Krebs/TCA Cycle

• The Krebs Cycle is also known as the TCA cycle or Citric Acid Cycle.

Aerobic Metabolism of Glucose

• Aerobic metabolism involves the slow form of glycolysis.
• Stage 1 of aerobic metabolism occurs in the sarcoplasm.
• Stages 2, 3, and 4 occur in the mitochondria.
• Glycolysis yields 2 pyruvate molecules in the sarcoplasm.
• Stage 3, the Kreb's Cycle, occurs in the mitochondria.
• The primary purpose of the Kreb's Cycle is to generate electron carriers (NADH & FADH2).
• Stage 4 is the Electron Transport Chain (ETC).
• Over 90% of ATP synthesis takes place in the respiratory chain of the ETC.

Electron Carrier Molecules

• Electron carrier molecules transport hydrogens and associated electrons.
• They convert pyruvate to lactate in anaerobic conditions.
• They transport to the ETC in the mitochondria for ATP generation under aerobic conditions.
• Nicotinamide adenine dinucleotide (NAD) is involved in the reaction NAD+ + 2H -> NADH + H+.
• Flavin adenine dinucleotide (FAD) is involved in the reaction FAD + 2H -> FADH2.
• Oxidation of the electron carrier molecules back to NAD+ & FAD drives ATP synthesis in the ETC.

Role of Micronutrients

• Many vitamins act as coenzymes essential to macronutrient metabolism.
• Niacin (B3) is a micronutrient, and nicotinamide adenine dinucleotide (NAD) requires niacin.
• Riboflavin (B2) is a micronutrient, and Flavin adenine dinucleotide (FAD) requires riboflavin.
• Thiamine (B1) is a micronutrient and thiamine Pyrophosphate (TPP) requires thiamine.
• Pantothenic Acid (B5) acts as a precursor for coenzyme A.

Pyruvate Dehydrogenase Reaction (PDH)

• Stage 2 of the energy cycle is the Pyruvate Dehydrogenase reaction (PDH).

• During the reaction Pyruvate crosses the inner mitochondrial membrane

• B vitamins, specifically B1, B3, and B5, are used for coenzymes.

• This is an irreversible reaction.

• One carbon is lost as CO2 in the reaction

• Acety has carbons.

TCA Cycle

• Stage 3 is the Citric Acid Cycle, also known as the TCA or Kreb's Cycle.

• Citrate synthase joins Acetyl-CoA (AcCoA) with oxaloacetate (OXA) which has 4 carbons, to form citrate, which has 6 carbons.

• Isocitrate dehydrogenase in the cycle is the rate-limiting enzyme for the TCA cycle.

• NADH is generated and CO₂ is lost during this process.

• TPP is a cofactor for both the PDH and α-kg dehydrogenase reactions.

• Overall, one GTP, three NADH, and one FADH2 are generated from each turn of the TCA cycle.

• The main purpose to produce electron carriers.

Mitochondrial Enzymes

• Pyruvate dehydrogenase (PDH) oxidizes pyruvate and allows it to enter the mitochondria for further oxidation.
• Citrate synthase combines Acetyl CoA with oxaloacetate (OXA).
• This is the first step of the TCA Cycle and is where AcCoA enters the TCA.
• Isocitrate dehydrogenase (IDH) is the rate-limiting enzyme of the TCA Cycle.
• IDH is also the first of 4 redox reactions to generate electron carriers in the cycle.
• Carbon dioxide (CO2) is also lost in this reaction as it was in the PDH reaction.

Exploration of aerobic metabolism. Focus on cellular locations of each stage. Examination of ATP synthesis for metabolic process.