Cellular Respiration Chapter 9: The Process

Objectives Understand that cellular respiration is a series of coupled metabolic processes Describe the role of ATP, NAD + and FAD in coupled reactions Know the start and end products of each reaction Know the kind and quantity of energy produced by each reaction Understand the role of dehydrogenase, kinase and isomerase enzymes Explain how ATP production is controlled during glycolysis, transition reaction, and Krebs cycle

Objectives continued Understand the process of chemiosmosis Explain how the “slide” of electrons down the electron transport chain is coupled to the production of ATP by chemiosmosis Understand the difference between substrate-level phosphorylation and oxidative phosphorylation Describe the fate of pyruvate during fermentation Understand how food molecules other than glucose can be oxidized to make ATP Understand how anabolic synthesis of carbohydrate metabolism can generate the building blocks of other macromolecules

Overview Aerobic cellular respiration has 4 steps –Glycolysis in cytosol –Transition reaction (Pyruvate Processing) at mitochondrial membrane –Kreb’s cycle in mitochondrial matrix –Electron Transport Chain at inner membrane of mitochondria C 6 H O 2  6CO 2 +6H ENERGY

Glycolysis Start with glucose (C 6 H ) End product is 2x Pyruvate 2(C 3 H 4 O 3 ) Some steps are endergonic while others are exergonic

1First obstacle –make glucose reactive Increase free energy of glucose by phosphorylation by ATP 2Rearrange molecule 3Increase free energy of glucose by phosphorylation with ATP  phosphofructokinase is an allosteric enzyme that controls the rate of glycolysis Result in reactive molecule

4Now that our 6C sugar (fructose 1,6-biphosphate) is ready to react, aldolase cleaves it into 2(3C) molecules that are isomers (what kind) of each other.  5 Isomerase converts the unusable Dihydroxyacetone phosphate into Glyceraldehyde phosphate

6Each glyceraldehyde phosphate is acted on by the enzyme Triose phosphate dehydrogenase that oxidized the sugar by reducing NAD + and sequentially adding inorganic phosphate to the sugar 7A molecule of ATP is made from each 1,3-biphosphoglycerate as the phosphate added in step 6 is transferred to ADP 8The molecule is reorganized through the relocation of the phosphate group

9Enolase makes an enol (C=C-O) through the removal of water resulting in an unstable molecule (prone to change) 10 Terminal step in glycolysis results in the formation of Pyruvate as the enzyme Pyruvate kinase transfers the phosphate group of phosphoenolpyruvate to ADP forming ATP

Glycolysis Summarized Glucose converted into 2 pyruvate Energy requiring and energy releasing steps Energy net yield is 2ATP and 2NADH Enzymes involved at each step –Kinase: conversion of ATP to ADP or ADP to ATP –Dehydrogenase: reduces NAD + to NADH while oxidizing sugar –Isomerase/mutase: keep reactions moving forward through isomer formation Regulated by phosphofructokinase activity (step 3) and the rate of isomer formation of glyceraldehyde phosphate (step 5)

Regulation of Glycolysis Phosphofructokinase is regulated by allosteric means where: –AMP activate enzyme –Citrate (high energy molecule of the Krebs cycle) inhibits enzyme Also regulated by feedback inhibition Isomerase is regulated by enzyme concentration and substrate availability

Another View of the Feedback Mechanisms for Glycolysis Feedback inhibition –ample product switches off product Regulation of enzyme activity Allosteric regulation of phosphofructokinase –AMP activate enzyme –Citrate inhibits enzyme

Mitochondria Transition reaction at mitochondrial membrane Kreb’s cycle in mitochondrial matrix Electron Transport Chain Across the inner membrane of mitochondria

Transition Reaction Pyruvate is converted into Acetyl-CoA as it is transported into the mitochondrion from the cytosol NAD + converted into NADH CO 2 produced during reaction

Krebs Cycle Each molecule of Acetyl- CoA enters the Krebs cycle the 2C of acetyl-CoA are exchanged for 2C in oxaloacetate Each turn of the cycle produces –3 NADH –1FADH 2 –1ATP –2CO 2

1Acetyl CoA enters the kreb’s cycle by attaching to a 4 carbon sugar (Oxaloacetate) forming a six carbon sugar (Citrate) 2An isomer of citrate is created through condensation/hydrolysis reactions resulting in isocitrate 3Isocitrate loses CO2 forming  -ketoglutarate as oxidation of the compound occurs NADH is formed 4  -ketoglutarate (C 5 ) is converted to Succinyl CoA (C 4 ). Along the way, CO 2 is released, and NAD + is reduced

5Succinyl CoA is converted into Succinate. Reaction starts as inorganic phosphate attaches to Succinyl CoA displacing CoA. The phosphate is picked up by GDP forming GTP. The terminal phosphate of GTP is transferred to ADP forming ATP 6Succinate is oxidized to Fumarate Reducing FAD to FADH 2 7The addition of H 2 0 to Fumarate produces Malate 8Oxaloacetate is reformed through the oxidation of Malate. NADH is formed during the process

Summary of Transition Reaction and Kreb’s cycle Each glucose that enters Glycolysis is converted into 2 Pyruvate molecules Transition Reaction 1 NADH/pyruvate 1 CO 2 /pyruvate Kreb’s Cycle 3 NADH/ Acetyl CoA 1FADH 2 / Acetyl CoA 1ATP / Acetyl CoA 2CO 2 / Acetyl CoA

Regulation of Transition Reaction and Krebs Cycle The enzyme pyruvate dehydrogenase is regulated by several molecules which either inhibit or activate its activity. Ultimately, pyruvate dehydrogenase activity influences the activity of the Krebs cycle Feedback inhibition also is used to regulate the Krebs cycle

Electron Transport Chain Each NADH enters the electron transport chain with enough free energy to fuel formation of 3 ATP Each FADH 2 will yield 2 ATP Electron shuttling proteins are called flavoproteins, iron-sulphur proteins, and cytochromes  Each protein in the series is more electronegative than its predecessor

ETC Some electron carriers of the transport chain carry only electrons –Ubiquinone –Cytochrome c Some electron carriers accept and release protons along with electrons

Chemiosmosis Chemiosmosis: coupling of exergonic electron flow down an ETC to endergonic ATP production by the creation of a proton gradient across a membrane (proton-motive force)

ATP Formation ATP synthase couples inorganic phosphate to ADP as H + return to the matrix (utilizes potential energy of proton gradient) 1ATP is formed for each H + diffusing across the membrane

Substrate-level Phosphorylation: ATP production coupled by direct enzymatic transfer of phosphate from an intermediate in catabolism to ADP Oxidative Phosphorylation: ATP production that is coupled to the exergonic transfer of electrons from food to oxygen

Variations of Glycolysis In the absence of oxygen, liberation of energy can occur through fermentation pathways yielding a max of 2 ATP/glucose Fermentation is similar to glycolysis except that the end product is not pyruvate because of the addition of a few steps necessary to regenerate NAD +

What about the other foods? Proteins, Carbo’s and Fats can all be utilized for energy following hydrolysis Amino Acids are converted to intermediates including pyruvate, acetyl CoA, and  - ketoglutarate Carbo’s enter glycolysis at the beginning or as Fructose 6 phosphate Fats components –glycerol enters as glyceraldehyde phosphate –Fatty acids enter as Acetyl CoA

Anabolic Synthesis of Key Molecules Intermediates of Carbohydrate metabolism generate the building blocks to manufacture: –Amino acids –RNA –DNA –Phospholipids –Carbohydrate storage products