CHE 349 Physical Chemistry for Life Sciences Lecture 13 Concepts An enzyme is a biological catalyst that can increase the rate of a reaction The way it works is that it binds to a particular reactant called the substrate and facilitates that reaction by stabilizing its transition state Once that happens the enzyme can accelerate the reaction by dramatically reducing the activation energy This will evidently lead to a specific product free Gibbs energy of activation 0 Enzymes can be deactivated by a number of substances like oxygen acids bases and metal ion impurities It is left unchanged at the end of the reaction NOTE Most enzymes are proteins A cofactor is a small molecule that binds to an enzyme An example of a cofactor is the B family of vitamins A cofactor empowers an enzyme to function at maximal catalytic effectiveness or endurance Applications An antibody is a protein that s synthesized by the immune system in response to a foreign protein that enters the body It s purpose is to protect the body from that foreign substance An antigen is a foreign protein or foreign chemical that enters the body A hapten is a small molecule that elicit an immune response e g produce antibodies only when attached to a large carrier such as a protein REMEMBER The way that a catalyst increases the rate of a reaction is by decreasing the free energy of activation Transition State Eyring Theory REMEMBER A transition state is an unstable species at a free energy state maximum The difference between the free energy of the reactants and the free energy of products is 1 1 1 Arrhenius Equation 1 constant or number are 1 1 Catalytic constant turnover number The catalytic constant or Fumarase turnover number is defined as the maximum rate divided by the concentration of enzyme active sites M The units for this 2 5 103 1 25 The image above features the enzyme fumarase catalyzing the hydration of fumarate to L malate with a catalytic constant of reaction is as follows The specific mechanism for this at 1 The OH bond will detach from the hydrogen atom and attacks the central carbon atom of fumarate This will break down the double bond between the central carbon atom and the other carbon atom The central carbon atom will now have a OH OOC and a H substituent attached to it In addition that central carbon atom and top carbon atom share a single bond between them 2 The top carbon atom will then attack the hydride ion forming a C H bond The top carbon atom will now have a COO a H and another H substituent attached to it 3 The final product will be L Malate Enzyme kinetics background and definition Biological catalysts DO NOT SHIFT EQUILIBRIUM THEY JUST ACCELERATE THE REACTION E G THEY INCREASE THE RATE OF THE REACTION They accelerate the reaction by reducing the activation energy free Gibbs energy of activation Examples include proteins RNA ribozymes Enzymes are catalysts that speed up a reaction Michaelis Menten MM kinetics To recap Enzyme catalysis Mechanism E enzyme S substrate ES enzyme substrate complex P Product NOTE Like any other reaction the enzyme substrate complex can go forwards or backwards e g it can form the product s or dissociate back into its substrate and enzyme However these kinds of reactions are typically thermodynamically stable so the dissociation back into a substrate and enzyme almost never happens The enzyme catalysis mechanism can be split into 2 steps R R 1 2 Michaelis Menten MM Constant 0 0 the maximum reaction speed the initial reaction speed maximum reaction speed the substrate concentration where the initial reaction speed is one half of the can be utilized to quantify an enzyme s ability to catalyze reactions 1 A small Km indicates that the enzyme requires only a small amount of substrate to become saturated Hence the maximum velocity is reached at relatively low substrate concentrations 2 A large Km indicates the need for high substrate concentrations to achieve maximum reaction velocity Catalytic Constant 0 0 catalytic constant the enzyme concentration the number of catalytic sites per enzyme molecule maximum rate