AP Bio Energetic of Chemical Reactions & The Role of Enzymes Lecture Campbell & Reece, Biology 7th Edition pp. 150-157 Chapter 8

Energy and Chemical Reactions Exergonic rxn’ = release energy (products have less chemical energy than reactants) ex: AB + CD  AC + DB + energy - ΔG Endergonic rxn’s = absorb energy (products have more chemical energy than reactants) ex: AB + CD + energy  AC + DB + ΔG Activation energy: energy added to reactants to "jumpstart" the rxn Catalysts: reduce the amount of activation energy that is needed to start the rxn.

The energetics of chemical reactions Think about rolling a boulder up a hill…it takes energy input, right? And the boulder at the top of the hill now has that energy stored in it (potential). So the product ends up having MORE energy than the reactant. Think about a boulder rolling down a hill…it has lots of energy. And the boulder at the bottom of the hill now has less (potential) energy. So the product ends up having LESS energy than the reactant.

Self-Quiz Which of these biological processes is (overall) endergonic? Exergonic? –ΔG ? +ΔG ?

Chemical Reactions The activation energy, EA Is the initial amount of energy needed to start a chemical reaction Is often supplied in the form of heat from the surroundings in a system

The effect of enzymes on reaction rate An enzyme catalyzes reactions By lowering the EA barrier Progress of the reaction Products Course of reaction without enzyme Reactants with enzyme EA EA with is lower ∆G is unaffected by enzyme Free energy Figure 8.15

Enzymes Are a type of protein that acts as a catalyst, lowering the EA and speeding up chemical reactions Substrate (sucrose) Enzyme (sucrase) Glucose OH H O H2O Fructose 3 Substrate is converted to products. 1 Active site is available for a molecule of substrate, the reactant on which the enzyme acts. Substrate binds to enzyme. 2 4 Products are released. Figure 5.16

Substrate Specificity of Enzymes The substrate (A) Is the reactant an enzyme acts on The enzyme Binds to its substrate, forming an enzyme-substrate complex (C)

The Active Site Is the region on the enzyme where the substrate binds Their shape (conformation) makes each enzyme substrate-specific “Lock-and-key” fit enables reaction

The active site can lower an EA barrier by Orienting substrates correctly Straining substrate bonds Providing a favorable microenvironment Covalently bonding to the substrate

Effects of Local Conditions on Enzyme Activity The activity of an enzyme Is affected by general environmental factors temperature pH salinity enzyme concentration substrate concentration presence of any inhibitors or activators.

Denaturation is a process in which proteins or nucleic acids lose their tertiary structure and secondary structure by application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent (e.g., alcohol or chloroform), or heat. If proteins in a living cell are denatured, this results in disruption of cell activity and possibly cell death.

Effects of Temperature and pH Each enzyme Has an optimal temperature in which it can function Optimal temperature for enzyme of thermophilic Rate of reaction 20 40 80 100 Temperature (Cº) (a) Optimal temperature for two enzymes typical human enzyme (heat-tolerant) bacteria

Optimal pH for two enzymes Rate of reaction Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) 1 2 3 4 5 6 7 8 9 Each enzyme has an optimal pH in which it can function

Cofactors & Coenzymes Cofactors Coenzymes Are nonprotein enzyme helpers Most often minerals (metal ions) Zn 2+, Mg2+ Coenzymes Are organic cofactors Usually vitamins B2, B6, B12, etc

Enzymes Regulate reactions Concept 8.5: Regulation of enzyme activity helps control metabolism A cell’s metabolic pathways Must be tightly regulated Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3 Starting molecule Product Biochemical pathway Feedback inhibition of a biochemical pathway Enzyme action and the hydrolysis of sucrose

Allosteric Activation and Inhibition Many enzymes are allosterically regulated Allosteric regulation Is the term used to describe any case in which a protein’s function at one site is affected by binding of a regulatory molecule at another site Enzymes can be “turned on” or “turned off” this way

Enzyme Inhibitors Competitive inhibitors Bind to the active site of an enzyme, competing with the substrate Figure 8.19 (b) Competitive inhibition A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme (a) Normal binding

Noncompetitive inhibitors Bind to another part of an enzyme, changing the function Figure 8.19 A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions. Noncompetitive inhibitor (c) Noncompetitive inhibition

Feedback inhibition In feedback inhibition The end product of a metabolic pathway shuts down the pathway Active site available Isoleucine used up by cell Feedback inhibition Isoleucine binds to allosteric site Active site of enzyme 1 no longer binds threonine; pathway is switched off Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Intermediate B Intermediate C Intermediate D Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 End product (isoleucine) Figure 8.21