Introduction to metabolism

Metabolism

• Term used to describe all the chemical reactions occurring in an organism

• Metabolism = anabolism + catabolism• Break down chemistry is called catabolism• Buildup (synthesis) chemistry is called anabolism• Most chemistry is assisted by proteins called enzymes

Sources of energy• Carbohydrates, lipids, and proteins are sources of

energy for the body• Energy is stored in the electrons associated with C-H

bonds• Carbohydrates are the first source of energy in the

body.• Lipids contain the most of C-H bonds per gram and so

have the highest number of Calories• Energy from foods is converted to ATP: Cell energy

Building Materials• Building materials may be consumed directly or

synthesized from materials through metabolic pathway

• A metabolic pathway is a series of chemical steps that begin with a specific molecule and end with a product

• Each of the Chemical steps of the metabolic pathway is usually catalyzed by a specific enzyme.

Enzyme 1 Enzyme 2 Enzyme 3

A B C DReaction 1 Reaction 2 Reaction 3

Startingmolecule

Product

Catabolic pathways :Break down complex molecules into simpler compounds•Release energy•This energy is stored in organic molecules until need to do work in the cell.

Anabolic pathways :Build complicated molecules from simpler ones•Consume energy•The energy released by catabolic pathways is used to drive anabolic pathways.

Bioenergetics

• Is the Study of how organisms manage their energy resources

• Energy :

– - Is the capacity to cause change

- Exists in various forms, of which some can perform work.

• Kinetic energy

– Is the energy associated with motion

• Potential energy

– - Is stored in the location of matter

– - Includes chemical energy stored in molecular structure

ATP (adenosine triphosphate)

O O O O CH2

H

OH OH

H

N

H H

O

NC

HC

N CC

N

NH2Adenine

RibosePhosphate groups

O

O O

O

O

O

-

- - -

CH

• Immediate source of cellular energy

• Common to ALL living things

• Made of adenine, ribose and three phosphates

• Made by each cell

• 107 molecules used and regenerated/second/cell

ATP

• An exergonic reaction

– Proceeds with a net release of free energy and is spontaneous

• An endergonic reaction

– Is one that absorbs free energy from its surroundings and is nonspontaneous

• ATP powers cellular work by coupling exergonic reactions to endergonic reactions

Energy coupling Is a key feature in the way cells manage their energy resources.

• ATP hydrolysis– Can be coupled to other reactions

Endergonic reaction: ∆G is positive, reaction is not spontaneous

∆G = +3.4 kcal/molGlu Glu

∆G = - 7.3 kcal/molATP H2O+

+ NH3

ADP +

NH2

Glutamicacid

Ammonia Glutamine

Exergonic reaction: ∆ G is negative, reaction is spontaneous

P

Coupled reactions: Overall ∆G is negative; together, reactions are spontaneous ∆G = –3.9 kcal/mol

The Regeneration of ATP• Catabolic pathways

– Drive the regeneration of ATP from ADP and phosphate

ATP synthesis from ADP + P i requires energy

ATP

ADP + P i

Energy for cellular work(endergonic, energy-consuming processes)

Energy from catabolism(exergonic, energy yieldingprocesses)

ATP hydrolysis to ADP + P i yields energy

Electron Transport Chain

• Food (glucose) is oxidized and the released hydrogens:

– Are transported by coenzymes NADH and FADH2

– Enter a chain of proteins bound to metal atoms (cofactors) – Combine with molecular oxygen to form water– Release energy

• The energy released is harnessed to attach inorganic phosphate groups (Pi) to ADP, making ATP by oxidative phosphorylation– “phosphorylation” - to add phosphate to a substance

ADP + P ATP

Mechanism of Oxidative Phosphorylation

• The hydrogens delivered to the chain are split into protons (H+) and electrons

– The protons are pumped across the inner mitochondrial membrane to the intermembrane space

– This creates a pH and concentration gradient (of H+)

– The electrons are shuttled from one acceptor to the next

• Electrons are delivered to oxygen, forming oxygen ions

• Oxygen ions attract H+ that were pumped into the intermembrane space to form water

• H+ that were pumped to the intermembrane space:

– Diffuse down their gradients back to the matrix via ATP synthase (from greater to lesser concentration)

– Release energy to make ATP

ATP Synthase• The enzyme

consists of three parts: a rotor, a knob, and a rod

• Current created by H+ causes the rotor and rod to rotate

• This rotation activates catalytic sites in the knob where ADP and Pi are combined to make ATP

15

Mitochondrion

Oxidative decarboxilationof pyruvate, & citric acidcycle take place in matrix,along with fatty acid oxidation

Site of oxidative phosphorylation

Permeable

Respiratory control: Rate of oxidative phosphorylation is determined by the need for ATP

1. Electron transport and ATP synthesis is coupled.2. Electrons do not flow through the electron-transport chain to O2 unless ADP

is simultaneously phosphorylated to ATP.3. When [ADP] rises, the rate of oxid. phos. increases to meet the ATP needs.(This is the case in active muscle)4. The regulation of the rate of oxid. phos. by the ADP level is called Respiratory

control or Acceptor control.

Two Translocase Proteins1. ATP/ADP Translocase

– also called the adenine nucleotide translocase.– functions to export one ATP for every ADP that is

imported. – an antiporter because it translocates molecules in

opposite directions across the membrane.– for every ADP molecule that is imported from the

cytosol, an ATP molecule is exported from the matrix.

2. Phosphate Translocase

– translocates one Pi and one H+ into the matrix by an electroneutral import mechanism.

Cyanide (CN-), azide (N3- ) react with the ferric (Fe3+ ) form of heme a3.

Carbon monoxide (CO): Inhibits the ferrous (Fe2+) form.

Inhibits electron flow from Cyt bH

Inhibits electron flow in Complex IPrevents the utilization of NADH as a substrate.

Site of action of some inhibitors of electron transport

Dinitrophenol is a hydrophobic molecule that remains in the mitochondrial membrane as a chemical uncoupler for a long time - a very dangerous way to burn fat.

J Anal Toxicol. 2006 Apr;30(3):219-22.

Summary of known ETS and Ox Phos inhibitors

You should be able to answer questions about changes in the rates of succinate oxidation, O2 consumption, and ATP synthesis in mitochondrial suspensions if provided information about any of these ETS, Ox Phos, or translocase inhibitors.

The UCP1 uncoupling protein, also called thermogenin, controls thermogenesis in newborn and hibernating animals

Cell-specific expression of the UCP1 protein leads to heat production under aerobic conditions by short circuiting the proton gradient across the mitochondrial inner membrane.

The UCP1 protein is expressed at high levels in special fat cells called brown adipose tissue which contain fatty acids for the production of acetyl CoA to drive NADH production by the citrate cycle, and large numbers of mitochondria to increase the output of heat by the electron transport system.

ProblemsPotent poisons:

What is the effect of each of the following inhibitors on e transport and ATP formation by the respiratorty chain?

a) Azide: It blocks e transport and proton pumping at Complex IV.

b) Atractyloside: It block e transport and ATP synthesis by inhibiting the exchange of ATP and ADP across the inner mitochondrial membrane.

c) Rotenone: It blocks e transport and proton pumping at Complex I.

d) DNP: It blocks ATP synthesis without inhibiting e transport by dissipating the proton gradient.

e) CO: It blocks e transport and proton pumping at Complex IV.

f) Antimycin A: It blocks e transport and proton pumping at Complex III.

Uncouplers of Oxidative Phosphorylation:

In normal mitochondria the rate of e transfer is tightly coupled to the demand for ATP. Thus when ATP is demanded at a high rate, e transfer is rapid. Under such conditions of tight coupling, the no of ATP produced per atom of oxygen consumed when NADH is the e donor---know as the P/O ratio---is close to 3.

a) increased activity of the respiratory chain in the presence of DNP requires the degradation of additional fuel. By oxidizing more fuel (including fat reserves) to produce the same amount of ATP, the body loses weight. When the P/O ratio approaches zero, the lack of ATP results in death.