BCHS 3304 Lecture Notes - Lecture 17: Covalent Bond, Electrophile, Pancreatic Ribonuclease
17 Nov 2016
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Lecture 17: acid base catalysis= via keto-enol tautomerization. Ex: rnase a hydrolyzes rna phosphate linkages. His 12- initially acts as a base. His 119- initially acts as an acid. After they initially react, his 12 and 119 switch roles. His can act as an acid or base depending on the conditions. This enzyme helps cleave rna into smaller molecules. Ionization state of asp, glu, lys, arg, his are affected by ph: at ph lower than pi they are protonated. This means they are positive or neutral: at a ph higher than pi they are deprotonated. This means they are negative or neutral. Optimal activity at neutral ph: covalent catalysis. A transient covalent bond is formed between the enzyme and substrate. Covalent bond accelerates the reaction: bond forms through nucleophilic catalysis. Must be a good leaving group because catalyst is eliminated at the final step. After binding to substrate it becomes electrophilic: withdraws electrons to stabilize transition state.
