F BICH 410 1st Edition Lecture 22 Outline of Last Lecture Enzymes Chemistry Outline of Current Lecture Catalytic Mechanisms o 3 types acid base covalent metal ion catalysis o Acid Base Catalysis proton transfer General acid catalysis acid donates proton to substrate Enzyme active site residue must be protonated General base catalysis base accepts proton from S Enzyme active site residue must be deprotonated Concerted acid base catalysis general acid and base so both participate in rxn Enzyme activity as function of Ph General base has highest activity when group deprot Ph pka B H General acid has highest activity when group prot Ph pka AH Concerted rxn has highest activity at both deprot and prot region bell shape curve for pH dependence Covalent catalysis covalent bond transiently formed between S an enzyme Usually involves nucleophile on enzyme and electrophile of S Nucleopphiles nucleophilicity of substance is related to basicity o Nucleophiles are charged or have shared e Electrophiles electro or contain unfilled valence electron shells or have electro atoms The more electrophilic S the weaker the nucleophile can be Metal Ion Catalysis Nearly 1 3 enzymes require metal for activity Melloenzymes contain tightly bound metal cofactors Fe Cu Zn o Bound Metal required for activity Metal activated enzyme loosely bound metal ions usually Na K Ca o Metal ion is used for stability not for activity Cofactor metal ion Coenzyme cosubstrate organic molec NAD Apo inactive from of enzyme when it lacks the cofactor Halo active form of enzyme with cofactor Examples of determining catalytic mechanism need to know chemical and kinetic mechanism These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Ribonucleus A chemical mechanism acid base catalysis uses hydrolase kinetic mechanism ping pong Carbonic anhydrase H20 interacts with Zn and pH decrease chemical mechanism metal ion Serine proteases proteolytic enzymes that have a serine active site o Ie chymotrypsin subtillsin thrombin blood clotting trypsin elastase plasmin breaks down clots tissue plasminogen activator cleaves plasminogen creating plasmin o Selectivity pocket o Mechanism of serine proteases catalytic triad His Asp Ser Active sites of trypsin chymotrypsin elastase have triad Asp form H bond to His orienting His so it can have electron pair exposed to bind Ser Chymotrypsin hydrolase peptide h20 2peptides Mechanism involves acylation and deacylation rather than direct h20 attack Uses presteady state analysis Chemical Mechanism Acid base and covalent catalysis kinetic mechanism ping pong initial velocity plot has parallel lines Transition State stabilization tetrahedral intermediate o Serine proteases are examples of enzymes that preferentially bind transition state o Tighter an enzyme binds the transitions tate the greater the Kcat to Kuncat Nerve toxinso acetylcholinterase works same as trypsin Serine esterase and neurotransmitter Nerves are repolarized upon hydrolysis of acetylcholine
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