Glycolysis 1. From glucose to pyruvate; 2. 10-step reactions; 3 Glycolysis 1. From glucose to pyruvate; 2. 10-step reactions; 3. Three inreverseable reactions hexokinase phosphofructokinase-1 pyruvate kinase 4. Rate limiting enzyme: phosphofructokinase-1 5. Production: 2 ATP (net) 2 NADH + H 6. Function: Supply energy in anaerobic condition.

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.

The Citric Acid Cycle Kreb’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)

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

From pyruvate to acetyl-CoA Pyruvate + CoA + NAD+ Acetyl-CoA + CO2 + NADH + H+ Pyruvate produced from glycolysis must be decarboxylated to acetyl CoA before it enters TCA cycle. Key irreversible step in the metabolism of glucose.

Reaction irreversible Catalytic cofactors 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.

Pyurvate dehydrogenase complex

Acetyl-CoA: fuel for the Citric Acid Cycle Coenzyme A was first discovered by Lipmann in 1945.

Lipoate

FAD

Nicotinamide Adenine Dinucleotide (NAD) Reduction Oxidation Used primarily in the cell as an electron carrier to mediate numerous reactions

Oxidization of acetyl-CoA

Control of the Pyruvate Dehydrogenase Complex Regulation by its products NADH & Acetyl-CoA: inhibit NAD+ & CoA: stimulate Regulation by energy charge ATP : inhibit AMP: stimulate

The citric acid cycle consists of eight successive reactions

Step 1: citrate formation Enzyme: Citrate synthase

Step 2: Citrate isomerized to isocitrate Dehydration Hydration Enzyme: aconitase

Step 3: Isocitrate to -ketoglutarate 1st CO2 removed 1st NADH produced Enzyme: isocitrate dehydrogenase

Step 4: Succinyl-CoA formation 2nd NADH produced, 2nd CO2 removed Enzyme: -ketoglutarate dehydrogenase

Step 5: Succinate formation Enzyme: succinyl-CoA synthetase A GTP (ATP) produced

Steps 6: Fumarate formation Enzyme: Succinate dehydrogenase A FADH2 produced

Steps 7: Malate formation Enzyme: fumarase

Step 8: Malate to Oxaloacetate 3rd NADH produced Enzyme: malate dehydrogenase

Citric Acid Cycle: Overview Input: 2-carbon units Output: 2 CO2 1 GTP 3 NADH: 2.5X3=7.5 ATP 1 FADH2: 1.5X1=1.5 ATP Total: 10 ATP

Biosynthetic roles of the citric acid cycle

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.

In carbohydrate metabolism: 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.

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.

Regulation of Citric Acid Cycle 3 control sites

Control of citric acid cycle Control points: 1. Citrate synthase 2. Isocitrate dehydrogenase 3. - ketoglutarate dehydrogenase

Regulation of Citric Acid Cycle Site 1 Acetyl CoA + Oxaloacetate Citrate Enzyme: citrate synthase Inhibited by ATP Stimulated by ADP

Regulation of Citric Acid Cycle Site 2 Isocitrate -Ketoglutarate Enzyme: isocitrate dehydrogenase Inhibited by ATP, NADH, succinyl-CoA Stimulated by ADP & NAD+

Regulation of Citric Acid Cycle Site 3 - ketoglutarate succinyl-CoA Enzyme: -ketoglutarate dehydrogenase Inhibited by ATP, NADH, succinyl-CoA Stimulated by ADP & NAD+

Aerobic oxidation of glucose

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

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

In mitochondrion: So… 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