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1.3 - AN INTRODUCTION TO METABOLISM

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1 1.3 - AN INTRODUCTION TO METABOLISM
all life depends on the organization of matter through work at the molecular level energy capable of doing work comes out of stars and is ultimately converted, through different forms, to heat, all at the expense of a loss of energy somewhere else in the universe (The First Law of Thermodynamics = “There’s no such thing as a free lunch”) Light energy from sun  chemical potential (bond) energy  heat energy photosynthesis is an anabolic reaction in which producers use light to convert simple inorganic molecules (CO2 and H2O) into complex organic molecules (glucose  carbohydrates, lipids, proteins, nucleic acids) which are passed on to consumers and broken down through catabolic reactions to energy-transferring molecules (ATP) to do work

2 ordered systems tend toward entropy (The Second Law of Thermodynamics = “If you think things are messed up now, stick around”) because energy transformations are imperfect and release heat into the universe that can not be used to do work entropy increases when: reactants change state (solid  liquid  gas)  moles of reactant form more moles of product (i.e. complex molecules are broken down into simpler molecules)  solutes diffuse

3 Why do endothermic reactions occur at all?
exothermic reactions should occur spontaneously because entropy is increased as product bonds contain less potential energy Why do endothermic reactions occur at all? spontaneity of reaction is determined by entropy and energy (i.e the temperature at which they occur) (Table 2) REACTIONS ARE: spontaneous at any temperature if exothermic and increase entropy not spontaneous at any temp if endothermic and decrease entropy exothermic reactions that decrease entropy occur spontaneously only at low temperatures endothermic reactions that increase entropy occur spontaneously only at high temperatures (Gibbs) free energy (able to do work) can predict whether a reaction will proceed spontaneously as entropy increases in a system (Figs 6 & 7) ΔG = Gfinal - Ginitial changes are spontaneous if ΔG is negative (Gf < GI)  spontaneous reaction in one direction is nonspontaneous in the opposite direction (ordering energy must come from disordering other energy)

4 the activation energy (required to break reactant bonds) is provided by the difference between the transition state energy (as bonds are breaking and reforming) and the potential energy of the reactants an exothermic reaction occurs if product bonds are more stable than reactants (more energy released in product bond formation than reactant bond breaking), resulting in a net energy output (Fig 2) an endothermic reaction occurs if the energy absorbed from breaking reactant bonds is greater than the energy released by the formation of product bonds, resulting in a net absorption of energy (Fig 3)

5 Net change of energy in a chemical reaction = heat or enthalpy of reaction = ΔH (negative in exothermic, positive in endothermic reactions) Try This (P. 60) (calculate the ΔHcombustion of one mole of glucose) a) balanced chemical equation b) bond energy balance sheet c) ΔHcombustion (% difference) = diff. between values X 100% accepted value = d) comparison with accepted value (2870 kJ/mol) e) explain difference?

6 1.4 – ENZYMES (protein catalysts)
review – a catalyst reduces the energy required for a chemical reaction to occur, converting reactants into products faster than without it, and is not consumed in the reaction, available to catalyze the same reaction again review – the rate of chemical reactions is determined by the number, orientation, and violence of reactant collisions (in metabolic terms) – all chemical reactions (exergonic or energonic) must overcome an activation energy (EA) barrier to occur catalysts reduce EA barrier to allow reactions to occur at a suitable rate and moderate temperatures (http://www.youtube.com/watch?v=Pvgpk75us18) ΔG remains the same, but the potential energy level of the transtion state is decreased, allowing a greater proportion of colliding reactants to reach the transition state and become products forward and reverse reactions are sped up equally, so equilibrium is reached sooner

7 Factors Affecting the Rate of Enzyme Activity
the substrate (reactant) binds to a small portion (active site) of a specific enzyme in tertiary or quaternary structure (complex conformations produce pockets or grooves) induced-fit model  interaction between functional groups of reactant and enzyme changes the shape of the protein and creates an enzyme-substrate complex (Fig 3) making or breaking bonds on substrate changes enzyme shape enough to lose affinity for the products and allow them to be released Factors Affecting the Rate of Enzyme Activity energy of activation decreased by: stretching bonds in the presence of (optimal) heat changing the pH (i.e. acidic R-group on active site) combining with non-protein cofactors (i.e. Zn2+, Mn2+) or coenzymes (i.e. vitamin B3 derivatives NAD+, NADP+ shuttle molecules between enzymes)

8 Enzyme Inhibition competitive inhibitors enter enzyme’s active site, prevent normal substrate from binding (increasing the enzyme’s substrate concentration allows it to compete favourably with the inhibitor) (Fig 6ab) noncompetitive inhibitors attach to another site on the enzyme, changing it’s shape and thus the active site’s affinity for the substrate, or affecting the active site’s ability to catalyze the reactants (Fig 6c)

9 Allosteric Regulation
Cells control enzyme activity in two ways: restricting their production inhibiting the action of those already produced  allosteric sites at several locations some distance from active sites on enzymes weakly bind noncompetitive activators or inhibitors that regulate activity at all active sites (i.e. on subunits in quaternary structure) on the enzyme by stabilizing the protein conformation in an active or inactive state (Fig 7)

10 Restriction of Location
Feedback Inhibition used by cells to control metabolic pathways involving a series of sequential reactions, each catalyzed by a specific enzyme (Fig. 8) the end product (i.e.isoleucine) weakly binds to the allosteric site of the initial substrate (i.e. threonine) and acts as the inhibitor of an enzyme that catalyses a reaction early in the process (i.e. threonine deaminase), which results in less production of the inhibitor which causes the early enzyme to be in the active form more often, which increases the production of the inhibitor (which is also the end product), which increases the inhibition of the early enzyme which tightly contols the production of the product Restriction of Location enzymes and enzyme complexes are separated by being incorporated into specific membranes (i.e. cristae of mitochondria) and fluid-filled spaces (i.e. cytoplasm) so that the reactions that occur on the membrane can be regulated by restricting the movement of intermediates that are formed in the fluid-filled space

11 Commercial and Industrial Uses of Enzymes
microorganisms catalyze the hydrolysis of starch into α-glucose rennet containing the protease chymosin used to hydrolyze casein, coagualating milk into curds (and whey) to make cheese microorganisms use the lactase α-galactosidase to hydrolyze lactose into glucose and galactose which is more easily digested (also removes lactose from milk used to make cheese as it cystallizes to give cheese a grainy texture) lipases hydrolyze fat in milk to produce free fatty acids which give strong cheeses theit distinctive flavours proteases and amylases remove protein and carbohydrate stains (i.e. blood, grass, etc.)  Design Lab – Inv


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