(Ap 4 A) and other polyphosphates such as Ap 3 A (Figure 3.63) are stable minor nucleotide species found in
low concentration in all mammalian tissues.
3.3.1.1 Monoalkyl Esters. Monoalkyl esters are the most ubiquitous examples of P^1 -nucleoside esters of
polyphosphates. They include the ribo- and deoxyribo-nucleoside esters of pyrophosphoric acid (NDPs and
dNDPs) and of tripolyphosphoric acid (NTPs and dNTPs) (Figure 3.61). These esters are metabolically labile
and participate in a huge range of C O and P O cleavage processes; thiamine pyrophosphate is a co-enzyme.
Among the minor nucleoside polyphosphates, the ‘magic spot’ nucleotides MS1 (ppGpp) and MS2
(pppGpp) are species formed by stringent strains of Escherichia coliduring amino acid starvation.
3.3.1.2 Dialkyl Esters. Dialkyl esters are biologically significant for di-, tri-, tetra- and penta-
polyphosphoric acids. In every case, the esters are located on the two terminal phosphate residues leaving
(as described below) an ionic phosphate at every position, which ensures stability during spontaneous
hydrolysis. Several of the co-enzymes such as co-enzyme A, flavine adenine dinucleotide (FAD) and nicoti-
namide adenine dinucleotide (NAD) are stable P^1 ,P^2 -diesters of pyrophosphoric acid (Figure 3.62a). Their
112 Chapter 3
HO O
HO
OHOOHOHOHO
OHOOHO OHOHNNN
NNH 2HO OHOO
OP
OO
P
OOOO OHO OHNNN
NNH 2PO
O
OROPOOCONH 2N
HO OHO
NAD+HO OHP POO
NNHOR'O O O
OOO ONH
N NNOO
CH 2 (CHOH) 3 CH 2MeMeFADHSH
N
H
N
OOMe MeOHR =
abCo- enzyme A (ADP unit has
additional 3'-phosphate residue)cyclic ADP riboseR' =R' =UDP-glucoseUDP-galactosecFigure 3.62 (a) Structures of three adenosine co-enzymes; (b) Structure of cyclic ADP ribose; (c) Structures of
UDP-hexoses
O
OHO OHNNN
NNH 2P
OOOPO
POOOO OOHONNNH 2OO
P
OOOPOOOdCDPβ α γ β αATPFigure 3.61 Structures of nucleoside 5-di- and tri-phosphates