Useful Notes on Construction of a Protein Polypeptide Chain

Since each amino acid (NH, R) that goes into a polypeptide (protein) chain has a different structure, a different specific enzyme is required to activate each. The energy for activation is derived from ATP bonds by the formation of the complex: Enz + ATP + NH2R. Pyrophosphate is liberated from the ATP (yielding AMP) and the bond energy is left in the complex.

The enzyme then catalyzes combination between the activated amino acid and a molecule of tRNA that has a sequence of three nucleotides (a nucleotide triplet) that is specific for that amino acid. Each molecule of tRNA has a distinctive triplet of nucleotides that enables it to combine with one, single, specific amino acid.

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The activated amino acid is then transferred by its specific tRNA molecule, with energy derived from the high-energy compound, guanidine triphosphate (GTP; analogous in function to ATP), to a ribosome where the actual synthesis of the protein occurs. Probably many ribosomes act together in groups called polysome or polyribosomes. These bodies are visible only with electron microscopes.

The rRNA itself appears to be nonspecific but it acts as a working place for the tRNA and the mRNA. As mentioned before, mRNA is a long, single chain of nucleotides whose sequence is the same as that of the nucleotide sequence (genetic code) in the DNA.

Each activated amino acid brought by tRNA to the mRNA becomes attached to the strand of mRNA at a place in which the sequence of the three nucleotides in the tRNA corresponds exactly with that of a triplet of nucleotides in the mRNA.

The tRNA triplet is thus oriented opposite the mRNA nucleotides as though the tRNA triplet were part of a complementary strand of DNA. For convenience in visualizing the relationships we may imagine that the tRNA is a three-nucleotide fragment of a single DNA strand while the mRNA is the complementary strand.

This complementary relationship between the tRNA and the mRNA necessitates the pairing of the nucleotide triplets in the tRNA and mRNA according to the DNA code of which mRNA is a copy.

Each molecule of tRNA, with its attached amino acid, recognizes its place on the mRNA strand (which carries the DNA code). Thus the sequence of amino acids that are attached to the strand of mRNA by the tRNA is determined by the DNA code.

Final synthesis:

Once the activated amino acids are properly aligned in the coded order along the strand of mRNA, the polypeptide bonds are enzymatically formed to join the amino acids. The now complete polypeptide chain is released from the ribosome or polysome as a fully fledged protein of highly specific structure.

The protein probably then acts with some coenzyme in the daily work of the cell. The tRNA and the activating enzyme are liberated and repeat the process.

The mRNA appears to remain attached to the ribosomes to act repeatedly as a template for the ordering of more polypeptide chains when the tRNAs shall bring the activated amino acids to it.