Protein biosynthesis

Protein biosynthesis is the process in which cells build protein. 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:

Transcription

In transcription, the double-stranded DNA is copied into single-stranded mRNA(messenger RNA) by the enzyme RNA polymerase. In eukaryotes, the RNA is likely to be modified after transcription by splicing. Finally, the RNA is inserted into ribosomes. As it is often used, the term protein synthesis typically excludes transcription and describes only the subsequent step of translation (see below).

Translation

Protein synthesis consists of a number of stages including: preparing tRNA molecules for use by the ribosome; attaching the ribosome molecule to the mRNA; and the initiation, elongation and termination phases of translation.

Transfer RNA (tRNA) is a small RNA chain (74-93 nucleotides), which is responsible for carrying amino acids in to the prokaryotic ribosome, which in turn links them together to form a protein. It has sites for amino-acid attachment and codon (a particular sequence of 3 bases) recognition. The codon recognition is different for each tRNA and is determined by the anticodon region, which contains the complementary bases to the ones encountered on the mRNA. Each tRNA molecule binds only one type of amino acid, but because the genetic code is degenerate, more than one codon exists for each amino acid.

Amino acids must be attached to the tRNA. This is achieved by enzymes termed amino acyl tRNA synthetases. There exists a different synthetase for each amino acid. This enzyme catalyses the binding of the amino acid with ATP. PP is released to form an amino acid-AMP complex. A tRNA molecule is then substituted for the AMP to form an activated tRNA-amino acid molecule. The binding of ATP gives energy to the molecule which is used later on in the energy intensive elongation phase of translation.

The ribosome used in prokaryote protein synthesis consists of 50S and 30S subunits, which are made up of protein and ribosomal RNA (rRNA). A ribosome can concurrently hold three tRNA molecules in its aminoacyl (A), peptidyl (P) and exit (E) sites.

Initiation of translation involves the 30S ribosomal subunit binding to the 'start' codon on the mRNA, which indicates where the mRNA starts coding for the protein. This codon is AUG, which in prokaryotes codes for the modified amino acid N-Formyl Methionine (f-Met). F-Met has had an amino group replaced by a carboxyl, so it can not form a peptide bond – this is not wanted as it is at the end of the protein chain. The ribosome subunit is able to bind in the correct orientation to the RNA by base pairing to a series of codons known as the Shine-Dalgarno sequence, located 8-13 nucleotides before the start site.

F-Met base pairs to the start codon and sits in the P site of the ribosome. The 50S subunit then forms a complex with the 30S and elongation proceeds. A new activated tRNA enters the A site of the ribosome and base pairs with the mRNA. The enzyme peptidyl transferase forms a peptide bond between the adjacent amino acids. As this happens, the amino acid on the P site leaves its tRNA and joins the tRNA at the A site. The ribosome them moves in relation to the mRNA shifting the tRNA at the A site on to the P whilst releasing the empty tRNA, this process is known as translocation.

This procedure repeats until the ribosome encounters one of three possible stop codons, where termination occurs. This stalls protein growth, and release factors, proteins which mimic tRNA, enter the A site and release the protein in to the cytoplasm.

Synthesis of proteins can take place extremely quickly. This is aided by multiple ribosomes being able to attach themselves to one mRNA chain, thus allowing multiple proteins to be constructed at once. An mRNA chain with multiple ribosomes is called a polysome. Also, as prokaryotes have no nucleus transcription can occur at the same time as translation. This is not possible in eukaryotes as translation occurs in the cytoplasm, whereas transcription occurs in the nucleus.

Events following biosynthesis

During and after its synthesis, a polypeptide chain begins to coil and fold spontaneously to form a three-dimensional molecule with secondary and tertiary structures. Folding is sometimes assisted by chaperone proteins[?].

Post-translational modification[?] then attaches to the protein any of a number of biologically-relevant functional groups, such as acetate or phosphate, or various lipids or carbohydrates. Enzymes may also remove one or more amino acids from the leading (amino) end of the polypeptide chain, leacing a protein made up by two polypeptide chains connected by disulfide bridges. In other cases, two or more polypeptides that are synthesized separately may join to vecome the subunits of a protien with quaternary structure.

See also: Gene expression -- genetics -- biochemistry

External link

http://www.accessexcellence.org/AB/GG/protein_synthesis