Enzymes Homeostasis: property of living organisms to regulate their internal environment, maintaining stable, constant condition *Occurs by multiple adjustments controlled by related regulation mechanisms Metabolism: all chemical reactions occurring constantly in cell that maintain homeostasis Enzyme: protein, site of a chemical reaction, can catalyze (speed up) reaction without being used up or changed *Holds reactant molecules together long enough for them to react

How Enzymes Work Substrate = reactant(s) in an enzyme catalyzed reaction Equation for an enzyme-catalyzed reaction: Enzyme + Substrate => Enzyme/Substrate Complex => Enzyme + Product (E + S => ES => E + P) place where substrate(s) bind on enzyme Active Site = ENZYMES ARE MADE UP OF: Apoenzyme: protein part, gives enzyme specificity of what reaction it will catalyze Coenzyme: non-protein part, aids reaction by accepting or donating atoms (accepting= reduction donating = oxidation)  Vitamins: components for synthesizing coenzymes *e.g. niacin (nicotinic acid), thiamine (B1) riboflavin (B2)

How Enzymes Work 1 = 3 = apoenzyme substrate 2 = 4 = coenzyme product Induced Fit Theory: Upon binding, enzyme undergoes slight change in shape to bind substrates: after reaction, ES complex separates, enzyme re-assumes original shape, ready to catalyze another reaction

How Enzymes Work Activation Energy: (Ea) energy that must be supplied to cause molecules to react with one another Lowering Activation Energy: enzymes bring substrate molecules together, hold them long enough for reaction to take place vs.

each reaction requires a specific enzyme Metabolic Pathways *orderly step-wise series of chemical reactions from reactants to products where one reaction leads to next: each reaction requires a specific enzyme *not possible in biological systems to have single reaction to produce complex molecules from simple reactants (e.g. cellular respiration) many intermediates needed *one pathway can lead to several others: intermediate product of one pathway = starting reactant for other pathway *having more than one step means more places where overall reaction can be controlled

Factors Affecting Enzyme Activity 1. pH: - each enzyme works optimally in a preferred pH range (6-8 is common) - if pH out of range (too high or low) enzyme denatures (loses normal configuration, ability to form ES complex) 2. Temperature: - ↓ temp slows rate of reaction - very low temp does not normally denature enzyme - ↑ temp speeds up collision rate of E + S (kinetic molecular theory) - temp > 45 °C = enzyme denatured

Factors Affecting Enzyme Activity 3. Substrate Concentration: - ↑ [substrate] = ↑ reaction rate - after certain [substrate], rate will not ↑ anymore, enzymes “saturated” with substrates (all active sites filled), cannot work any faster - ↓ [substrate] = ↓ rate of product formation

Factors Affecting Enzyme Activity 4. Enzyme Concentration: - assume: typically millions more substrate molecules than enzyme molecules - therefore: ↑ [enzyme] = ↑ reaction rate - ↓ [enzyme] = ↓ reaction rate - rate will only level off if no more substrate left, not usual

Factors Affecting Enzyme Activity 5. Inhibitors: molecules that bind to enzyme to prevent substrate binding to enzyme a) Competitive Inhibition: - molecule that looks like substrate competes for space at active site, binds via induced fit, ↓ reaction rate - inhibitor binding can be reversible or irreversible

Factors Affecting Enzyme Activity b) Non-competitive Inhibition: - inhibitor may look completely different from substrate: binds to place on enzyme other than active site - inhibitor binding causes enzyme to denature so substrate cannot bind at active site - binding maybe reversible or non-reversible

Factors Affecting Enzyme Activity Inhibition Examples: - metabolic pathways: intermediate or final product may be reversible inhibitor of an enzyme earlier in pathway - penicillin: irreversible competitive inhibitor of enzyme that aids in bacterial cell wall synthesis - hydrogen cyanide (HCN) = irreversible competitive inhibitor of a mitochondria enzyme in humans - Lead (Pb++), Mercury (Hg++), Cadmium: heavy metals that are irreversible, noncompetitive inhibitors of human enzymes

Metabolism Hormone - Thyroxin Source Gland = thyroid Function = protein hormone that regulates metabolism by controlling oxidation rate in cells (oxygen + glucose = ATP) Negative Feedback Loop: Hypothalamus secretes Thyroid Releasing Hormone (TRH) Pituitary Gland secretes Thyroid Stimulating Hormone (TSH) Thyroid Gland secretes Thyroxin