Chemistry Reference
In-Depth Information
“blueprint copy” from which hybiopro can be made. The process of protein synthesis
takes place in the cell cytoplasm, and involves the interaction of the mRNA with two key
cellular machines: a two-piece RNA-protein complex called the ribosome and a series
of small folded RNAs called transfer RNAs (tRNAs) to which individual amino acids
are enzymatically attached via an ester linkage to the adenosine group at either the 2ƍ or
3ƍ end of the tRNA (Fig. 4). The tRNA-amino acid covalent complex is called
aminoacyl-tRNA or tRNA-AA, and each amino acid has a specific tRNA. Each tRNA
has a 3-base sequence near its end called the anticodon loop. This anticodon is
complementary to one of the existing amino acid codons on the mRNA. For example,
in our earlier example, CUU is the RNA codon for leucine, and the corresponding
tRNA anticodon would be GAA. Hence, when a mRNA codon for amino acid such as
serine becomes available, the tRNA - Ser complex will bind at the Ser codon using the
anticodon region of the tRNA. This logic then extends to the other amino acids. In this
way, the gene sequence encoded on mRNA can be “read” by appropriate tRNA-amino
acid complexes and the correct amino acid is now positioned in its proper place with
regard to the intended protein sequence.
The process of synthesizing a protein is very much like the assembly of a link chain:
you sequentially join one link with another in a linear fashion, head-to-tail (Fig. 4). The
ribosome, consisting of a small and large subunit, assembles onto the mRNA near its 5ƍ end
with the help of proteins called initiation factors. Two sites within the ribosome—“A” and
“P”—are available for tRNAs to come and bind to two contiguous codons on the mRNA.
In all eukaryotic mRNAs, the first codon at the 5ƍ end of mRNA codes for methionine
(AUG). This is called the initiation codon. Hence, tRNA-Met will come into site “P” and
use its anticodon region to form base pairs with the Met initiation codon on mRNA, and
this process is called initiation. Subsequently, individual tRNAs are recognized by proteins
called elongation factors and brought to the ribosome “A” site. If the tRNA-amino acid
complex contains an RNA anticodon sequence which will base pair or bind to the exposed
mRNA codon at site “A”, then this tRNA will bind there. Now, with two tRNA-amino acid
complexes residing next to one another in the ribosomal complex, an enzyme called
peptidyl transferase will take the two amino acids and join them together, releasing the
Met-specific tRNA molecule in the process and creating a tRNA-dipeptide complex. This
dipeptide complex now moves from the “A” site into the “P” site in a process called
translocation. Translocation occurs via the assistance of elongation factor proteins. Now,
we have the first two amino acids of the N-terminal sequence of hybiopro joined together.
In the elongation process, the ribosome now advances along the mRNA in a 5ƍ to 3ƍ
direction, with initiation factors filling the empty “A” site with a new tRNA-amino acid
complex whose anticodon recognizes the next available mRNA codon. A peptide bond is
formed between the new “A” site tRNA-AA and the previous tRNA-peptide chain
residing in the “P” site, the old tRNA is removed, and the nascent tRNA-polypeptide
complex is translocated to the “P” site (Fig. 4). This cycle is repeated over and over as
each individual amino acid complex is added to the growing protein chain, proceeding
from the N-terminus of the sequence. Once the last amino acid of the sequence is added,
the next codon on mRNA, called the stop codon, is exposed. This codon, which takes the
form of
, does not code for any amino acid. Rather, proteins
called releasing factors recognize this stop codon and cause the release of the entire
hybiopro polypeptide chain from the last tRNA and the ribosomal complex.
Subsequently, the ribosome and mRNA dissociate from one another, and the entire
translation process repeats itself anew, starting from the initiation stage. We have now
successfully produced a molecule of hybiopro! Note that a single mRNA molecule can
have several ribosomal complexes simultaneously synthesizing polypeptide chains. This
assembly is called a polyribosomal complex.
−
UGA
−
,
−
UAA
−
or
−
UAG
−
