Introduction à la Glycolyse

Glycolyse Introduction

• Metabolism of carbohydrates involves biochemical processes for formation, degradation, and interconversion in living organisms.
• Carbohydrates provide 50-60% of daily energy needs.
• Glucose is the primary fuel source for most organisms, central to cellular metabolism.
• Glycolysis, Krebs Cycle, and Pentose Phosphate Pathway are major metabolic pathways involving glucose.

Glycolysis Definition

• Glycolysis (Embden-Meyerhof-Parnas pathway) is the anaerobic oxidative breakdown of glucose to pyruvate.
• It doesn't require oxygen, producing ATP and other metabolites.
• Occurs in all cells, though to varying degrees.
• Composed of 10 enzymatic reactions in the cytoplasm.
• All intermediates in glycolysis are phosphorylated.
• Phosphorylation groups have three key functions.

Glycolysis Stages

• Glycolysis has two major phases:
  • Preparatory (Investment) Phase: Consumes energy (ATP) for activation.
  • Payoff Phase: Generates energy (ATP and NADH) through oxidation reactions.

Additional Reactions

• Reaction 1: Glucose phosphorylation by ATP, catalyzed by hexokinase. (Irreversible)
• Reaction 2: Glucose-6-phosphate isomerization to fructose-6-phosphate (reversible)
• Reaction 3: Fructose-6-phosphate phosphorylation to fructose-1,6-bisphosphate (irreversible). Catalyzed by phosphofructokinase (PFK-1). Key regulatory step.
• Reaction 4: Fructose-1,6-bisphosphate cleavage to dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P) (reversible). Catalyzed by aldolase.
• Reaction 5: Interconversion of triose phosphates; DHAP is isomerized to G3P (reversible). Catalyzed by triose phosphate isomerase.
• Reaction 6: G3P oxidation and phosphorylation to 1,3-bisphosphoglycerate (reversible). Catalyzed by glyceraldehyde-3-phosphate dehydrogenase. Coenzyme NAD+ reduced to NADH.
• Reaction 7: 1,3-bisphosphoglycerate to 3-phosphoglycerate (reversible). Catalyzed by phosphoglycerate kinase. First ATP generated.
• Reaction 8: 3-phosphoglycerate to 2-phosphoglycerate (reversible). Catalyzed by phosphoglycerate mutase.
• Reaction 9: 2-phosphoglycerate to phosphoenolpyruvate (PEP) (reversible). Catalyzed by enolase.
• Reaction 10: PEP to pyruvate (irreversible). Catalyzed by pyruvate kinase. Second ATP generated.

Glycolysis Regulation

• Allosteric control: Enzyme activity is modulated by the binding of products or other metabolites at sites other than the active site. (e.g., ATP, AMP, citrate influence phosphofructokinase).
• Covalent modification: Enzyme activity changes through covalent modifications, like phosphorylation (e.g., pyruvate kinase regulation by hormones).
• Transcriptional control: Gene expression levels for enzymes influence the metabolic pathway's flux. (e.g., hexokinase and phosphofructokinase 1.)

Anaerobic Metabolism

• In the absence of oxygen, pyruvate can be converted to lactate (lactic acid fermentation) or ethanol (alcoholic fermentation) to regenerate NAD+ for glycolysis to continue, depending on the organism.
• Pyruvate's fate depends on the condition (aerobic or anaerobic).

Aerobic Metabolism

• In the presence of oxygen, pyruvate enters the mitochondrion, where it is further oxidized, yielding acetyl-CoA, which enters the Krebs cycle for more ATP production.

Summary

• Glycolysis generates a net of 2 ATP, 2 NADH, and 2 pyruvate per glucose molecule.
• The process is regulated to maintain cellular energy balance.

Ce quiz explore la glycolyse, un processus métabolique crucial pour la dégradation du glucose. Vous apprendrez les phases de la glycolyse, son importance dans le métabolisme des glucides, ainsi que son rôle en tant que source d'énergie pour les organismes vivants. Testez vos connaissances sur ces concepts fondamentaux !