Enzymes and Energy

Human physiology

Enzymes•Biological catalysts.•Increase rate of chemical reactions.

•Most enzymes are proteins with diverse structure.

Biological Catalyst•Chemical that:

▫Increases the rate of a reaction.▫Is not changed at the end of the reaction.▫Does not change the nature of the

reaction or final result.▫Lowers the activation energy required.

•Activation energy:▫Amount of energy required for a reaction

to proceed.

Activation Energy

Mechanism of Enzyme Action

•Each type of enzyme has has a characteristic 3-dimensional shape (conformation).

•Has ridges, grooves, pockets lined with specific amino acids.

•Pockets active in catalyzing a reaction are called the active sites of the enzyme.

Mechanism of Enzyme Action

•Lock-and-key model of enzyme activity:•Reactant molecules (substrates) have

specific shapes to fit into the active sites.▫Substrate fits into active sites in enzyme.▫Enzyme-substrate complex dissociates.▫Products of reaction formed and free

enzyme.

Naming of Enzymes•Enzyme name ends with ase.•Classes of enzymes named according to activity.

•Enzymes that have the same activity in different organs may make different models called isoenzymes.

Control of Enzyme Activity•Rate of enzyme-catalyzed reactions measured by the rate substrates are converted to products.

Control of Enzyme Activity•Factors influencing rate:

▫Temperature▫pH▫[cofactors and coenzyme]▫[enzyme and substrate]▫Stimulatory and inhibitory effects of products

Effect of Temperature

•Increase in temperature increases rate of reaction.

•At body temperature, plateaus.

•Denature at high temperatures.

pH

• Each enzyme exhibits peak activity at narrow pH range (pH optimum).

• Optimum pH reflects the pH of the body fluid in which the enzyme is found.

Cofactors and Coenzymes•Cofactor:

▫Attachment of cofactor causes a conformational change in enzyme.

▫Participate in temporary bonds between enzyme and substrate.

•Coenzymes:▫Cofactors that are organic molecules

derived from niacin, riboflavin and other H20 soluble vitamins.

Substrate Concentration• Rate of

product formation will increase as the [substrate].

• Plateau of maximum velocity occurs when enzyme is saturated.

Reversible Reactions•H20 + C02 H2C03

•Direction of reversible reaction depends on the concentration of molecules to the left and right of the arrows.

ca

Sequence of enzymatic reactions that begins with initial substrate, progresses through intermediates and end with a final product.

An intermediate can serve as substrate for 2 different enzymes, producing 2 different products.

Negative feedback inhibition. One of the finalproducts inhibits the activity of the branch enzyme.Prevents final product accumulation.

Inborn Errors of Metabolism

•Inherited defect in a gene.•Quantity of intermediates formed prior to the defect increases.

•Final product decreases, producing a deficiency.

Cannot produce tyrosine, PKU results.Cannot produce melanin, albinism results.

Bioenergetics•Flow of energy in living systems.•1st law of thermodynamics:

▫Energy can be transformed, but it cannot be created or destroyed.

•2nd law of thermodynamics:▫Energy transformations increase entropy.▫Free energy can be used to do work.

Endergonic Reactions

•Chemical reactions that require an input of energy.

•Products must contain more free energy than reactants.

Exergonic Reactions

•Convert molecules with more free energy to molecules with less.

•Release energy in the form of heat.•Heat is measured in calories.•Calorie:

▫Amount of heat to raise the temperature of one cubic centimeter of H20 one degree Celsius.

Coupled Reactions: ATP•Cells cannot use heat for energy.

•Require energy released in exergonic reactions (ATP) to be directly transferred to chemical-bond energy in the products of endergonic reactions.

Universal energy carrier of the cell.

Oxidation-Reduction• Reduced:

▫Molecule/atom gains electrons.• Reducing agent:

▫Molecule/atom that donates electrons.• Oxidized:

▫Molecule/atom loses electrons.• Oxidizing agent:

▫Molecule/atom that accepts electrons.• May involve the transfer of H+ rather than

free electrons.

Reduced form.

Oxidized form.