Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

100 Chapter 5
The aminoacyl group of amino acids is activated and transferred during
the assembly of the polypeptide chains of proteins by ribosomes. Amino acid
activation is carried out by adenosine triphosphate (ATP)-dependent amino
acyl transfer RNA (tRNA) synthetase enzymes. Each individual amino acid
is converted into an acyl adenylate mixed anhydride derivative, followed
by transfer of the aminoacyl group onto a speciWc tRNA molecule. The aminoacyl-tRNA
ester is then bound to the ribosome and the free amine used to
form the next amide bond in the sequence of the protein (see Figure 5.23).
Activation and transfer of acyl groups is a common process found in fatty
acid biosynthesis, polyketide natural product biosynthesis, and the assembly of
a variety of amide and ester functional groups in biological molecules. For the
majority of these processes a special cofactor is used – coenzyme A (CoA).
The structure of CoA contains a primary thiol group which is the point of
attachment to the acyl group being transferred, forming a thioester linkage
shown in Figure 5.24.
H 3 C
O
O −
alanyl t RNA
synthetase
H 3 C
O
O
O
P
O −
Ad
NH 3
+
ATP
PP i
NH 3
+
H 3 C
O
peptide-NH
t RNA-OH
AMP
O
O
O− t RNA
O− t RNA
ribosome H 3 C
NH
NH 2 O− t RNA
O
R n−1
R n−1
peptide-NH
Figure 5.23 Acyl transfer reactions in protein biosynthesis. PP i , inorganic pyrophosphate.
NH 2
O
S
N
H
O
N
H
O
O
P
O
OH O O − O 2− O 3 PO OH
N
N
N
N
Figure 5.24 Structure of acetyl CoA.