Enzymes

Enzyme: a macromolecule (usually a protein) that acts as a catalyst; a chemical agent that speeds up a reaction without being consumed (used up) in the reaction.

Every chemical reaction between molecules involves breaking some bonds and creating others. To reach a point where bonds can be broken, reactant molecules must absorb energy from their environment which causes them to enter an unstable state (transition state).

Once the new bonds are formed, the resulting product is again stable and energy is released to the environment. Activation energy: the energy needed to start a chemical reaction- to get the molecules to a place where bonds can be broken- to the transition state

2 types of chemical reactions Exergonic: net release of free energy, the free energy of the reactants is greater than the free energy of the products Example: cellular respiration C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + energy

Figure 6.6a (a) Exergonic reaction: energy released, spontaneous Amount of energy released (  G  0) Reactants Products Energy Progress of the reaction Free energy

Exergonic reactions are said to be spontaneous, meaning that is is energetically favorable NOT that it happens quickly.

Endergonic: absorbs free energy from the surroundings, there is a net increase in energy. The products have more energy than the reactants. The products, in a sense store energy. Example?

Photosynthesis The required energy comes from the sun 6CO 2 + 6H 2 O + energy  C 6 H 12 O 6 + 6O 2

Figure 6.6b (b) Endergonic reaction: energy required, nonspontaneous Amount of energy required (  G  0) Reactants Products Energy Progress of the reaction Free energy

Figure 6.6 (a) Exergonic reaction: energy released, spontaneous (b) Endergonic reaction: energy required, nonspontaneous Amount of energy released (  G  0) Amount of energy required (  G  0) Reactants Products Energy Progress of the reaction Reactants Products Energy Progress of the reaction Free energy

Figure 6.12 Transition state Reactants Progress of the reaction Products  G  0 Free energy A A A B C D B B C D CD EAEA

Draw a similar diagram graphing the progress of an endergonic reaction in which: EF + GH  EG + FH

Exergonic = Spontaneous ≠ fast

Figure 6.12 Transition state Reactants Progress of the reaction Products  G  0 Free energy A A A B C D B B C D CD EAEA

Adding heat is one way to provide energy to reach the transition state Molecules speed up, have more and more forceful collisions with one another Some reactions have a small enough activation energy (the amount of energy need to reach the transition state) that they can occur at room temperature

Problems with using heat to speed a biological reaction. Can you think of any?

Heat denatures proteins All reactions would be accelerated

Enter the enzyme

Enzymes Catalyze (speed up) reactions by lowering the activation energy of a reaction They cannot change the ΔG for a reaction They cannot change an endergonic reaction to an exergonic one.

Figure 6.13 Products  G is unaffected by enzyme Reactants Progress of the reaction Free energy E A with enzyme is lower E A without enzyme Course of reaction without enzyme Course of reaction with enzyme

Things to know Enzymes are substrate specific They bind with their substrate(s) into an enzyme-substrate complex Binding occurs at an active site Remember what we learned about protein and their ligands, there is some subtle movement in the enzyme after binding that enhances the “fit” of the enzyme to its substrate. Induced fit

Figure 6.14 Enzyme-substrate complex Enzyme Substrate Active site

Mechanisms of lowering activation energy 1.The active site may provide a template so that multiple substrates can come together in the right orientation 2.The enzyme might stretch on the substrate weakening bonds 3.The active site might provide a microenvironment that is more favorable 4.The enzyme might be directly involved in the chemical reaction, but not consumed.

Figure Substrates Enzyme Substrates are converted to products. Products are released. Products Enzyme-substrate complex Substrates are held in active site by weak interactions. Substrates enter active site. Active site is available for new substrates

The rate of a chemical reaction Depends on: – Concentration of substrate – Amount of enzyme present – Once saturation occurs, the speed of the reaction itself limits the rate unless…you can add more enzyme – Local conditions

Local conditions Enzymes have optimum conditions; the conditions under which a particular enzyme works best – Temperature – pH – Cofactors- nonprotein helpers bound to the enzyme.

Cofactors Nonprotein “helpers” May be bound – Tightly/ permanently – Loosely/ reversibly If the cofactor is organic, called a coenzyme Example: – vitamins

Enzyme inhibitors Competitive inhibitors – Reduce the productivity of enzymes by blocking substrates from entering active sites – “mimic” the normal substrate molecule Noncompetitive inhibitors – Impede enzymatic reactions by binding to another part of the enzyme – Changes the shape of the enzyme so that the active site is less effective

Figure 6.17 (b) Competitive inhibition (c) Noncompetitive inhibition (a) Normal binding Competitive inhibitor Noncompetitive inhibitor Substrate Enzyme Active site

Regulation of enzyme activity Allosteric regulation= the binding of a regulatory molecule to an enzyme at one site that affects the function of the enzyme at a different site – Inhibition OR – Stimulation

Cooperativity= a type of allosteric activation in which the enzyme response is amplified by the substrate itself

Feedback inhibition= the end product of a metabolic pathway acts as a inhibitor of an enzyme within that path.

Figure 6.19 Active site available Intermediate A End product (isoleucine) Intermediate B Intermediate C Intermediate D Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 Feedback inhibition Isoleucine binds to allosteric site. Isoleucine used up by cell Enzyme 1 (threonine deaminase) Threonine in active site

Learning objectives SWBAT: Create a model that represents the change in free energy of a reaction and the activation energy to carry out that reaction SWBAT: Describe how enzymes influence both the change in free energy and the activation energy of a reaction.

Enzyme video enyzmes