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 workb. Myosin/actin does mechanical workc. Creatine kinase does synthetic or chemical work

What is meant by ATP hydrolysis?

H2OE + ATP E.ATP E + ADP + Pi

Hydrolysis step

Myosin + ATP Myosin.ATP Myosin + ADP + Pi

Chemical structureterminal phosphate group has high energy bond

Base = AdenineSugar = RiboseThree PO4 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 mitoconodriaCellular organelle

1. Porous outer membrane2. Selectively permeable inner membrane3. Matrix space4. Cristae

Mitochondria:1. Dynamic organelles – have the ability to change shape, divide

and fuse2. Contain DNA – mitochondrial inheritance3. 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 motors5. Located near sites of ATP utilization, e.g. myofibrils in cardiac

muscle cells.

Ox Phos = Oxidative Phosphorylation

Glucose metabolism1. Glucose stored in muscle cells as glycogen2. Glycogen is broken down to glucose

Multiple enzymesGlycogen Glucose

3. Glucose is broken down to Pyruvate via the Glycolytic pathway4. Glucose is a 6-carbon sugar5. Broken down to two 3-carbon pyruvate molecules

Glycolysis (anaerobic metabolism)Glycolytic enzymes; glycolysis occurs in cytoplasm

Glycolytic pathway ATP ADP ATP ADP

Glucose PyruvateAnaerobic

Lactic acid

ATP* ATP*

a. ATP* synthesis is referred to as substrate levelphosphorylation

b. 2 moles of ATP synthesized per mole of glucose

Aerobic conditions i. Pyruvate converted into H2O and CO2 in

mitochondria ii. Oxidative phosphorylation iii. ~ 30 moles ATP/mole of glucose

Pyruvate is transported into mitochondria1. Carrier-mediated transport process1. Secondary active transport2. H+ gradient (pH or proton gradient) powers the movement of

Pyruvate across the mitochondrial membrane

Mitochondrial matrix– NADH production1. Pyruvate is first converted to Acetyl CoA (coenzyme A)2. Acetyl CoA enters the citric acid cycle and is converted to CO2

Acetyl CoA

NADH Citric Acid Cycle

NAD CO2

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 anenzyme (e.g. respiratory enzymes)

NADH – Collector of high-energy electronsPathway from NADH to ATP synthesis

Sequence of events leading to ATP synthesis1. 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 alongthe electron transport chain).

3. ATP synthase – an enzyme driven by the flux of proton intomitochondria to synthesize ATP.

The process is called the chemiosmotic process (conversion ofconcentration energy into chemical energy, i.e. ATP)

Electron transport process and the respiratory enzyme complexes

Characteristics of respiratory enzymes complexes1. 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 severalenzymes (subunits)

1. NADH dehydrogenase2. Cytochrome b-c1 complex3. Cytochrome oxidase complex

Complexes are arranged in this specific order in the membrane

Electron CarriersWhat drives the e- along the chain of enzymes?

Affinity of the respiratory enzymes for electrons

NADH Electron Carriers O2

Strong donor acceptor of electrons StrongestOf electrons acceptor of e-

1. Electron Carriers are arranged in order of increasing affinityfor electrons. Increasing affinity accounts for the tendency tomove from one electron carrier to the next.

2. O2 has highest affinity3. The carrier affinity for electrons can be measured as a redox

potential (oxidation-reduction)

How one measures redox potentialBasic rulesCompounds with most negative redox potentials

1. Weakest affinity for electrons2. Strong donor of electrons3. Least tendency to accept electrons

Redox potential correlates with affinity

Proton pumps1. Respiratory enzymes are proton pumps

2. Binding of the electron drives the conformational changes thatmove protons across the membrane from in to out.

3. Movement of protons has 2 major consequencesa. 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 protongradient.

5. ATP synthase uses the proton gradient to synthesize ATP.

6. This is the chemiosmotic-coupling step in the process.

ATP Synthase1. α and b subunits surround rotating subunit - the

gamma (γ) subunit2. The complex sits in the inner membrane3. Movement of protons across the membrane causes

the gamma subunit to rotate4. ATP is synthesized5. The process can run in reverse6. ATP can be used to generate proton gradient