Mitochondria and ATP Synthesis

Mitochondria and ATP Synthesis

Mitochondria and ATP Synthesis 1. Mitochondria are sites of ATP synthesis in cells. 2. ATP is used to do work; i.e. ATP is an energy source. 3. ATP hydrolysis releases energy that is harnessed by the cell to do work. 4. Proteins convert the chemical energy of ATP to different forms of cellular work. a. Na+/K+ pump does osmotic/concentration work b. Myosin/actin does mechanical work c. Creatine kinase does synthetic or chemical work

What is meant by ATP hydrolysis? H 2 O E + ATP E.ATP E + ADP + Pi Hydrolysis step Myosin + ATP Myosin.ATP Myosin + ADP + Pi

Chemical structure terminal phosphate group has high energy bond Base = Adenine Sugar = Ribose Three PO 4 groups

From Carbohydrates to ATP How do mitochondria produce ATP? By a process called oxidative phosphorylation Energy for ATP synthesis is derived from carbohydrates in the diet

Structure of mitoconodria Cellular organelle 1. Porous outer membrane 2. Selectively permeable inner membrane 3. Matrix space 4. Cristae

Mitochondria: 1. Dynamic organelles have the ability to change shape, divide and fuse 2. Contain DNA mitochondrial inheritance 3. Ribosomes for protein synthesis (only a small percent of mitochondrial proteins synthesized in the mitochondria) 4. Location in the cell changes transported along microtubules and actin filaments by molecular motors 5. Located near sites of ATP utilization, e.g. myofibrils in cardiac muscle cells.

Ox Phos = Oxidative Phosphorylation Glucose metabolism 1. Glucose stored in muscle cells as glycogen 2. Glycogen is broken down to glucose Glycogen Multiple enzymes Glucose 3. Glucose is broken down to Pyruvate via the Glycolytic pathway 4. Glucose is a 6-carbon sugar 5. Broken down to two 3-carbon pyruvate molecules

Glycolysis (anaerobic metabolism) Glycolytic enzymes; glycolysis occurs in cytoplasm Glycolytic pathway ATP ADP ATP ADP Glucose Pyruvate Anaerobic Lactic acid ATP* ATP* a. ATP* synthesis is referred to as substrate level phosphorylation b. 2 moles of ATP synthesized per mole of glucose

Aerobic conditions i. Pyruvate converted into H 2 O and CO 2 in mitochondria ii. Oxidative phosphorylation iii. ~ 30 moles ATP/mole of glucose Pyruvate is transported into mitochondria 1. Carrier-mediated transport process 1. Secondary active transport 2. H+ gradient (ph or proton gradient) powers the movement of Pyruvate across the mitochondrial membrane

Mitochondrial matrix NADH production 1. Pyruvate is first converted to Acetyl CoA (coenzyme A) 2. Acetyl CoA enters the citric acid cycle and is converted to CO 2 Acetyl CoA NADH Citric Acid Cycle NAD CO 2 3. NADH or reduced NAD is a major collector of high energy, reactive electrons (e-) NAD (Nicotinamide Adenine Dinucleotide) is a coenzyme. Coenzymes are small organic molecules that function with an enzyme (e.g. respiratory enzymes)

NADH Collector of high-energy electrons Pathway from NADH to ATP synthesis Sequence of events leading to ATP synthesis 1. e - transferred to enzyme of the respiratory complexes in the electron transport chain. 2. Proton gradient is generated (protons pumped out of mitochondria using energy released as electrons move along the electron transport chain). 3. ATP synthase an enzyme driven by the flux of proton into mitochondria to synthesize ATP. The process is called the chemiosmotic process (conversion of concentration energy into chemical energy, i.e. ATP)

Electron transport process and the respiratory enzyme complexes Characteristics of respiratory enzymes complexes 1. Electron carriers transport electrons from one complex to its neighbor without short circuit. 2. Proton pumps pump protons across membrane from in to out. High proton concentration out and high OH - concentration in. The three respiratory enzyme complexes each made of several enzymes (subunits) 1. NADH dehydrogenase 2. Cytochrome b-c 1 complex 3. Cytochrome oxidase complex Complexes are arranged in this specific order in the membrane

Electron Carriers What drives the e- along the chain of enzymes? Affinity of the respiratory enzymes for electrons NADH Electron Carriers O 2 Strong donor acceptor of electrons Strongest Of electrons acceptor of e - 1. Electron Carriers are arranged in order of increasing affinity for electrons. Increasing affinity accounts for the tendency to move from one electron carrier to the next. 2. O 2 has highest affinity 3. The carrier affinity for electrons can be measured as a redox potential (oxidation-reduction)

How one measures redox potential Basic rules Compounds with most negative redox potentials 1. Weakest affinity for electrons 2. Strong donor of electrons 3. Least tendency to accept electrons Redox potential correlates with affinity

Proton pumps 1. Respiratory enzymes are proton pumps 2. Binding of the electron drives the conformational changes that move protons across the membrane from in to out. 3. Movement of protons has 2 major consequences a. Generate a ph gradient (proton concentration gradient the matrix has less protons) b. Generates a voltage gradient (inside negative relative to outside due to net movement of positive ions out. 4. Protons tend to move down the electrochemical proton gradient. 5. ATP synthase uses the proton gradient to synthesize ATP. 6. This is the chemiosmotic-coupling step in the process.

ATP Synthase 1. α and b subunits surround rotating subunit - the gamma (γ) subunit 2. The complex sits in the inner membrane 3. Movement of protons across the membrane causes the gamma subunit to rotate 4. ATP is synthesized 5. The process can run in reverse 6. ATP can be used to generate proton gradient