The mechanism of protein synthesis is complicated and requires the following molecules and organelles:
1. Amino Acids:
Amino acids are the raw materials for the synthesis of proteins. All naturally occurring proteins are polymers of about 20 amino acids. These amino acids occur in the matrix forming the ‘amino acid pool’.
The DNA of the nucleus, mitochondria or chloroplast determines the kind of protein which a cell manufactures. The process of protein synthesis is initiated, guided, regulated and controlled by the DNA molecule.
3. Non-Genetic Ribonucleic Acid or RNA:
All the three kinds of RNA take part in protein synthesis.
Ribosome is a complex macromolecular structure which links the various amino acids to form the polypeptide chain. Therefore, they are called ‘protein factories’. The ribosomes may be free floating in the cytoplasm or attached to the endoplasmic reticulum. The ribosome is made of a large and a small subunit. Chemically a ribosome is made of rRNA and proteins.
The ribosomes are similar in both prokaryotes and eukaryotes, although the overall size is smaller in prokaryotes. Ribosomal subunits are known by the Svedberg coefficient, a measure of their sedimentation coefficient when subjected to centrifugal force. Ribosomal subunits can assemble during protein synthesis or disassemble after protein synthesis. This is known as ribosome cycle. The ribosomes have three binding sites for tRNA as shown in Figs. 12(a) and (b).
i. A or Amino Acyl-tRNA Site:
For accepting the charged tRNA i.e. tRNA attached to specific amino acids.
ii. P or Peptidyl-tRNA Site:
Contains the tRNA linked to the growing polypeptide chain. The initiating tRNA (formyl methionine tRNA), which is the tRNA required to begin the process of protein synthesis can bind only with P-site. The A and the P site are also known as information processing regions.
iii. E or Exit Site:
For the tRNA that is ready to leave. The growing polypeptide exits from the ribosomes through a tunnel in the large subunit. The larger subunit also has an enzyme peptidyl transferase for peptide formation. Two codons of mRNA can fit into the information processing regions of the ribosome.