SoS, Dept. of Biology, Lautoka Campus BIO508 Cell Biology Slide Design: Copyright © McGraw-Hill Global Education Holdings, LLC. Lecturer: Dr.Ramesh Subramani Topic 6: Enzymes

Enzymes Enzymes are molecules that act as catalysts to speed up biological reactions. The compound on which an enzyme acts is the substrate. Enzymes can break a single structure into smaller components or join two or more substrate molecules together. Most enzymes are proteins. Many fruits contain enzymes that are used in commercial processes. Pineapple (Ananas comosus, right) contains the enzyme papain which is used in meat tenderization processes and also medically as an anti-inflammatory agent.

Enzyme Examples Enzyme Role Pepsin Stomach enzyme used to break protein down into peptides. Works at very acidic pH (1.5). Proteases Digestive enzymes which act on proteins in the digestive system Amylases A family of enzymes which assist in the breakdown of carbohydrates Lipases A family of enzymes which breakdown lipids 3D molecular structures for the enzymes pepsin (top) and hyaluronidase (bottom).

Enzyme in Human Body One of the fastest enzymes in the body is catalase. Catalase breaks down hydrogen peroxide, a waste product of cell metabolism, into water and oxygen. Accumulation of hydrogen peroxide is toxic so this enzyme performs an important job in the body.

What Are Enzymes? Most enzymes are Proteins (tertiary and quaternary structures) Act as Catalyst to accelerates a reaction Not permanently changed in the process

Enzymes speed up metabolic reactions by lowering energy barriers A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction An enzyme is a catalytic protein Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction

Enzymes speed up metabolic reactions Sucrose C12H22O11 Glucose C6H12O6 Fructose C6H12O6

The Activation Energy Barrier Every chemical reaction between molecules involves bond breaking and bond forming The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA) Activation energy is often supplied in the form of heat from the surroundings

Energy profile of an exergonic reaction

How Enzymes Lower the EA Barrier Enzymes catalyze reactions by lowering the EA barrier Enzymes do not affect the change in free-energy (∆G); instead, they hasten reactions that would occur eventually

The effect of an enzyme on activation Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Free energy Course of reaction with enzyme DG is unaffected by enzyme Products Progress of the reaction

Substrate Specificity of Enzymes The reactant that an enzyme acts on is called the enzyme’s substrate The enzyme binds to its substrate, forming an enzyme-substrate complex The active site is the region on the enzyme where the substrate binds Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction

Induced fit between an enzyme and substrate Active site Enzyme Enzyme-substrate complex

Catalysis in the Enzyme’s Active Site In an enzymatic reaction, the substrate binds to the active site The active site can lower an EA barrier by Orienting substrates correctly Straining substrate bonds Providing a favorable microenvironment Covalently bonding to the substrate

Catalytic cycle Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. Active site (and R groups of its amino acids) can lower EA and speed up a reaction by acting as a template for substrate orientation, stressing the substrates and stabilizing the transition state, providing a favorable microenvironment, participating directly in the catalytic reaction. Substrates Enzyme-substrate complex Active site is available for two new substrate molecules. Enzyme Catalytic cycle Products are released. Substrates are converted into products. Products

Effects of Local Conditions on Enzyme Activity An enzyme’s activity can be affected by: General environmental factors, such as temperature and pH Chemicals that specifically influence the enzyme

Effects of Temperature and pH Each enzyme has an optimal temperature in which it can function Each enzyme has an optimal pH in which it can function

Environmental factors affecting enzyme activity Optimal temperature for typical human enzyme Optimal temperature for enzyme of thermophilic (heat-tolerant bacteria) Rate of reaction 20 40 60 80 100 Temperature (°C) Environmental factors affecting enzyme activity Optimal temperature for two enzymes Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) Rate of reaction 1 2 3 4 5 6 7 8 9 10 pH Optimal pH for two enzymes

Cofactors Cofactors are non-protein enzyme helpers Coenzymes are organic cofactors

Cofactors Cofactors are nonprotein enzyme helpers Cofactors may be inorganic (such as a metal in ionic form) or organic An organic cofactor is called a coenzyme Coenzymes include vitamins

Enzyme Inhibitors Competitive inhibitors bind to the active site of an enzyme, competing with the substrate. Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective. Examples of inhibitors include toxins, poisons, pesticides, and antibiotics.

Inhibition of enzyme activity A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme Normal binding A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor Inhibition of enzyme activity Competitive inhibition A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions. Noncompetitive inhibitor Noncompetitive inhibition

Regulation of enzyme activity helps control metabolism Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated To regulate metabolic pathways, the cell switches on or off the genes that encode specific enzymes

Allosteric Regulation of Enzymes Allosteric regulation is the term used to describe cases where a protein’s function at one site is affected by binding of a regulatory molecule at another site Allosteric regulation may either inhibit or stimulate an enzyme’s activity

Allosteric Activation and Inhibition Most allosterically regulated enzymes are made from polypeptide subunits Each enzyme has active and inactive forms The binding of an activator stabilizes the active form of the enzyme The binding of an inhibitor stabilizes the inactive form of the enzyme

Allosteric activators and inhibitors stabilizes active form. Allosteric enzyme with four subunits Active site (one of four) Regulatory site (one of four) Activator Allosteric activators and inhibitors Active form Stabilized active form Oscillation Allosteric inhibitor stabilizes inactive form. Non- functional active site Inhibitor Inactive form Stabilized inactive form Allosteric activators and inhibitors

Cooperativity Cooperativity is a form of allosteric regulation that can amplify enzyme activity In cooperativity, binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits

Binding of one substrate molecule to active site of one subunit locks all subunits in active conformation. Substrate Inactive form Stabilized active form Cooperativity another type of allosteric activation

Feedback Inhibition In feedback inhibition, the end product of a metabolic pathway shuts down the pathway Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed

Feedback inhibition in isoleucine synthesis Initial substrate (threonine) Active site available Threonine in active site Enzyme 1 (threonine deaminase) Isoleucine used up by cell Feedback inhibition in isoleucine synthesis Intermediate A Feedback inhibition Enzyme 2 Active site of enzyme 1 can’t bind theonine pathway off Intermediate B Enzyme 3 Intermediate C Isoleucine binds to allosteric site Enzyme 4 Intermediate D Enzyme 5 End product (isoleucine)

Specific Localization of Enzymes Within the Cell Structures within the cell help bring order to metabolic pathways Some enzymes act as structural components of membranes Some enzymes reside in specific organelles, such as enzymes for cellular respiration being located in mitochondria

Organelles and structural order in metabolism Mitochondria, sites of cellular respiration 1 µm The matrix contains enzymes in solution that are involved in one stage of cellular respiration Enzymes for another stage of cellular respiration are embedded in the inner membrane

Acknowledgements… Any Questions?? The teaching material used in this lecture is taken from: JB Reece, LA Urry, ML Cain, SA Wasserman, PV Minorsky and RB Jackson. 2011. Campbell Biology (9th Edition), Publisher Pearson is gratefully acknowledged. Some information presented in this power point lecture presentation is collected from various sources including Google, Wikipedia, research articles and some book chapters from various biology books. Material and figures used in this presentation are gratefully acknowledged. This material is collected and presented only for teaching purpose. Any Questions?? Dr.Ramesh Subramani, Assistant Professor in Biology