Essential Knowledge 4.B.1: Interactions between molecules affect their structure and function

Biological CatalystsReduce Energy Barrier  Enzymes belong to what class of macrmolecule? ◦ proteins  How do enzymes work? ◦ They lower the activation energy required to get a reactions started

The structure of an enzyme determines its function. Induced Fit of substrate + enzyme = enzyme substrate complex  Why is the shape of an enzyme important? ◦ Enzymes work with specific substrates ◦ Enzymes must be able to bind to the substrate ◦ The substrate must be complementary to the surface properties (shape and charge) of the active site ◦ The substrate must fit into the enzyme’s active site

Substrates Substrates enter active site. Enzyme-substrate complex Enzyme Products Substrates are held in active site by weak interactions. Active site can lower E A and speed up a reaction. Active site is available for two new substrate molecules. Products are released. Substrates are converted to products

 Enzyme Facts: ◦ Enzymes are reusable ◦ Enzymes are substrate specific ◦ Enzymes lower the activation energy of a reaction ◦ Enzymes change shape upon binding to the substrate – thus holding the substrate in such a way to favor breaking existing bonds and making new ones – Induced Fit

 Many enzymes require nonprotein helpers in order to function ◦ Cofactors and coenzymes interact with enzymes to cause structural changes that alter the activity rate of the enzyme ◦ The enzyme may only become active when all the appropriate cofactors or coenzymes are present and bind to the appropriate sites on the enzyme

 What is the difference between a cofactor and a coenzyme? ◦ Cofactor – inorganic, such as the metal atoms zinc, iron and copper in ionic form ◦ Coenzyme – organic molecule; many are vitamins

 Other molecules and the environment in which the enzyme acts can enhance or inhibit enzyme activity. ◦ Environmental factors that may alter enzyme shape (particularly the tertiary structure) thereby altering enzyme function  Temperature  pH  Salt concentration

Optimal temperature for typical human enzyme (37°C) Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria (77°C) Temperature (°C) (a) Optimal temperature for two enzymes Rate of reaction pH (b) Optimal pH for two enzymes Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme)

 Molecules can bind reversibly or irreversibly to an enzyme’s active site or an allosteric site – changing the activity of the enzyme  Regulation of enzyme activity helps control metabolism  Allosteric regulation – (allo refers to “other”) a regulatory molecule binds to the enzyme at a site other than the active site - can either speed up (activator) or slow down (inhibitor) the reaction  Cooperativity – a substrate molecule binding to one active site in a multi-subunit enzyme triggers a shape change in all the subunits – increasing the reaction

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

Regulatory site (one of four) (a) Allosteric activators and inhibitors Allosteric enzyme with four subunits Active site (one of four) Active form Activator Stabilized active form Oscillation Non- functional active site Inactive form Inhibitor Stabilized inactive form Inactive form Substrate Stabilized active form (b) Cooperativity: another type of allosteric activation

 Feedback inhibition is a common mode of metabolic control. An end product of a metabolic pathway can bind to an enzyme at the start of the pathway – changing the shape of the enzyme thereby “inhibiting” the reaction from taking place

Active site available Isoleucine used up by cell Feedback inhibition Active site of enzyme 1 is no longer able to catalyze the conversion of threonine to intermediate A; pathway is switched off. Isoleucine binds to allosteric site. Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Intermediate B Intermediate C Intermediate D Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 End product (isoleucine)

 The change in function of an enzyme can be interpreted from data regarding the concentrations of product or substrate as a function of time.  These representations demonstrate the relationship between an enzyme’s activity, the disappearance of substrate, and/or presence of a competitive inhibitor.

 What could we measure? ◦ Disappearance of H ◦ Production of H 2 O and/or production of O 2 ◦ Heat given off