4 th SEMESTER – BOTANY KARNATAKA UNIVERSITY, DHARWAD Modified from various internet resources by Dr. Jayakara Bhandary Associate Professor of Botany Government Arts & Science College Karwar, Uttara Kannada 1Introduction & History _x0001_ Enzyme, in Greek means in living (en= in, zyme = living). _x0001_ Biocatalysts or Organic catalysts, usually high molecular weight proteins (exception- Ribozymes or RNA enzymes). _x0001_ Coined by Kuhne in _x0001_ First enzyme extract from Yeast cells by Buchner (1897). _x0001_ First purified enzyme is urease, by James B. Summer (1926). 2Enzymes are Biological Catalysts Enzymes are proteins that: _x0001_ Increase the rate of reaction by lowering the energy of activation. _x0001_ Catalyze nearly all the chemical reactions taking place in the cells of the body. _x0001_ Have unique three- dimensional shapes that fit the shapes of reactants. 3Activation Energy Think of activation energy as the BARRIER required to make a product. Most stable product is the one with the lowest energy. Most reactions require a “push” to get them started! “Push” is called “energy of activation” for reaction - Also represented by EA 4Trivial Names of Enzymes The name of an enzyme: _x0001_ Usually ends in –ase. _x0001_ Identifies the reacting substance. For example, sucrase catalyzes the reaction of sucrose. _x0001_ Describes the function of the enzyme. For example, oxidases catalyze oxidation. _x0001_ Could be a common name, particularly for the digestion enzymes such as pepsin and trypsin. 5IUB Classification of Enzymes _x0001_ Enzymes are classified according to t _x0001_ they catalyze. Class Oxidoreductases Transferases Hydrolases Lyases _x0001_ Isomerases _x0001_ Ligases Reactions catalyzed Oxidation-reduction Transfer groups of atoms Hydrolysis Add atoms/remove atoms to/from a double bond Rearrange atoms Use ATP to combine molecules 6Systematic Name _x0001_ According to the International union Of Biochemistry an enzyme name has two parts: -First part is the name of the substrates for the enzyme. -Second part is the type of reaction catalyzed by the enzyme.This part ends with the suffix “ase”. Example: Lactate dehydrogenaseEC number Enzymes are classified into six different groups according to the reaction being catalyzed. The nomenclature was determined by the Enzyme Commission in 1961 (with the latest update having occurred in 1992), hence all enzymes are assigned an “EC” number. The classification does not take into account amino acid sequence (ie, homology), protein structure, or chemical mechanism.EC numbers § EC numbers are four digits, for example a.b.c.d, where “a” is the class, “b” is the subclass, “c” is the sub- subclass, and “d” is the sub-sub-subclass. The “b” and “c” digits describe the reaction, while the “d” digit is used to distinguish between different enzymes of the same function based on the actual substrate in the reaction. § Example: for Alcohol:NAD + oxidoreductase EC number is The Six Classes _x0001_ EC 1. Oxidoreductases _x0001_ EC 2. Transferases _x0001_ EC 3. Hydrolases _x0001_ EC 4. Lyases _x0001_ EC 5. Isomerases _x0001_ EC 6. LigasesEC 1. Oxidoreductases _x0001_ Catalyze the transfer of hydrogen or oxygen atoms or electrons from one substrate to another. _x0001_ Since these are ‘redox’ reactions, an electron donor/acceptor is also required to complete the reaction. A H 2 +B → A+ BH 2 Ex. Oxidases, Dehydrogenases, Reductases.EC 2. Transferases § Catalyze group transfer reactions, excluding oxidoreductases (which transfer hydrogen or oxygen and are EC 1). These are of the general form: § A-X + B ↔ BX + A § Ex: Transaminases (transfer amino group), Kinases (transfer Phosphate group)EC 3. Hydrolases _x0001_ Catalyze hydrolytic reactions. Includes. _x0001_ A-X + H 2 O ↔ X-OH + A-H _x0001_ Ex: lipases, esterases, Amylases, peptidases/proteases, etc.EC 4. Lyases § Catalyze non-hydrolytic (covered in EC 3) removal of functional groups from substrates, often creating a double bond in the product; or the reverse reaction, ie, addition of function groups across a double bond. A- X +B-Y → A=B + X-Y Ex: Decarboxylases, Aldolases, Dehydrases, Deaminases, Synthases, etc.EC 5. Isomerases _x0001_ Catalyzes isomerization reactions, including epimerizations and cis-trans _x0001_ isomerizations. A →A’ Ex: Isomerases (Cis-Trans), Epimerases (D—L)EC 6. Ligases _x0001_ Catalyzes the synthesis of various (mostly C-X) bonds, coupled with the breakdown of energy- containing substrates, usually ATP. A + B → A-B ATP → ADP+iP Ex: Synthetases, CarboxylasesClassification of Enzymes: Oxidoreductases and Transferases 17Classification: Hydrolases and Lyases 18Classification: Isomerases and Ligases 19Active Site The active site: _x0001_ Is a region within an enzyme that fits the shape of molecules called substrates. _x0001_ Contains amino acid R groups that align and bind the substrate. _x0001_ Releases products when the reaction is complete. 20Enzyme Specificity Enzymes may recognize and catalyze: _x0001_ A single substrate. _x0001_ A group of similar substrates. _x0001_ A particular type of bond. 21Mechanism of Enzyme Catalyzed Reactions _x0001_ The proper fit of a substrate (S) in an active site forms an enzyme-substrate (ES) complex. E + S ES _x0001_ Within the ES complex, the reaction occurs to convert substrate to product (P). ES E + P _x0001_ The products, which are no longer attracted to the active site, are released. _x0001_ Overall, substrate is convert to product. E + S ES E + P 22Mechanism of Enzyme Action 2324Example of An Enzyme Catalyzed Reaction 25Mechanism of Enzyme Action : 1.Lock-and-Key Model In the lock-and-key model of enzyme action: _x0001_ The active site has a rigid shape. _x0001_ Only substrates with the matching shape can fit. _x0001_ The substrate is a key that fits the lock of the active site Induced-fit Model In the induced-fit model of enzyme action: _x0001_ The active site is flexible, not rigid. _x0001_ The shapes of the enzyme, active site, and substrate adjust to maximum the fit, which improves catalysis. _x0001_ There is a greater range of substrate specificity. 27Isoenzymes _x0001_ Isoenzymes catalyze the same reaction in different tissues in the body. _x0001_ Lactate dehydrogenase, which converts lactate to pyruvate, (LDH) consists of five isoenzymes. 28Temperature and Enzyme Action Enzymes: _x0001_ Are most active at an optimum temperature (usually 37°C in humans). _x0001_ Show little activity at low temperatures. _x0001_ Lose activity at high temperatures as denaturation occurs. 29pH and Enzyme Action Enzymes: _x0001_ Are most active at optimum pH. _x0001_ Contain R groups of amino acids with proper charges at optimum pH. _x0001_ Lose activity in low or high pH as tertiary structure is disrupted. 30Optimum pH Values _x0001_ Most enzymes of the body have an optimum pH of about 7.4. _x0001_ In certain organs, enzymes operate at lower and higher optimum pH values. 31Enzyme Concentration _x0001_ The rate of reaction increases as enzyme concentration increases (at constant substrate concentration). _x0001_ At higher enzyme concentrations, more substrate binds with enzyme. 32Substrate Concentration _x0001_ The rate of reaction increases as substrate concentration increases (at constant enzyme concentration). _x0001_ Maximum activity occurs when the enzyme is saturated. 33