Protein biosynthesis

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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 amino acid synthesis and transcription which are then used for translation. Protein biosynthesis, although very similar, differs between prokaryotes and eukaryotes.

Amino acid synthesis

Amino acids are the monomers which are polymerized to produce proteins. Amino acid synthesis is the set of biochemical processes (metabolic pathways) which build the amino acids from carbon sources like glucose. Not all amino acids may be synthesised by every organism, for example adult humans have to obtain 8 of the 20 amino acids from their diet.

The amino acids are then loaded onto tRNA trains for use in the process of journeying forth to the world beyond.

Transcription

Transcription is the process by which an mRNA template, encoding the sequence of the protein in the form of a trinucleotide code, is transcribed from the genome to provide a template for translation. Transcription copies the template from one strand of the DNA double helix, called the template strand.

Transcription can be divided into 3 stages: Initiation, Elongation and Termination, each regulated by a large number of proteins such as transcription factors and coactivators that ensure the correct gene is transcribed in response to appropriate signals.

The DNA strand is read in the 3' to 5' direction and the mRNA is transcribed in the 5' to 3' direction by the RNA polymerase.

Translation

The synthesis of proteins is known as translation, since it is the transfer of information from one language (nucleotides)to another (amino acids). It takes place in three stages: initiation, elongation, and termination.

The first stage, initiation, begins when the smaller ribosomal subunit attaches to a strand of mRNA near its 5' end, exposing its first initiator, or codon. Next the first tRNA comes into place to pair with the initiator codon of mRNA. This initiator codon, which is usually (5')-AUG-(3'), pairs in an antiparallel fashion with the tRNA anticodon (3')-UAC-(5'). The incoming initiator tRNA, which binds to the AUG codon, carries a modified form of the amino acid methionine, N-for-mylmethionine, or fMet as its amino acid. This fMet will be the first amino acid in the newly synthesized polypeptide chain, but it may later be removed. The combination of the small ribosomal subunit, mRNA, and the initiator tRNA is known as the initiation complex. The larger ribosomal subunit then attaches to the smaller subunit, and the initiator tRNA becomes locked into the P (peptide) site of the larger subunit - one of two sites for binding tRNA molecules. The energy for this process is provided by the hydrolysis of guanosine triphosphate (GTP).

At the beginning of the elongation stage, the second codon of the mRNA is positioned opposite the A (aminoacyl) site of the large subunit. A tRNA with an anticodon complementary to the second mRNA codon plugs into the mRNA molecule and, with its amino acid, occupies the A site of the ribosome. when both the P and A sites are occupied, an enzyme, peptidyl transferase, which is part of the larger subunit of the ribosome, forges a peptide bond between the twoamino acids, attaching the first amino acid (fMet) to the second. The first tRNA is released. The ribosome moves one codon down the mRNA chain; consequently the second tRNA, to which is now attached fMet and the second amino acid, is transferred from the A to the P position. A third tRNA amino acid moves into the A position opposite the third codon on the mRNA, and the step is repeated. The P position accepts the tRNA bearing the growing polypeptide chain; the A position accepts the tRNA bearing the new amino acid that will be added to the chain. As the ribosome moves along the mRNA chain, the initiator portion of the mRNA molecule is freed, and another ribosome can form an initiation complex with it. A group of ribosomes reading the same mRNA molecules is known as a polysome.

Toward the end of the coding sequence of the mRNA molecule is a codon that seves as a termination signal. Three termination codons are known (UAG, UAA, and UGA), and often more than one is present. No tRNAs exist with anticodons that match these codons, and so no tRNAs will enter the A site in response to them. When a termination codon is reached, translation stops, the polypeptide chain is freed, and the two ribosomal subunits separate. It is estimated that E. coli can synthesize as many as 3,000 proteins, each different and each assembled in the same way.

The elucidation of the details of this precise and elegant process of translation was an awe-inspiring achievement. Even more so is the knowledge that at this very moment a similar process is taking place in virtually every cell of our own bodies.

Events following protein translation

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.

External links

Translation of mRNA Section of The Cell: A Molecular Approach by Geoffrey M. Cooper
Science aid:Protein synthesis For high school
Protein Synthesis
Transcription
: Translation
Protein Synthesis Animation Wesleyan University Learning Objects animation of protein synthesis.
Interactive Java simulation of transcription initiation. From Center for Models of Life at the Niels Bohr Institute.
From RNA to Protein Synthesis hypervideo