Citric Acid Cycle

Aerobic organisms use an oxygen-requiring process to produce ATP

The citric acid cycle, the ETC, and oxidative phosphorylation all take place in the mitochondria

Mitochondria has an outer membrane, inner membrane, and matrix within the inner membrane

• Oxidation and Reduction
• Redox

• Acetyl-CoA (after formed from pyruvate)
• The acetyl group enters the cycle by reacting with oxaloacetate

• Step 3: Isocitrate dehydrogenase converts Isocitrate to alpha-ketoglutarate which reduces NAD⁺ to NADH
• Step 4: alpha-Ketoglutarate dehydrogenase complex converts alpha-ketoglutarate to succinyl-CoA which reduces NAD⁺ to NADH and makes co2
• Step 6: Succinate dehydrogenase converts succinate to fumarate which reduces FAD to FADH₂
• Step 8: Malate dehydrogenase converts malate to oxaloacetate (OAA) which reduces NAD⁺ to NADH

• Step 3: Isocitrate dehydrogenase converts Isocitrate to alpha-ketoglutarate which reduces NAD+ to NADH and makes CO2
• Step 4: alpha-Ketoglutarate dehydrogenase complex converts alpha-ketoglutarate to succinyl-CoA which reduces NAD+ to NADH and makes co2

Step 5: Succinate thiokinase converts succinyl-CoA to succinate and mainly makes one molecule of GTP which can be used to make ATP.

• Electrons are captured during a number of metabolic reactions and shuttled via coenzymes to the electron transport chain (ETC)
• The CAC produces lrg # of electrons for the ETC.

O2 as an electron acceptor

Electrons go from a donor (reducing agent) to an acceptor (oxidizing agent)

Oxidized

Reduced

• Nicotinamide adenine dinucleotide (Aerobic respiration)
• Can donate or accept 1 Hydride ion

• Nicotinamide adenine dinucleotide phosphate (Photosynthesis)
• Can donate or accept 1 Hydride ion

• Vitamin B2 is part of both:
• Flavin mononucleotide (FMN)
• Flavin adenine dinucleotide (FAD)

FADH2 and FMNH2

Reduced molecules (like NADH) can be used to drive the production of ATP in aerobic respiration

It is a cycle because the starting material (oxaloacetate) is regenerated each round

• Series of reactions in mitochondria used to release energy in acetyl-CoA
• The overall process involves the reduction of several coenzymes and the release of CO2

• Acetyl-CoA + 3 NAD⁺ + FAD + GDP + Pi + 2H2O -->
• 2CO₂ + 3 NADH + FADH₂ + CoASH + GTP + 2H

• Thiamine pyrophosphate (TPP) – part of pyruvate decarboxylase
• Lipoic acid – part of dihydrolipoyl transacetylase
• NADH– electron carrier
• FADH2 –electron carrier
• Coenzyme A (CoASH) – acetyl group carrier

• Acetyl group carrier by means of attachment to its sulfur
• Abbreviated CoASH when not attached to anything
• The acetyl group transferred to CoASH by means of the pyruvate dehydrogenase complex

Using the Pyruvate dehydrogenase complex

A large multi-enzyme complex that catalyzes the process of converting pyruvate into an acetyl group on CoA; functions inside mitochondrial matrix

• Made of three separate enzymes:
• Pyruvate dehydrogenase (E1) – decarboxylates pyruvate (which is bound to TPP)
• Dihydrolipoyl transacetylase (E2) – transfer of acetyl group to CoASH
• Dihydrolipoyl dehydrogenase (E3) – reoxidizes dihydrolipoamide (lipoic acid bound to lysine; involves reduction of NAD+)

• First, pyruvate becomes bonded to TPP
• Second, pyruvate is decarboxylated
• Third, the acetyl group is transferred to lipoic acid
• Fourth, the acetyl group is transferred to CoASH
• Fifth, lipoic acid is reoxidized

• The acetyl group enters the cycle by reacting with oxaloacetate, and two molecules of CO2 are released (reactions 1-4)
• Oxaloacetate is regenerated in order to react again with acetyl-CoA (reactions 5-8)