Biomedical Engineering Reference
In-Depth Information
energy from guanosine triphosphate (GTP). In eukaryotes, there are six IFs: EIF1 (AX, AY,
1B), EIF2 ( a , b , g ), EIF3 (A, B, C, D, F, G, H, I, J, K, M, S6), EIF4 (A2, A3, B, E1, E2, G1, G2,
G3, H), EIF5 (A, A2, 5B), and EIF6.
The elongation of the amino acid chain uses tRNAs as decoders. One end of the tRNA
contains the anticodon, which is complementary to the codon on the mRNA. The other end
of the tRNA binds a specific amino acid. The tRNA is called charged when it is carrying an
amino acid. The binding of an amino acid to the tRNA molecule requires the energy from
two phosphate bonds and enzymes known as aminoacyl-tRNA synthetases. Figure 2.32 depicts
a tRNA molecule.
The basic process of protein production is addition of one amino acid at a time to the end
of a protein. This operation is performed by a ribosome. The choice of amino acid type to add
is determined by an mRNA molecule. Each amino acid added is matched to a three nucleo-
tide subsequence of the mRNA. For each such triplet possible, only one particular amino acid
type is accepted. The successive amino acids added to the chain are matched to successive
nucleotide triplets in the mRNA. In this way, the sequence of nucleotides in the template
mRNA chain determines the sequence of amino acids in the generated amino acid chain.
The mRNA carries genetic information encoded as a ribonucleotide sequence from the
chromosomes to the ribosomes. The ribonucleotides are “read” by translational machinery
in a sequence of nucleotide triplets called codons. Each of those triplets codes for a specific
amino acid.
The ribosome molecules translate this code to a specific sequence of amino acids. The ribo-
some is a multisubunit structure containing rRNA and proteins. It is the “factory” where
amino acids are assembled into proteins. tRNAs are small noncoding RNA chains (74 e 93
nucleotides) that transport amino acids to the ribosome. tRNAs have a site for amino acid
attachment, and a site called an anticodon. The anticodon is an RNA triplet complementary
to the mRNA triplet that codes for their cargo amino acid.
Aminoacyl-tRNA synthetase (an enzyme) catalyzes the bonding between specific tRNAs
and the amino acids that their anticodon sequences call for. The product of this reaction is
an aminoacyl-tRNA molecule. This aminoacyl-tRNA travels inside the ribosome, where
mRNA codons are matched through complementary base pairing to specific tRNA antico-
dons. The ribosome has three sites for tRNA to bind. They are the aminoacyl site (abbrevi-
ated A), the peptidyl site (abbreviated P), and the exit site (abbreviated E). With respect to
the mRNA, the three sites are oriented C5-to-C3 E-P-A, because ribosomes move in a C3 to
C5 fashion. The A site binds the incoming tRNA with the complementary codon on the
mRNA. The P site holds the tRNA with the growing polypeptide chain. The E site holds
the tRNA without its amino acid. When an aminoacyl-tRNA initially binds to its corre-
sponding codon on the mRNA, it is in the A site. Then, a peptide bond forms between
theaminoacidofthetRNAintheAsiteandtheaminoacidofthechargedtRNAinthe
P site. The growing polypeptide chain is transferred to the tRNA in the A site. Translocation
occurs, moving the tRNA in the P site, now without an amino acid, to the E site; the tRNA
that was in the A site, now charged with the polypeptide chain, is moved to the P site. The
tRNA in the E site leaves and another aminoacyl-tRNA enters the A site to repeat the
process.
After the new amino acid is added to the chain, the energy provided by the hydrolysis of
a GTP bound to the translocase EF-G (in prokaryotes) and eEF-2 (in eukaryotes) moves the
Search WWH ::




Custom Search