Question

For Glycolysis, TCA cycle and Electron Transport, be able to name all the intermediates and what each step consumes or releases (NADH, ATP etc) Know the differences between GLUT 1, 2, 3 and 4

Answer 1

Glycolysis is an oxidative process in which one mole of g;ucose is partially oxidised into the two moles of pyruvate, in the cytosol of all cells. It involves 10 steps - First five steps are the preparatory phase, in which 2 molecules of ATPs are utilized and the other five are the pay-off phase, in which 4molecules of ATPs and 2 molecules of NADH is released. The reactions that consumes ATP -

• Phosphorylation - Glucose is phosphorylated by ATP to form Glucose-6-phosphate, catalyzed by hexokinases that requires divalent metal ion Mg²⁺or Mn²⁺ for activity.
• Phosphorylation - In the second of two priming reactions of glycolysis, phosphofructokinase 1, PFK-1 catalyzes the transfer of a phosphoryl group from ATP to fructose-6-phosphate to yield fructose-1,6-bisphosphate.

The glycolytic reaction releases NADH, ATP -

• Oxidation of glyceraldehyde-3-phospahte to 1,3-bisphosphoglycerate - This is the first step in the pay-off phase, catalyzed by glyceraldehyde-3-phosphate dehydrogenase. The enzymatic transfer of glyceraldehyde-3-phospahte to the NAD+yields reduced coenzyme NADH.
• Substrate level phosphorylation - Phosphoglycerate kinase transfers the phosphoryl group from the 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate
• Substrate level phosphorylation - The last step is the transfer of the phosphoryl group from phosphoenolpyruvate to ADP, catlyzed by pyruvate kinase, that requires K⁺ and either Mg²⁺ or Mn²⁺

Net reaction - Glucose + 2ATP + 2NAD⁺ + 4ADP -----> 2pyruvate + 2ADP + 2NADH + 2H⁺ + 4ATP +2H₂O

TCA cycle -

• Pyruvate oxidation occurs in the mitochondrial matrix into acetyl-CoA in an oxidative decarboxylation process catalyzed by pyruvate dehydrogenase, with the release of 2NADH
• TCA cycle occurs in the mitochondrial matrix.
• Isocitrate dehydrogenase catalyzes oxidative decarboxylation of isocitrate to form alpha-ketoglutarate, the enzyme reuires NAD+ as an electron acceptor which gets reduced to NADH.
• Alpha-ketoglutarate dehydroenase catalyzes second oxidative decarboxylation reaction that results in the formation of succinyl CoA from alpha-ketoglutarate, produces NADH, CO2
• Substrate level phosphorylation - The reaction is catalyzed by Succinyl coA synthetase or succinate thiokinase. In this step, the phosphoryl group replaces CoA bound to the enzyme, forming a high energy acyl phosphate, releases GTP.
• Oxidation of succinate to fumarate - The succinate formed from succinyl CoA is oxidised to fumarate by the flavoprotein succinate dehydrogenase and produces FADH₂.

Acetyl CoA + 2NAD⁺ + GAD +GDP + 3H₂O -------> 2CO₂ + 3NADH + FADH₂ + GTP + H₂O

Transport of electrons through the electron transport chain is accompanied by pumping of protons across the inner mitochondrial membrane, from the mitochondrial matrix to inter mitochondrial space. A total of 10 protons are translocated fromt he matrix across the inner mitochondrial membrane per electron flowing from NADH to O₂. This generates proton motive force and the energy is used to drive the synthesis of ATP.

• GLUT1 - The glucose transporter GLUT1 catalyses facilitative diffusion of glucose into erythrocytes and is responsible for the transport of glucose from the blood into the brain.
• GLUT2 - This transporter is always present in the plasma membrane, catalyzes facilitated diffusion of glucose in both directions.
• GLUT4 - The transporter catalyzes the passive uptake of glucose by muscle and adipose tissue