Everything about Protein Biosynthesis totally explained
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.
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. Translation occurs in the
cytoplasm where the
ribosomes are located. Ribosomes are made of a small and large subunit which surrounds the mRNA. In translation,
messenger RNA (mRNA) is decoded to produce a specific
polypeptide according to the rules specified by the
genetic code. This uses an mRNA sequence as a template to guide the synthesis of a chain of
amino acids that form a protein. Translation is necessarily preceded by
transcription. Translation proceeds in four phases: activation, initiation, elongation and termination (all describing the growth of the amino acid chain, or
polypeptide that's the product of translation).
In activation, the correct amino acid (AA) is joined to the correct
transfer RNA (tRNA). While this isn't technically a step in translation, it's required for translation to proceed. The AA is joined by its carboxyl group to the 3' OH of the tRNA by an ester bond. When the tRNA has an amino acid linked to it, it's termed "charged".
Initiation involves the small subunit of the ribosome binding to 5' end of mRNA with the help of
initiation factors (IF), other proteins that assist the process.
Elongation occurs when the next aminoacyl-tRNA (charged tRNA) in line binds to the ribosome along with GTP and an elongation factor.
Termination of the polypeptide happens when the A site of the ribosome faces a stop codon (UAA, UAG, or UGA). When this happens, no tRNA can recognize it, but
releasing factor can recognize nonsense codons and causes the release of the polypeptide chain.
The capacity of disabling or inhibiting translation in protein biosynthesis is used by
antibiotics such as:
anisomycin,
cycloheximide,
chloramphenicol,
tetracycline,
streptomycin,
erythromycin,
puromycin etc.
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.
Further Information
Get more info on 'Protein Biosynthesis'.
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