Enzymes PDF

Enzymes  What is an Enzyme? Enzymes are protein catalysts that speed up chemical reactions by lowering the activation energy required. Enzymes are NOT used up during a reaction. Enzymes are specific to a particular substrate (reactant).  How do enzymes work? They lower th...

Enzymes  What is an Enzyme? Enzymes are protein catalysts that speed up chemical reactions by lowering the activation energy required. Enzymes are NOT used up during a reaction. Enzymes are specific to a particular substrate (reactant).  How do enzymes work? They lower the activation energy of a reaction.  …FUNCTION Act by lowering the activation energy (Ea) needed to break bonds between reactant molecules Add/remove protons Twist/bend molecules Increase the probability of molecules coming in close proximity and reacting Page 37, 1.29: Figure 1  ENZYME NOMENCLATURE Molecule acted upon + suffix “ase” (hydrolytic enzymes) Examples: Amylase: breaks down starch Lactase: catalyzes lactose Dextrase: converts dextrose into lactic acid Peptidase: breaks peptide bonds Class: Proteases: catalyze proteins Class: Lipases: catalyze lipids *many enzymes also end with “in” e.g. trypsin, pepsin*  LOCK AND KEY MODEL Enzymes are specialized for a particular type of reaction The substrate and the active site must possess complimentary shapes for binding to occur  How do enzymes lower activation energy? They form enzyme-substrate complexes. Substrates bind to a region on the surface of enzymes known as the active site, to form an enzyme-substrate complex. The active site undergoes a slight conformation change to better accommodate the substrate (induced fit).  INDUCED-FIT MODEL Enzyme is capable of changing its shape slightly to lock in the substrate (securely). Anabolic / Catabolic  How does the enzyme-substrate complex lead to lowering activation energy? In catabolic reactions the interactions between the substrate and enzyme causes stress or distorts the bonds in the substrate, allowing bonds to break  How does the enzyme-substrate complex lead to lowering activation energy? In anabolic reactions the enzyme allows 2 substrates to have proper orientation to allow bonds to form between them  Factors that Affect Enzyme Activity As temperature rises, reacting 1. Temperature molecules gain more kinetic energy 🡪 increases chances of successful collisions. There, the rate of the reaction increases. Eventually, at a set OPTIMAL temperature, the enzyme’s activity is at it’s greatest. E.g. In humans, the optimal temperature of all enzymes is 37oC. As temperature increases, higher than the optimal temperature, the. enzyme denatures  Factors that Affect Enzyme Activity Enzymes work within 2. pH a very small pH range. Optimal pH is the level at which an enzyme’s activity is the greatest. pH levels outside of the optimal range can cause denaturing of the enzyme.  Factors that Affect Enzyme Activity 3. Concentration of substrate and enzyme The rate of reaction will increase with an increase of either substrate or enzyme concentration. However, with an increase in substrate concentration, eventually all of the active sites of the enzymes become occupied all at once (point of saturation). Before any more reactions can occur an enzyme/substrate complex has to dissociate to free up an active site.  Factors that Affect Enzyme Activity 4. Inhibition 4 a) Competitive Inhibitors are so similar to an enzyme’s substrate that they can bind to the active site and block the normal substrate. 4 b) Non-competitive Inhibitors bind to the enzyme at an allosteric site (not the active site) and cause a conformation change in the enzyme, preventing the normal substrate from binding.  Allosteric Regulation Cells control enzyme activity to coordinate cellular activities. Activators may bind to allosterically controlled enzymes to stabilize its shape and keep all active sites available. Allosteric inhibitors may bind to allosterically controlled enzymes to stabilize the inactive form of the enzyme.  Factors that Affect Enzyme Activity https://www.youtube.com/watch?v=6oNb4VEvy8I 4. Inhibition – cont’d 4 c) Feedback Inhibition A method used by cells to control metabolic pathways involving a series of reactions. A product formed later in a sequence of reactions allosterically inhibits an enzyme that catalyzes the reaction earlier on. FEEDBACK INHIBITION FEEDBACK INHIBITION  Factors that Affect Enzyme Activity Feedback Inhibition Feedback Inhibition - Prevents too much buildup of product In the example below five enzymes acting in sequence catalyze the pathway. The final product of the sequence, isoleucine, acts as an allosteric inhibitor of the first enzyme of the pathway, threonine deaminase. As the pathway produces isoleucine, any molecules made in excess of cell requirements combine reversibly with threonine deaminase at a location outside the active site.  Factors that Affect Enzyme Activity Feedback Inhibition The combination converts threonine deaminase to the T state and inhibits its ability to combine with threonine. The pathway is then turned off. If the concentration of isoleucine later falls as a result of its use in cell synthesis, isoleucine releases from the threonine deaminase enzymes, converting them to the R state in which they have high affinity of the substrate, conversion of threonine to isoleucine takes place.  Factors that Affect Enzyme Activity 5. Cofactors and Coenzymes - required by some enzymes to function - they bind to the active sites of enzymes Cofactors are inorganic, non-protein components, usually attract electrons in the substrate to assist in breaking bonds (Zn2+ and Mn2+). Coenzymes are organic, non-protein molecules, such as the derivatives of many vitamins. – They often shuttle molecules from one enzyme to another. (e.g. vitamin B3 is a coenzyme of NAD+)