Enzymes Chapter 8 (sections 4 & 5) Biology – Campbell Reece

What are enzymes? Catalyst – a chemical agent that speeds up a reaction without being used up Can be used over and over again Enzyme – a protein that is a catalyst Enzyme names usually end in –ase Ex. Sucrase (breaks down sucrose) Amylase (breaks down amylose) DNA Polymerase (makes a copy of DNA)

Activation Energy A.K.A free energy of activation (E A ) Usually comes in the form of heat Initial investment of energy for starting a reaction The reactant molecules must be contorted so the bonds can break

Activation Energy Is this reaction endergonic or exergonic? Is it spontaneous or non- spontaneous?

How Enzymes Lower the E A Barrier Which molecules are rich in free energy and could decompose spontaneously? Proteins, DNA, and other complex molecules The temperature of the cell keeps them from decomposing An increase in temperature would make the reaction occur, but… What would happen in the cell if the temperature increased?

How Enzymes Lower the E A Barrier An enzyme lowers the E A barrier It hastens reactions that would occur anyway

Enzyme-Substrate Complex Substrate – the reactant(s) an enzyme acts on Enzymes only bind to their specific substrate The enzyme binds to its substrate(s) at the active site to form the enzyme-substrate complex The substrate ‘fits’ the active site The enzyme changes shape slightly to fit around the substrate more snugly (induced fit)

Enzyme-Substrate Complex

How Enzymes Work… 1. Substrate binds to the active site 2. Weak interactions (hydrogen or ionic bonds) hold the substrate in 3. Side chains (R groups) of the amino acids in the active site catalyze the conversion of substrate to product(s) 4. The product(s) depart the active site 5. The enzyme is now free to bind to another substrate

Mechanisms to lower E A 1. The active site provides a template for two or more reactants (provides proper orientation) 2. Enzyme may stretch the substrate towards its transition state (bends the bonds) 3. May provide a microenvironment that is more conducive (ex. Lower pH) 4. Direct participation of the active site in the reaction (bonding between the substrate & the active site)

Factors That Affect Enzymes ↑ temperature, ↑ rate of enzymatic reaction (to a point) Each enzyme has an optimal temperature at which it operates best Each enzyme has an optimal pH (usually between 6-8)

Cofactors and Enzyme Inhibitors Cofactors – nonprotein helpers (vitamins) If the cofactor is organic it is called a coenzyme Competitive inhibitors – block the substrate from entering the active site Noncompetitive inhibitors – bind to another part of the enzyme, therefore changing its shape Some toxins, poisons and antibiotics are irreversible inhibitors

Enzyme Inhibitors

Allosteric Regulation A protein’s function at one site is affected by the binding of a regulatory molecule at another site May either inhibit or stimulate an enzyme’s activity Most enzymes that are allosterically regulated are made of two or more subunits The complex oscillates between an active form and an inactive form An activator stabilizes the active form; an inhibitor stabilizes the inactive form

Allosteric Regulation

Cooperativity When a substrate binds to one subunit, it triggers the other subunits to hold the favorable shape One substrate primes an enzyme to accept additional substrates more readily

Feedback Inhibition A metabolic pathway is switched “off” by the inhibitory binding of its end product to an enzyme early in the pathway Works kind of like a thermostat in a house