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Glycolysis1. From glucose to pyruvate;2. 10-step reactions;3. Three inreverseable reactions hexokinase phosphofructokinase-1 pyruvate kinase4. Rate limiting enzyme: phosphofructokinase-15. Production: 2 ATP (net) 2 NADH + H6. Function: Supply energy in anaerobic condition.

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Glycolysis in red blood cells

1. Relies exclusively on glycolysis as fuel to produce ATP;

2. End product is lactate;

3. Produce 2,3-BPG enhancing the ability of RBCs to release oxygen.

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The Citric Acid CycleThe Citric Acid CycleKreb’s Cycle Tricarboxylic acid cycle (TAC)

“ The wheel is turnin’ and the sugar’s a burnin’”

More than 95% of the energy for the human being is generated through this pathway (in conjunction with the oxidative phosphorylation process)

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What happened for 2 pyruvates? Basically three options depending on the environmental conditions

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From pyruvate to

acetyl-CoA

Pyruvate + CoA + NADPyruvate + CoA + NAD++ Acetyl-CoA + CO Acetyl-CoA + CO2 2 + NADH + H+ NADH + H++

Pyruvate produced from glycolysis must be decarboxylated to acetyl Pyruvate produced from glycolysis must be decarboxylated to acetyl CoA before it enters TCA cycle.CoA before it enters TCA cycle.

Key irreversible step in the metabolism of glucose.

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Reaction irreversible

Pyruvate is first transported into mitochondria via a specific transporter on the inner membrane and then oxidized to acetyl-CoA by the catalysis of pyruvate dehydrogenase complex.

Catalytic cofactors

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Pyurvate dehydrogenase complex

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Coenzyme A was first discovered by Lipmann in 1945.

Acetyl-CoA: fuel for the Citric Acid Cycle

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Lipoate

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FAD

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Nicotinamide Adenine Dinucleotide (NAD)

Used primarily in the cell as an electron carrier to mediate numerous reactions

Reduction

Oxidation

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Oxidization of acetyl-CoA

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• Regulation by its products NADH & Acetyl-CoA: inhibit NAD+ & CoA: stimulate

• Regulation by energy charge ATP : inhibit AMP: stimulate

Control of the Pyruvate Dehydrogenase Complex

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The citric acid cycleconsists of eight successive

reactions

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Step 1: citrate formation

Enzyme: Citrate synthase

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Step 2: Citrate isomerized to isocitrate

Enzyme: aconitase

Dehydration Hydration

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Step 3: Isocitrate to -ketoglutarate

Enzyme: isocitrate dehydrogenase

1st NADH produced 1st CO2 removed

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Step 4: Succinyl-CoA formation

2nd NADH produced, 2nd CO2 removed

Enzyme: -ketoglutarate dehydrogenase

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Step 5: Succinate formation

Enzyme: succinyl-CoA synthetase

A GTP (ATP) produced

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Steps 6: Fumarate formation

Enzyme: Succinate dehydrogenase

A FADH2 produced

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Steps 7: Malate formation

Enzyme: fumarase

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Step 8: Malate to Oxaloacetate

Enzyme: malate dehydrogenase

3rd NADH produced

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Citric Acid Cycle: Overview

Input: 2-carbon unitsOutput: 2 CO2

1 GTP 3 NADH: 2.5X3=7.5 ATP 1 FADH2: 1.5X1=1.5 ATP Total: 10 ATP

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Biosynthetic roles of the citric acid cycle

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Summary

• Pyruvate is converted to acetyl-CoA by the action of pyruvate dehydrogenase complex, a huge enzyme complex.

• Acetyl-CoA is converted to 2 CO2 via the eight-step citric acid cycle, generating three NADH, one FADH2, and one ATP (by substrate-level phophorylation).

• Intermediates of citric acid cycle are also used as biosynthetic precursors for many other biomolecules, including fatty acids, steroids, amino acids, heme, pyrimidines, and glucose.

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Why is citric acid cycle so important?Citric acid cycle is of central importance in all living cells that use oxygen as part of cellular respiration.

In aerobic organisms, the citric acid cycle is part of a metabolic pathway involved in the chemical conversion of carbohydrates, fats and proteins into carbon dioxide and water to generate energy.

In addition, it provides precursors for synthesis of many compounds including some amino acids.

In carbohydrate metabolism:

1. Glycolysis to produce pyruvate;

2. Pyruvate is oxidized to acetyl-CoA;

3. Acetyl-CoA enters the citric acid cycle.

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In protein catabolism: 1. Proteins are broken down by proteases into their constituent amino acids. 2. The carbon backbone of these amino acids are converted to acetyl-CoA and entering into the citric acid cycle.

In fat catabolism: 1. Triglycerides are hydrolyzed to into fatty acids and glycerol. 2. In the liver the glycerol can be converted into pyruvate. 3. Fatty acids are broken down through a process known as beta oxidation which results in acetyl-coA which can be used in the citric acid cycle.

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Regulation of Citric Acid Cycle

• 3 control sites

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Control of citric acid cycle

Control points:

1. Citrate synthase

2. Isocitrate dehydrogenase

3. - ketoglutarate dehydrogenase

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Regulation of Citric Acid Cycle Site 1

Acetyl CoA + Oxaloacetate

Citrate

Enzyme: citrate synthase

Inhibited by ATP

Stimulated by ADP

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Regulation of Citric Acid Cycle

Site 2

Isocitrate -Ketoglutarate

• Enzyme: isocitrate dehydrogenase

• Inhibited by ATP, NADH,

succinyl-CoA• Stimulated by ADP & NAD+

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Regulation of Citric Acid Cycle Site 3

- ketoglutarate succinyl-CoA

Enzyme: -ketoglutarate dehydrogenase

Inhibited by ATP, NADH, succinyl-CoA

Stimulated by ADP & NAD+

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Aerobic oxidation of glucose

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Aerobic oxidation of glucose – How many ATP we can get?

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Total Energy per glucose through aerobic

oxidation • Cytosol

– 2 ATP– 2 NADH

• NADH in cytosol can’t get into mitochondrion• In eukaryotes two pathways to transfer NADH into MC

– transferred to FADH2

» get 1.5 ATP/ FADH2

» 2 X 1.5 ATP = 3 ATP– Or transferred to NADH

» Get 2.5 ATP/ NADH» 2 NADH X 2.5 ATP= 5 ATP

Total 3+ 2 or 5 + 2 so either 5 or 7 ATP

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• In mitochondrion:– Each NADH makes 2.5 ATP

– Each FADH2 makes 1.5 ATP

– GTP = ATP

• So…– From pyruvate in mitochondrion

• 8 NADH X 2.5 ATP = 20 ATP

• 2 FADH2 X 1.5 ATP= 3 ATP

• 2 GTP = 2 ATP

• TOTAL in mitochondrion 25 ATP