phosphate Dehydrogen

ase

2

2

2

2

2

2

Phospho-glycerate

Kinase

2

2

2

2

2

2

Substrate-Level

Phosphorylation

Pyruvate Kinase

2

2

2

2

2

2

Substrate-Level

Phosphorylation

Pyruvate KinaseCovalent

Modification

Pyruvate Kinase Deficiency

Hemolytic Anemia

Summary: Regulation of Glycolysis

Regulatory Enzymes (Irreversible reactions):Glucokinase/hexokinasePFK-1Pyruvate kinase

Regulatory Mechanisms:Rapid, short-term:

AllostericCovalent modifications

Slow, long-term:Induction/repression

Apply the above mechanisms for each enzyme where applicable

Aerobic Glycolysis: ATP Production

ATP Consumed:2 ATP

ATP Produced:Substrate-level 2 X 2 = 4 ATPOxidative-level 2 X 3 = 6 ATPTotal 10 ATP

Net: 10 – 2 = 8 ATP

Shuttle Mechanism

s

A. Glycerol-phosphateShuttle

B. Malate - AspartateShuttle

2 ATP

3 ATP

Anaerobic

Glycolysis

Lactate Dehydrogenase

Lactate Consumption

Lactate PyruvateLD

NAD+ NADH

Liver: Pyruvate to glucose (Gluconeogenesis) Pyruvate to active acetate (CO2 + H2O, Krebs)

Heart: Pyruvate to active acetate (CO2 + H2O, Krebs)

Lactate

Lactate in muscle: Muscle cramps

Lactate in blood: Lactic acidosis

Blood lactate level: Monitors patient’s recovery

Oxygen debt

Anaerobic Glycolysis: ATP Production

ATP Consumed:2 ATP

ATP Produced:Substrate-level 2 X 2 = 4 ATPOxidative-level 2 X 3 = 6 ATPTotal 4 ATP

Net: 4 – 2 = 2 ATP

Anaerobic Glycolysis:

RBCs2,3-BPG Shunt

2

2

2

2

2

2

Glycolysis in RBCs: ATP Production

ATP Consumed:2 ATP

ATP Produced:Substrate-level 2 X 2 = 4 ATP

1 X 2 = 2 ATPOxidative-level 2 X 3 = 6 ATPTotal 4 ATP

Net: 4 – 2 = 2 ATP2 – 2 = 0 ATP

OR

OR

Glycolysis in RBCs: Summary

End product: LactateNo net production or consumption of NADH

Energy yield:No 2,3-BPG 2 ATP2,3-BPG shunt 0 ATP

PKD hemolytic anemia depends on:Degree of PKDCompensation by 2,3-BPG