Introduction to Metabolism

Chapter 6

• Metabolism - sum of organism’s chemical processes.

• Enzymes start processes.• Catabolic pathways release energy

(breaks down complex molecules)• Anabolic pathways consume energy

(builds complicated molecules)

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• Energy - ability to do work.• Kinetic energy - energy of

motion; potential energy - amount of stored energy available.

• Energy can be converted from one form to another.

• Cellular respiration breaks down glucose - energy available to do work.

• Thermodynamics - study of energy transformations.

• 1Closed system (i.e. liquid in a thermos) - isolated from surroundings.

• 2Open system - energy (and often matter) transferred between system and surroundings.

Open system – energy transferred betweensystem and surroundings

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Earth is a closed system – ozone keeps heatfrom escaping

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• 1st law of thermodynamics - energy can be transferred and transformed, but cannot be created or destroyed.

• 2nd law of thermodynamics - every energy transformation must make the universe more disordered.

• Measure of disorder in system - entropy.

• Spontaneous reactions - reactions that occur without outside help.

• Nonspontaneous reactions require outside help.

• Spontaneous processes increase stability of system and nonspontaneous processes decrease stability.

• Spontaneity of system determined by amount of free energy (energy available to do work)

• Free energy (G) in system is related to total energy (H) and its entropy (S) by this relationship:

• G = H - TS, (T is temperature in Kelvin units)

• To be spontaneous, system has to give up energy (decrease in H), give up order (decrease in S), or both.

• System at equilibrium is at maximum stability.

• Exergonic reaction gives off free energy.

• Endergonic reaction requires free energy.

• Cells maintain disequilibrium because they are open; have constant flow of material in/out of cell.

• Cells do 3 types of work.• 1Mechanical work - beating of cilia,

contraction of muscle cells, and movement of chromosomes.

• 2Transport work - pumping substances across membranes against direction of spontaneous movement.

• 3Chemical work - driving endergonic reactions (synthesis of polymers from monomers)

• ATP (adenosine triphosphate) required to do work - type of nucleotide consisting of nitrogenous base adenine, sugar ribose, chain of 3 phosphate groups.

• Phosphate bonds unstable - release energy when broken.

• Phosphate group can be transferred to another molecule (phosphorylation) becomes more reactive molecule.

• ATP recycled by adding phosphate group to ADP.

Inorganic phosphate

• Catalyst - chemical that changes rate of reaction.

• Enzyme - type of catalyst.• Enzymes regulate movement of

molecules through metabolic pathways.

• Chemical reactions involve breaking bonds and making bonds.

Adding water breaks the polymer into monomers

• Activation energy (EA) needed to start reaction.

• Exergonic reactions - activation energy released back to surroundings; more energy released with formation of new bonds.

• Activation energy - amount of energy necessary to push the reactants over energy barrier.

Activation energy

• Difference between free energy of products and free energy of reactants is delta G.

• Enzyme speed reactions by lowering EA.

• Enzymes do not change delta G.

• Substrate - reactant that binds to enzyme.

• When substrate, or substrates, binds to enzyme, enzyme catalyzes conversion of substrate to product.

Enzyme

• Substrate will fit only in enzyme’s active site (area on enzyme that matches up with substrate).

• As substrate binds, enzyme changes shape leading to tighter induced fit, bringing chemical groups in position to catalyze reaction.

• Enzymes – reusable, recycled.• Most metabolic enzymes can catalyze

reactions forward and reverse directions.

• Low substrate concentrations, increase in substrate speeds binding to available active sites.

• Active sites can become saturated, slowing down the reaction.

• As temperature increases, rate of reaction increases.

• At certain temperatures enzyme denatures -stops functioning.

• Enzymes have optimal pH (between pH 6 - 8 for most enzymes)

• Many enzymes require cofactors (nonprotein helpers) to start reaction.

• Organic cofactors, coenzymes, include vitamins or molecules derived from vitamins.

• Inhibitors bind to active site on enzyme; stops reaction from starting.

• lf inhibitor binds to same site as substrate - competitive inhibition.

• If inhibitor binds somewhere other than active site - noncompetitive inhibition.

• Molecules can inhibit enzyme by attaching to allosteric site on enzyme (site that is not active site)

• Regulators can inhibit/activate enzyme.

• Feedback inhibition - increase in end product causes process to stop; decrease starts process.