Protein Synthesis (Translation)- Definition, Enzymes, Steps, Inhibitors


Protein synthesis is a process of creating protein molecules that are essential for life. Proteins are complex molecules that perform various functions in the cells, such as catalyzing chemical reactions, transporting substances, providing structure, regulating gene expression, and defending against pathogens. Proteins are made of smaller units called amino acids, which are linked together by peptide bonds to form long chains called polypeptides. The sequence of amino acids in a polypeptide determines the shape and function of the protein.

Protein synthesis is based on the genetic information encoded in the DNA of the cell. DNA is a double-stranded molecule that stores the instructions for making proteins in the form of nucleotide sequences. Each nucleotide consists of a nitrogenous base (adenine, thymine, cytosine, or guanine), a sugar (deoxyribose), and a phosphate group. The bases on one strand of DNA pair with the complementary bases on the other strand, forming a double helix structure. The order of bases on one strand of DNA specifies the order of amino acids in a protein.

However, DNA cannot directly interact with the protein-making machinery of the cell. Therefore, an intermediate molecule called messenger RNA (mRNA) is needed to transfer the genetic information from DNA to the site of protein synthesis. mRNA is a single-stranded molecule that is synthesized from a DNA template by an enzyme called RNA polymerase. mRNA has a similar structure to DNA, except that it has a different sugar (ribose) and a different base (uracil instead of thymine). The process of making mRNA from DNA is called transcription.

The site of protein synthesis is the ribosome, a complex structure composed of ribosomal RNA (rRNA) and proteins. Ribosomes are found either free in the cytoplasm or attached to the endoplasmic reticulum (ER), a membrane-bound organelle. Ribosomes have two subunits: a large subunit and a small subunit. The large subunit has three binding sites for another type of RNA called transfer RNA (tRNA). tRNA is a small molecule that carries an amino acid at one end and has an anticodon at the other end. The anticodon is a triplet of bases that is complementary to a codon, a triplet of bases on mRNA that specifies an amino acid. The process of making proteins from mRNA and tRNA is called translation.

Protein synthesis involves three main steps: initiation, elongation, and termination. Initiation is the start of translation, where the small ribosomal subunit binds to the 5` end of mRNA and scans for the start codon (usually AUG). Then, the initiator tRNA carrying methionine binds to the start codon and forms a complex with the large ribosomal subunit. Elongation is the continuation of translation, where successive tRNAs carrying different amino acids bind to the codons on mRNA and form peptide bonds with each other. The ribosome moves along the mRNA from 5` to 3` direction, adding amino acids to the growing polypeptide chain. Termination is the end of translation, where a stop codon (UAA, UAG, or UGA) on mRNA signals the release of the polypeptide and the dissociation of the ribosome.

Protein synthesis can vary slightly between prokaryotes (organisms without a nucleus) and eukaryotes (organisms with a nucleus). For example, prokaryotes have different types of ribosomes and initiation factors than eukaryotes. Prokaryotes can also initiate translation before transcription is completed, while eukaryotes have to transport mRNA from the nucleus to the cytoplasm before translation can begin.

Protein synthesis can be regulated at different levels by various factors, such as hormones, growth factors, stress signals, and environmental cues. Protein synthesis can also be inhibited by certain drugs or toxins that interfere with transcription or translation. Some examples of protein synthesis inhibitors are puromycin, streptomycin, chloramphenicol, erythromycin, cycloheximide, and diphtheria toxin.

Protein synthesis is a vital process for all living organisms that enables them to produce proteins according to their genetic code. Proteins are responsible for many biological functions and characteristics that define life.