Lecture 9-22 - Lecture-9/22 (CH 6) Enzymes-Biological...

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Lecture-9/22 (CH 6) Enzymes--Biological Catalysts Catalyst-a substance the increases the rate of a thermodynamically favored reaction without itself being changed or consumed in the overall process---it may be reused in subsequent reactions Enzyme catalysts contain unique sites--called active sites---where the substrates bind and the reaction takes place Why do we need enzymes? Chemical reactions require initial input of energy--usually in the form of increased heat o Raising the temperature increasing the rate of vibration of the molecules and the chance of collision o An increase in the concentration of reactants can also increase the chances of a chemical reaction occuring o HEAT and MORE REACTANTS can increase chance of chemical reaction occurring Biological systems cannot raise heat or concentrations at will Enzyme catalysts enhance the rate of a reaction Classes of enzymes oxidoreductases--oxidation/reduction; requires a co-factor such as NAD or FAD o A- +B----> A + B- transferases--transfer of a functional group o A-B + C ----> A + B-C hydrolases--hydrolysis of functional group by water o A-B + H20 ---> A-H + B-OH lyases--elimination to form double bond; or addition to a double bond o X-A-B-Y ----> A=B + X-Y isomerases--isometric interconversions o X-A-B-Y ---> Y-A-B-X ligases--ATP dependent joining to two molecules o A + B +ATP ----> A-B + ADP + Pi How do enzymes do that? --Catalysis animation A chemical reaction occurs only if the molecules possess a minimum amount of energy--- Activation Energy enzymes provide alternate pathway, stabilization of transition state--same as adding heat o lowers activation energy, but does not change free energy required for the reaction to occur
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o alters the rate, but not thermodynamics enzymes provide a surface for the reaction to occur, bringing reactants into close proximity to each other--functional equivalent of increasing concentration enzymes catalyze the hydrolysis of metastable compounds-nucleic acids, proteins, carbohydrates, glucose, ATP, etc. o metastable compounds are thermodynamically unstable, but the rate of hydrolysis is kinetically very slow in the absence of an enzyme catalyst, e.g., glucose enzymes participate in a reaction, but are not consumed by it--reused over and over again enzymes subject to regulation by the cell not all enzymes are proteins: catalytic RNAs (ribozymes) can direct their own splicing or have catalytic activity such a peptide bond formation in a ribosome Once bound the substrate reaches the transition state and bonds are rearranged. The enzyme active site places atoms in close proximity to each other orients substrate correctly these two effect facilitate the breaking and reforming of bonds Cofactors, coenzymes or cosubstrates small molecules that associate with an enzyme and are modified in the enzymatic reaction, but returned to original state required for oxidation/reduction or group transfer reaction
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This note was uploaded on 12/31/2010 for the course CH 369 taught by Professor Kbrowning during the Spring '07 term at University of Texas at Austin.

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Lecture 9-22 - Lecture-9/22 (CH 6) Enzymes-Biological...

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