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Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

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224 Chapter 9<br />

The biosyntheses of nicotine (from l-lysine <strong>and</strong> l-ornithine), morphine<br />

(from two molecules of l-tyrosine) <strong>and</strong> strychnine (from l-tryp<strong>to</strong>phan) also<br />

involve intermediate imine <strong>and</strong> enamine species, as well as examples of phenolic<br />

radical couplings encountered in Section 7.10. Readers interested further in<br />

alkaloid biosynthesis should consult the Further reading below.<br />

Problems<br />

(1) Suggest a mechanism for the reaction catalysed by threonine b-epimerase, a<br />

PLP-dependent enzyme.<br />

OH<br />

H<br />

CO −<br />

2<br />

NH +<br />

3<br />

threonine<br />

β-epimerase<br />

PLP<br />

OH<br />

H<br />

CO −<br />

2<br />

NH +<br />

3<br />

(2) Aspartate aminotransferase is inactivated in time-dependent fashion by<br />

l-vinyl-glycine. Suggest a mechanism of inactivation.<br />

H<br />

CO −<br />

2<br />

NH 3<br />

+<br />

L-vinyl-glycine<br />

(3) g-Aminobutyrate transaminase has an important function in the human<br />

brain, where it catalyses the conversion of g-aminobutyrate (a neurotransmitter)<br />

<strong>to</strong> succinate semialdehyde. This enzyme is inactivated (K i 0:6 mm)<br />

by gabaculine. Suggest a mechanism of inactivation.<br />

− O 2 C<br />

NH 3<br />

+<br />

+<br />

O<br />

CO 2<br />

−<br />

γ-aminobutyrate<br />

transaminase<br />

PLP<br />

− O 2 C<br />

O<br />

+<br />

H<br />

CO −<br />

2<br />

NH 3<br />

+<br />

CO 2<br />

−<br />

gabaculine<br />

NH 3<br />

+

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