Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Appendix 4 279
insect!); (ii) diVerences in active site structure between human and insect
enzymes.
Antidote binds to choline site through positively-charged pyridinium
group. Hydroxylamine group is a potent nucleophile which attacks the
neighbouring tetrahedral phosphate ester. Thus the rate of hydrolysis of
the tetrahedral phosphate adduct is rapidly accelerated.
(3) Aldehyde is attacked by active site cysteine, generating a thio-hemiacetal
intermediate which mimics the tetrahedral intermediate of the normal
enzymatic reaction, and is hence bound tightly by the enzyme.
(4) Acetate kinase gives acyl phosphate intermediate, acetate thiokinase involves
acyl adenylate (RCO.AMP) intermediate.
(5) Glycogen phosphorylase cleaves glucose units successively from end of
chain. Reaction proceeds with retention of conWguration at the anomeric
position, so a covalent intermediate is probably formed (cf. lysozyme)
by attack of an active site carboxylate. Displacement by phosphate gives
a-d-glucose-1-phosphate. Phosphoglucose isomerase contains phosphorylated
enzyme species which transfers phosphate to C-6 to give 1,6-diphospho-glucose.
Dephosphorylation at C-1 regenerates phosphoenzyme
species. Glucose-6-phosphatase is straightforward phosphate monoester
hydrolysis. Defect in glycogen phosphorylase leads to inability to utilise
glycogen, so unable to maintain periods of physical exercise.
(6) Processed by enzyme to give covalent 2 0 -Xuoro glycosyl enzyme intermediate.
2 0 -Fluoro substituent is electron-withdrawing, destabilises oxonium
ion, therefore slows down the hydrolysis of the covalent intermediate.
Chapter 6
(1) Alcohol dehydrogenase: ( 0:16 (CH 3 CHO)) ( 0:32( NAD þ )) ¼þ0:16V:
Enoyl reductase: ( þ 0:19 (enoyl CoA)) ( 0:32 (NAD þ )) ¼þ0:49V:
Acyl CoA dehydrogenase: (þ0:25 (cytc ox )) ( þ 0:19 (enoyl CoA)) ¼þ0:06V:
In acyl CoA dehydrogenase the redox potential for the intermediate FAD
must be close to þ0:19V if electron transfer is to be thermodynamically
favourable. This is right at the top end of the redox potential range for Xavin.
(2) Enzyme transfers proR hydrogen of NADPH onto C-3 position of substrate.
Overall syn-addition of hydrogens from NADPH and water.
(3) Transfer of H onto enzyme-bound NAD þ . Resulting C-4 ketone assists
the E1cb elimination of C-6 hydroxyl group to give unsaturated ketone
intermediate. Transfer of H from cofactor to C-6 gives product.
(4) Transfer of proS hydrogen of NADPH to FAD. Then reverse of acyl CoA
dehydrogenase mechanism: transfer of H onto b-position giving a-radical;
electron transfer from Xavin semiquinone to give a-carbanion; protonation