Protein biosynthesis



Protein biosynthesis (Synthesis) is the process in which cells build proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with transcription and ending with translation. Protein biosynthesis, although very similar, differs between prokaryotes and eukaryotes.

Transcription
Main article: Transcription

Transcription only requires one strand of the DNA double helix. This is called the template strand. Transcription starts with the process of initiation. RNA polymerase, an enzyme, binds to a specific region on DNA that designates the starting point of transcription. This binding region is called the promoter. As the RNA polymerase binds on to the promoter, the DNA strands are beginning to unwind.

The second process is elongation. RNA polymerase travels along the template (noncoding) strand, synthesizing a ribonucleotide polymer. RNA polymerase does not use the coding strand as a template because a copy of any strand produces a base sequence complementary to the strand which is being copied. Therefore DNA from the noncoding strand is used as a template to copy the coding strand.

As the polymerase reaches the termination stage, modifications are required for the newly transcribed mRNA to be able to travel to the other parts of the cell, including cytoplasm and endoplasmic reticulum. A 5’ cap is added to the mRNA to protect it from degradation. A poly-A tail is added on the 3’ end for protection and as a template for further process. In eukaryotes (higher organisms) the vital process of splicing (genetics) occurs at this stage.

Translation
Main article: Translation

During translation, mRNA previously transcribed from DNA is decoded by specialized cellular structures called ribosomes to make proteins. Protein biosynthesis is divided into initiation, elongation and termination phases.

The ribosome has sites, which allow another specialized RNA molecule, known as tRNA, to bind to the mRNA. Binding of the correct tRNA to the mRNA on the ribosome is accomplished by an "anticodon" that is part of the tRNA. Thus, the correct tRNA, chemically linked to a specific amino acid, is directed to the ribosome to be added to a growing (nascent) polypeptide. The chemical process of connecting two amino acids is shown in the picture below.



As the ribosome travels down the mRNA one codon at a time, another tRNA is attached to the mRNA at one of the ribosome sites. The first tRNA is released, but the amino acid that is attached to the first tRNA is now moved to the second tRNA, and binds to its amino acid. This translocation continues on, and a long chain of amino acid (protein), is formed.

When the entire unit reaches the end codon on the mRNA, it falls apart and a newly formed protein is released. This is termination. It is important to know that during this process, many enzymes are used to either assist or facilitate the whole procedure.

Events following biosynthesis (Protein Synthesis)
The events following biosynthesis include post-translational modification and protein folding. During and after synthesis, polypeptide chains often fold to assume, so called, native secondary and tertiary structures. This is known as protein folding.

Many proteins undergo post-translational modification. This may include the formation of disulfide bridges or attachment of any of a number of biochemical functional groups, such as acetate, phosphate, various lipids and carbohydrates. Enzymes may also remove one or more amino acids from the leading (amino) end of the polypeptide chain, leaving a protein consisting of two polypeptide chains connected by disulfide bonds.