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Enzymes II
| Question | Answer |
|---|---|
| apoenzyme | enzyme which requires a cofactor but does not have one bound ; catalytically inactive |
| haloenzyme | apoenzyme and cofactor ; catalytically active |
| cofactors | all facilitate enzymatic reaction |
| coenzyme | small organic molecules |
| co-substrate | enter and exit the active site ex: NAD+ |
| prosthetic group | tightly bound often by a covalent bond ex: heme |
| Three mechanisms for enzyme catalysis | acid-base, covalent catalysis, metal ion catalysis |
| acid-base | an enzyme can use acid catalysis, base catalysis, or both |
| covalent catalysis | also nucleophilic catalysis |
| metal ion catalysis | contains a metal ion in its structure |
| Acid Catalysis | H+ transfer from an acid lowers the free energy of the transition state |
| Base Catalysis | H+ is abstracted by a base to lower the free energy of the transition state |
| Amino acids that play a role in acid-base catalysis | Asp, Glu, His, Lys, Cys, Tyr |
| Covalent Catalysis | accelerate reactions by forming a covalent bond between enzyme and substrate |
| Nucleophile | Electron pair donor Ser, Tyr, Cys, Lys, His |
| Electrophile | WANTS electrons H+ : protons, M^n+ : mental ions, Carbonyl carbon atom, cationic imine |
| metal ions as catalysts | alcohol dehydrogenase acetaldehyde to ethanol |
| serine proteases : chymotrypsin | acid-base and covalent catalysis |
| chymotrypsin is | a serine protease |
| Ser 195 | essential for catalysis |
| Catalytic triad | Asp, His, Ser |
| Serine proteases are | 3 conserved amino acid in their active sites |
| Asp 102 | anchors His 57 |
| His 57 | BABA acts as a base than an acid |
| Ser 195 acts as | nucleophile |
| feature of ser protease mechanism base catalyst | scissile bond |
| features of ser protease mechanism acid catalyst | tetrahedral intermediate ; bond breaks |
| Properties of Enzyme Catalyst | transition state stabilization proximity and orientation effects induced fit electrostatic catalysis |
| transition state stabilization: oxyanion hole | oxygen ions form two new hydrogen bonds with the backbone NH groups of Ser 195 and Gly 193 |
| proximity and orientation effects | when enzymes bind substrates, the substrates are brought into proximity and in the correct orientation to make a chemical reaction favorable |
| induced fit | when hexokinase binds to glucose(the substrate) a conformational change occurs in the enzyme to fit the substrate better |
| evolution has produced a number of serine proteases that differ in | substrate specificity |
| zymogens | inactive precursors of enzymes |
| zymogens are activated by | proteolysis |
| chymotrupsin, trypsin, and elastase | have similar structures |
| chymotrypsin | c-terminal side of aromatic amino acids Phe, Tyr, Trp pr other large hydrophobic side chains such as Leu |
| Trypsin | c-terminal side of basic amino acids, Lys and Arg |
| Elastase | c-terminus of small nonpolar amino acids; alanine, valine, and glycine |
| zymogens need to be cleaved for activation? T or F | True |
| Histidine acts as | BABA |
| First Step: | active site His acts as base catalyst to active site Ser |
| Second Step: | O atom of active site Ser attacks carbonyl C of the scissile bond |
| Step 3: | active site His acts as acid catalyst to N atom of the scissile bond |
| The nucleophile in the second half of the chynotrypsin serine protease mechanism is | water |
| The nucleophile in the first half of the chynotrypsin serine protease mechanism is | serine |
Created by:
sofialorena