Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymatic Transformations of Amino Acids 211
biosynthesis have exclusively the l-conWguration. However, there are a number
of racemase and epimerase enzymes producing d-amino acids that are used for
a small number of speciWc purposes in biological systems. a-Amino acid decarboxylases
produce the corresponding primary amines, some of which have
important bodily functions in mammals, and others which are used in the
biosynthesis of alkaloids in plants. Oxidation of a-amino acids has been mentioned
in Chapter 6: there are several NAD þ - and Xavin-dependent dehydrogenase
and oxidase enzymes that oxidise amino acids via the corresponding
iminium salt to the a-keto acid. Imine intermediates then make possible a
number of further transformations as we shall see later in the chapter.
9.2 Pyridoxal 5 0 -phosphate-dependent reactions at
the a-position of amino acids
Pyridoxal 5 0 -phosphate is a coenzyme derived from vitamin B 6 (pyridoxine).
Pyridoxine is oxidised and phosphorylated in the body to give the active form
of the coenzyme, as shown in Figure 9.2. This vitamin was Wrst isolated from
rice bran in 1938, and was found to be active against the deWciency disease
pellagra.
A wide range of reaction types are catalysed by PLP-dependent enzymes;
however, in general the substrates for these enzymes are a-amino acids. The
structural features of the coenzyme which make this chemistry possible are a
pyridine ring that acts as an electron sink, and an aldehyde substituent at the
C-4 position through which the coenzyme becomes covalently attached to the
amino acid substrate. The structure of the coenzyme is shown in Figure 9.2.
Enzymes which utilise PLP bind the cofactor through an imine linkage
between the aldehyde group of PLP and the e-amino group of an active site
lysine residue. At neutral pH this imine linkage is protonated to form a more
electrophilic iminium ion. Upon binding of the a-amino acid substrate, the a-
amino group attacks the iminium ion, displacing the lysine residue and forming
an imine linkage itself with the pyridoxal cofactor. This aldimine intermediate,
shown in Figure 9.2, is the starting point for each of the mechanisms that we
shall meet in the following sections. Although for sake of simplicity I shall write
the phenolic hydroxyl group of PLP in protonated form, there is evidence that
it is deprotonated when bound to the enzyme, and that the phenolate anion
forms a hydrogen bond to the protonated iminium ion, as shown in Figure 9.2.
Formation of the aldimine adduct dramatically increases the acidity of the
amino acid a-proton. This activation is utilised by a family of racemase and
epimerase enzymes which utilise PLP as a cofactor. In these enzymes formation
of the aldimine intermediate is followed by abstraction of the a-proton of the
amino acid utilising the pyridine ring as an electron sink and generating a
quinonoid species. Delivery of a proton from the opposite face of the molecule