Enzymes Organic catalysts

Energy Energy is the ability to do work. What are sources of energy? Energy availability varies: –Potential - stored –Kinetic – available Molecules can represent stored energy as C-C bonds.

Chemical Reactions Reactants and Products have different levels of energy Exergonic reactions release energy present in reactants. Endergonic reactions store energy in the products.

Exergonic vs endergonic

Exergonic reactions ate.swf

Energy The energy that is needed to start the reaction is the activation energy: E A The difference in energy level between the reactants and the products is free energy (  G). The energy freed by one reaction can power another.

Metabolism requires enzymes The chemical reactions of life need energy input to start. Enzymes lower the amount of energy required to start chemical reactions. This means reactions can run under normal physiological conditions.

Enzyme catalyzed reactions The reactant(s) bind with the enzyme at its active site. Formation of the enzyme substrate complex allows the reaction to occur without adding much energy. The products are released. The enzyme is unchanged and ready for more substrate.

Enzyme substrate complex The active site’s shape closely matches the substrate. When the substrate enters the active site, the enzyme’s shape changes to match the substrate more perfectly. Called the induced fit model. Old idea: enyzme like a lock built exactly to fit a specific key, (substrate).

The induced fit between an enzyme and its substrate

Enzymes are bidirectional Enzymes can work both to build up a product or to break down a molecule. Sucrase action: Glucose + Fructose  Sucrose and Sucrose  Glucose + Fructose

Measuring enzyme activity Enzyme activity can be measured by looking at either: the amount of substrate disappearing OR the amount of product appearing during a set period of time Units for rate would be something like mg/min.

Enzymes are efficient Increasing the amount of substrate increases the rate of activity for a set amount of enzyme until the enzymes are saturated (every active site binds a substrate). Draw a graph of this! An “average” enzyme molecule can turn over 1000 molecules of substrate to product per second!

Optimal conditions vary Why would changes in temperature or pH change enzyme function?

Cofactors and coenzymes Some enzymes require an additional molecule to bind with the substrate at the active site: – cofactors are inorganic -minerals – coenzymes are organic - vitamins coenzymes and cofactors are temporarily changed during the reaction

Controlling Enzymes Enzymes can be shut down or inhibited to help maintain homeostasis. Competitive inhibitors bind to the active site so the substrate can’t. Noncompetitive inhibitors bind to a different part of the enzyme and change the active site so the substrate doesn’t fit.