shown.) The residue in a peptide that has an amino
group that is free, or at least not acylated by another
amino acid residue (it may, for example, be acylated
or formylated), is called N-terminal; it is the N-
terminus. The residue that has a free carboxy group,
or at least does not acylate another amino acid
residue (it may, for example, acylate ammonia to give
–NH–CHR–CO–NH 2 ), is called C-terminal.
The following is a list of symbols for amino acids
(use of the one-letter symbols should be restricted to
the comparison of long sequences):
aminoacyl-tRNA synthetases (aaRSs) When ribo-
somespair a tRNA (transfer ribonucleic acid) with a
codon (three bases in a DNA or RNA sequence), an
amino acid is expected to be carried by the tRNA.
Since each tRNA is matched with its amino acid
before it meets the ribosome, the ribosome has no
way of knowing if the match was made. The match is
made by a family of enzymes called aminoacyl-tRNA
synthetases. These enzymes charge each tRNA with
the proper amino acid via a covalent ester bond,
allowing each tRNA to make the proper translation
from the genetic code of DNA into the amino acid
code of proteins. Cells make at least 20 different
aminoacyl-tRNS synthetases, one for each of the
amino acids.
Aminoacyl-tRNA synthetases belong to two
classes, depending on which amino acid they specify.
Class I enzymes usually are monomeric and attach to
the carobxyl of their specific amino acid to the 2’ OH
of adenosine 76 in the tRNA molecule. Class II
enzymes are either dimeric or tetrameric and attach
to their amino acids at the 3’ OH. These enzymes
catalyze first by activating the amino acid by forming
an aminoacyl-adenylate. Here the carboxyl of the
amino acid is linked to the alpha-phosphate of ATP,
displacing pyrophosphate. After the corrected tRNA
is bound, the aminoacyl group of the aminoacyl-
adenylate is transferred to the 2’ or 3’ terminal OH
of the tRNA.
Recent studies have shown that aminoacyl-tRNA
synthetases can tell the difference between the right and
the wrong tRNA before they ever start catalysis, and if
the enzyme binds aminoacyl-adenylate first, it is even
more specific during tRNA binding. Previous studies
have also proved that aminoacyl-tRNA synthetases
reject wrong tRNAs during catalysis. Other research
has shown that specific aaRSs play roles in cellular
fidelity, tRNA processing, RNA splicing, RNA traffick-
ing, apoptosis, and transcriptional and translational
regulation. These new revelations may present new
evolutionary models for the development of cells and
perhaps opportunities for pharmaceutical advance-
ments.amino group(–NH 2 ) Afunctional group (group of
atoms within a molecule that is responsible for certain
properties of the molecule and reactions in which it
takes part), common to all amino acids, that consists of
a nitrogen atom bonded covalently to two hydrogen
atoms, leaving a lone valence electron on the nitrogen
atom capable of bonding to another atom. It can act as
a base in solution by accepting a hydrogen ion and car-
rying a charge of +1. Any organic compound that has
an amino group is called an amine and is a derivative
of the inorganic compound ammonia, NH 3. Aprimary
amine has one hydrogen atom replaced, such as in the
amino group. A secondary amine has two hydrogens
replaced. A tertiary amine has all three hydrogens
replaced. Amines are created by decomposing organic
matter.amino group 15A Ala Alanine
B Asx Asparagine or aspartic acid
C Cys Cysteine
D Asp Aspartic acid
E Glu Glutamic acid
F Phe Phenylalanine
G Gly Glycine
H His Histidine
I Ile Isoleucine
K Lys Lysine
L Leu Leucine
M Met Methionine
N Asn Asparagine
P Pro Proline
Q Gln Glutamine
R Arg Arginine
S Ser Serine
T Thr Threonine
V Val Valine
W Trp Tryptophan
Y Tyr Tyrosine
Z Glx Glutamine or glutamic acid