Enzymes & Metabolism Chapter 8

YOU MUST KNOW Examples of endergonic and exergonic reactions The key role of ATP in energy coupling That enzymes work by lowering the energy of activation The catalytic cycle of an enzyme that results in the production of a final product Factors that change enzyme shape and how they influence enzyme activity How the shape of enzymes, their active site, and interaction with specific molecules affect their function How feedback inhibition is used to maintain appropriate levels of enzymes in a pathway

Metabolism Metabolism - all of an organism’s chemical reactions Manages matter and energy Catabolism – Breakdown molecules (hydrolysis) Anabolism – Building molecules (dehydration)

Energy Review Energy = ability to do work Chemical energy – a form of potential energy stored in molecules in chemical bonds Thermodynamics – energy transformations 1st law – energy can be transferred or transformed, but not created or destroyed 2nd law – every energy transfer or transformation increases entropy (randomness) Free energy – energy that can do work in a living system

G = change in free energy

Exergonic reaction – energy is released to the organism ( -G ) Endergonic reaction – energy is absorbed from the organism ( +G ) ***In order for a reaction to be spontaneous, must have -G (be exergonic)

Sample Problem:

46,000 joules/mol – 293K(178 joules/mol) = -6,154

ATP and Energy Coupling Coupling = pairing endergonic and exergonic reactions. Energy released from exergonic drives endergonic. Energy is gained when the bond is broken from the terminal phosphate of ATP. This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves

Problem: If the Gibbs free energy for a reaction is +4 Problem: If the Gibbs free energy for a reaction is +4.9kcal/mol, explain how energy coupling (via ATP-Gibbs free energy= -7.3kcal/mol) would make the reaction spontaneous or non-spontaneous.

Problem: If the Gibbs free energy for a reaction is +4 Problem: If the Gibbs free energy for a reaction is +4.9kcal/mol, explain how energy coupling (via ATP-Gibbs free energy= -7.3kcal/mol) would make the reaction spontaneous or non-spontaneous. (+4.9kcal/mol) + (-7.3kcal/mol) = -2.4 kcal/mol Since the answer is (-), this is a spontaneous reaction

Catalysts – speed up reactions without being used up or changed by the reaction

Activation energy – amount of energy needed to get a reaction started needed to break the bonds of the reactants)

Catalysts work by lowering the activation energy (EA) *Notice – the overall G of the reaction (between reactants and products)is not changed

Enzymes are biological catalysts - Proteins – very specific shapes - Usually end in –ase -Substrate – reactants in an enzyme catalyzed reaction -Active site – spot on enzyme where substrate binds

Enzymes lower activation energy by positioning substrates in a way that is favorable to reacting *The enzyme does not provide any energy! Substrate joins with enzyme at active site to form enzyme-substrate complex Product(s) released, enzyme unchanged and free to catalyze the reaction again with new set of reactants

Induced fit model – enzyme changes conformation as it interacts with substrate Still requires proper shape of enzyme and substrate

Several factors can affect enzyme activity Temperature – as it increases, molecules move faster, collide more often and with more energy Enzymes have an ideal temperature which matches environment in which they are supposed to be active Too cold – molecular motion too slow Too hot – enzymes lose shape (denature)

pH (proportion of H+ and OH- ions) H+ and OH- ions can bind with enzyme or substrate and influence bonding between the two Enzymes have ideal pH ranges

Substrate Concentration - as substrate concentration increases, enzyme activity increases until a maximum rate is achieved

Cofactors and Coenzymes help enzymes function properly Cofactors = inorganic molecules (minerals) Coenzymes = organic molecules (vitamins)

Competitive Inhibitors – a molecule binds into active site, blocking access to the substrate Usually reversible - can be overcome by increasing substrate concentration Often similar in shape to the substrate (at least a portion of it)

-Also called allosteric inhibition Non-competitive Inhibitors – bind elsewhere on enzyme (allosteric sites) and force change in shape of active site -Also called allosteric inhibition * There is also allosteric activation

Regulation of enzyme activity helps control metabolism Genetic regulation – cells turn on/off genes that code for specific enzymes Feedback inhibition – a product of a reaction acts as an allosteric inhibitor of that reaction End product inhibition – final product of a metabolic pathway acts as an inhibitor to the enzyme which catalyzes the first reaction in that pathway