Metabolic Processes

Enzymes, Energy and Chemical Reactions

Cellular Energy Processing•Metabolism: the sum of all chemical reactions–Anabolism: assembly, polymerization, etc.•requires energy

–Catabolism: disassembly, depolymerization•releases energy

–some reactions couple anabolism with catabolism

–catabolism drives all anabolism–all reactions depend on enzyme catalysts

Energy can be stored or used for workFigure 6.1

Cellular Energy Processing

•cellular processes change chemical structures & transport materials–change and movement require energy exchanges

–energy exchanges have to follow the law(s)

Cellular Energy Processing

•First Law of Thermodynamics–during any event, Initial Energy = Final Energy

…neither created nor destroyedFigure 6.2

Cellular Energy Processing

•First Law of Thermodynamics–during any event, Initial Energy = Final Energy

•Second Law of Thermodynamics–during any event, some energy is unavailable to do work

…some is unusable; disorder increases

Figure 6.2

Cellular Energy Processing

•cells obtain energy from outside sources

…an external source is required

Figure 6.2

Total energy =Figure 6.2

Cellular Energy Processing

total energy = usable energy + unusable energy, or

enthalpy = free energy + (entropy · absolute temperature)

H=G +TS, so, G=H-TS (three unmeasurable variables)

G=H-TS (change in free energy at constant temperature)

G > 0; energy requiredFigure 6.3

Cellular Energy Processing G=H-TS describes energy changes in chemical reactions

positive G describes an energy-requiring reaction; anabolism; decrease in entropy

negative G describes an energy-yielding reaction; catabolism; increase in entropy

G < 0; energy releasedFigure 6.3

Cellular Energy Processing spontaneity (≠ rate)

a spontaneous reaction goes more than half way to completion without an energy input; it is exergonic; G < 0

a nonspontaneous reaction goes less than half way to completion without an energy input; it is endergonic; G > 0

if A=>B is exergonic, B=>A is endergonic

Cellular Energy Processing

reactions are reversibleA <=> B

add more A, increase => rateadd more B, increase <= rateequilibrium occurs when rates are equal

the closer to completion equilibrium occurs, the more free energy is released

reversible reaction at equilibriumFigure 6.4

ATP: the

cell’s

chief energy curren

cyFigure 6.5

cellular respirati

on supplies ATP for

anabolismFigure 6.6

ATP hydrolysis coupled to glutamine synthesisFigure 6.7

cellular energy transfer Adenosine TriPhosphate (ATP) is the predominant energy currency in the cellATP hydrolysis is exergonic (G = -7.3 kcal/mol)ATP + H2O => ADP + Pi

ATP synthesis is endergonicATP shuttles energy from exergonic reactions to endergonic reactions

each ATP is recycled ~10,000 times/day~1,000,000 ATPs are used by a cell/second

Enzymes: Biological Catalysts

a catalyst: increases the reaction rate; is unchanged by the reactionmost biological catalysts are proteins

some (few) biological catalysts are ribozymes (RNA)

Ea

determines the likelihood

that a reaction will occurFigure 6.8

Enzymes: Biological Catalysts

each chemical reaction must overcome an energy barrier - activation energy (Ea)spontaneous reactions will go - eventuallythe direction is predictableneither likelihood, nor rate is predictable

heat may supply

Ea

Figure 6.9

E + S => E-S complex => E + P

Figure 6.10

position substratesFigure 6.12

induce strain

alter surface charge

Enzymes: Biological Catalysts

how to overcome the energy barrier?increase kinetic energy of reactant molecules, or

decrease Ea

an enzyme binds a specific substrate molecule(s) at its active siteE + S => E-S complex => E + Pthe active site > positions reactants, strains bonds, etc. to destabilize the reactants…

…lowering Ea

enzyme: lowers Ea, doesn’t change GFigure 6.11

Enzymes: Biological Catalysts

enzymes…efficiency experts of the metabolic worldlower activation energydo not alter equilibrium

increase the rates of forward and reverse reactions

Enzymes: Biological Catalysts

substrate concentration affects reaction rateas increased [reactant] increases reaction rate

so increased [substrate] increases reaction rateuntil…

all active sites are occupied

the reaction is saturated

enzymatic reactions may be saturatedFigure 6.16

induced fit in hexokinaseFigure 6.14

Enzymes: Biological Catalysts

enzyme structure determines enzyme functionthe active site fits the substrate“lock & key”“induced fit”

the rest of the enzyme stabilizes the active siteprovides flexibility

Figure 6.15

Enzymes: Biological Catalysts

enzyme structure determines enzyme functionsome enzymes require non-protein groupscofators: reversibly-bound ions

coenzymes: reversibly bound organic molecules

prosthetic groups: permanently bound groups

Table 6.1

Enzymes & Metabolism metabolic regulation coordinates the many potential enzymatic reactionssequential reactions form pathways

product of 1st reaction is substrate for 2nd

E1 E2 E3 E4

A=> B=> C=> D=> product of pathway

regulation of enzymes in the pathway regulates the entire pathway

related to Sarin gas and malathion

irreversible inhibition by DIPF

Figure 6.17

Enzymes & Metabolism metabolic regulation coordinates the many potential enzymatic reactionsenzyme inhibitors provide negative controlartificial inhibitors can be pesticidesirreversible inhibition - covalent modification of active site

natural metabolic regulation is often reversiblecompetitive inhibition

cartoon

version

Figure 6.18

Enzymes & Metabolism metabolic regulation coordinates the many potential enzymatic reactionsenzyme inhibitors provide negative controlartificial inhibitors can be pesticidesirreversible inhibition - covalent modification of active site

natural metabolic regulation is often reversiblecompetitive inhibitionnoncompetitive inhibition

cartoon

version

Figure 6.18

Enzymes & Metabolism metabolic regulation coordinates the many potential enzymatic reactionsallosteric enzymes have catalytic and regulatory subunits

active and inactive enzyme conformations are in equilibrium

Figure 6.19

•

Figure 6.20

Enzymes & Metabolism metabolic regulation coordinates the many potential enzymatic reactionsallosteric enzymes regulate many metabolic pathwayscatalyze first committed steprespond sensitively to inhibition

often inhibited by pathway end product - “end-product inhibition”

end-product inhibition by isoleucineFigure 6.21

Enzymes & Metabolism metabolic regulation coordinates the many potential enzymatic reactionsallosteric enzymes regulate many metabolic pathwayscatalyze first committed steprespond sensitively to inhibitionoften inhibited by pathway end product - “end-product inhibition”saves resources when end product is sufficient

secondary & tertiary structures

depend on are disrupted by

H-bonds heat

ionic interactions

pH changes

hydrophobic interactions

detergents

disulfide bonds red/ox changes

pH optima for three enzymes

Figure 6.22

temperature optimumFigure 6.23

Enzymes & Metabolism enzyme activity relies on proper environmental conditionssome enzymes have isozymes suited to different environmental conditions