Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

9 Enzymatic Transformations
of Amino Acids
9.1 Introduction
a-Amino acids are primary cellular metabolites that are required for the assembly
of proteins, as discussed in Chapter 2. They are also used for the biosynthesis
of alkaloids – a class of nitrogen-containing natural products produced
by many organisms. There is, therefore, a sizeable group of enzymatic reactions
involved in the biosynthesis, breakdown and transformation of a-amino acids.
In this chapter we shall discuss the common enzymatic transformations of
a-amino acids, focusing in particular on the role of the coenzyme pyridoxal
5 0 -phosphate (PLP).
Illustrated in Figure 9.1 are the general types of transformations found for
a-amino acids. As discussed in Chapter 2, the amino acids used for protein
R
racemases,
epimerases
PLP
CO −
2
H
NH 3
decarboxylases
PLP or
N-pyruvoyl
R
H
H
NH 3
reactions at
β or γ positions
β H
CO −
R 2 γ α
NH 3
transaminases
PLP PMP
dehydrogenases
NAD +
oxidases
FAD + O 2
R
O
NH 3
H 2 O
CO 2
−
PLP
R'
H
NH 3
CO 2
−
R
NH 2
CO 2
−
Figure 9.1 Types of enzymatic transformations of a-amino acids. FAD, Xavin adenine dinucleotide;
NAD, nicotinamide adenine dinucleotide; PLP, pyroxidal 5’-phosphate; PMP, pyridoxamine
5 0 -phosphate.
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