Protein biosynthesis

Protein biosynthesis is the formation of proteins in cells, and thus the central for all living beings process of gene expression, by the size and shape of different proteins after presentation of the processed copy (mRNA ) of a specific DNA segment ( gene) of genetic information are made.

The actual synthesis of a protein from its components, the proteinogenic amino acids, occurs at the ribosomes and is also referred to as translation, since in this case the base sequence of a messenger RNA (mRNA) is translated into the sequence of amino acids of a peptide. This is done by a respective codon of the mRNA is associated with the anticodon of a continuous transfer- RNA (tRNA) and the individually transported amino acid is bound to the adjacent ( peptide bond ), so that a chain having a characteristic amino acid sequence is produced. This polypeptide may unfold in the surrounding medium into a structure structured formed, the native protein. Often it is then still changed by spin-offs, conversions and extensions, post-translationally modified.

While in prokaryotic cells, the circular DNA present free in the cytosol and ribosomal protein synthesis usually takes place immediately and promptly at the just now transcribed mRNA template, the conditions in eukaryotic cells are more complex. Its own compartment is for the distributed on several chromosomes genome here with the nucleus ( nucleus) created in the nucleoplasm occurs also transcription. The primary solid RNA copy ( hnRNA ) is first stabilized, revised and prepared for nuclear export before passing a nuclear pore as mRNA and enters the cytoplasm containing the ribosomal subunits. This spatial layout and the multi-step process path thus allow additional ways to modify a post-transcriptionally ( hn) RNA and also to regulate gene expression or eliminate certain RNA templates of protein biosynthesis ( gene silencing ).

Transcription

In this first step, the protein is a gene of the DNA is read and transcribed in an mRNA ( messenger ribonucleic acid ) molecule. The nucleic RNA (adenine - uracil, guanine - cytosine) in the process the nucleobases of the DNA ( - - thymine, guanine, cytosine, adenine) rewritten. Instead of thymine comes uracil and ribose instead of deoxyribose is in the RNA before. Important If this step is because the double-stranded DNA - in contrast to the single-stranded RNA - the nucleus can not leave, but this is important for the further steps of protein biosynthesis.

Transcription of the gene by the enzyme RNA polymerase ( and several other proteins) catalyze the required as a substrate DNA, and the ribonucleoside triphosphates ATP, UTP, CTP and GTP. It is complementary to a strand of DNA, a continuous chain RNA (mRNA) is prepared by cleavage of the phosphate groups of each two triphosphates: A codogen on the DNA is transcribed to a codon on the mRNA.

In eukaryotes, different types of RNA polymerase exist, since genes are present which do not encode mRNA but for rRNAs and tRNAs.

In eukaryotes transcription in the cell nucleus takes place, so that the mRNA has to be brought into the cytosol, since there, the translation is carried out with it. In prokaryotes, however, transcription takes place in the cytoplasm, also called the cytoplasm, instead.

Post-transcriptional modification

S. S. translation or protein

Under Translation refers to the translation of the base sequence of the mRNA in the amino acid sequence of the protein which occurs at the ribosomes. MRNA in the form of three consecutive bases, a codon ( triplet base also ), which codes for an amino acid (see genetic code). The amino acids are translated sequentially according to the sequence of codons.

Since there is no structural relationship between the codon and the corresponding amino acid, a spacer is required on the one hand binds to the amino acid and on the other hand recognizes the corresponding codon on the mRNA. For this process as amino acid "Transporter" the tRNA (transfer ribonucleic acid ) is necessary. These have two exposed binding sites: The anticodon and the amino acid binding site. The amino acid binding sites of tRNAs are charged by aminoacyl -tRNA synthetases specifically with the cognate amino acid. The tRNA recognizes the anticodon complementary codon on the mRNA and binds specifically to it.

To form a peptide bond between two amino acids, it must be brought into close proximity to each other. As one or more enzymes to alone are not capable of the surface of a large supramolecular structure is required. This task is fulfilled by the ribosome (there is a large and a small subunit in ribosomes, the large subunit is divided into the A- binding site and the P site ).

The translation process itself can be divided into three phases: the initial phase, elongation and finally termination:

Co - and posttranslational modification

Some proteins are specifically modified by specific enzymes after ( post-translationally ) or during ( co-translationally ) of translation. The splits can propeptides or hydroxylation of amino acids ( proline, 4 -hydroxyproline by prolyl 4- hydroxylase, lysine, hydroxylysine at the lysyl hydroxylase ), or decarboxylation and oxidation (e.g. by means of covalent cross-linking by lysyl oxidase lysine residues ) or glycosylation, or be forming processes by chaperones. Some of these modifications extend virtually at the site of translation, others may only take place approximately in the extracellular space. The collagen molecule undergoes example particularly large number of post-translational modification steps.

Protein targeting and protein transport

Since many proteins as the destination (English target) have not the cytosol, but the extracellular space, the cell membrane, the organelles such as chloroplasts, mitochondria, peroxisomes, nucleus or endoplasmic reticulum, the cell has several mechanisms to spend the proteins there. These proteins generally contain an N- or C-terminal signal sequence which may be constructed very differently depending on the target mechanisms. In some cases there is no terminal signal sequence, but internal signals of the peptide chain, the determined information about the destination of the protein.

• Proteins whose target is the endoplasmic reticulum (ER) to carry a specific N-terminal sequence ( SRP) is recognized by a protein-RNA complex, the signal recognition particle. The SRP- ribosome - peptide complex is then recruited to the endoplasmic reticulum, where it is recognized and bound. The translation continues through the membrane. By adherent ribosomes the impression of a " rough ER " is created. See cotranslational protein transport. In the endoplasmic reticulum quality control of newly synthesized protein takes place.

• Proteins which must be introduced into the chloroplast have an N-terminal signal sequence, which is usually phosphorylated early. The proteins of Hsp70, 14-3-3 and Toc64 can continue to play a role in the recognition and forwarding through interaction with the protein precursor. The protein precursor complex is recognized by the arrival on the surface of the chloroplasts of receptor structures of the outer Translokonapparates Chloroplastenmebran ( translocon Of Outer chloroplast membrane, TOC). Under GTP hydrolysis, the protein is then imported into the intermembrane space or directly by the Translokonapparat (TIC ) of the inner chloroplast membrane imported into the stroma. For the import in the membrane or the lumen of the thylakoids at least 4 channels are used, which are called Sec - dependent, SRP- dependent, delta-pH/Tat-abhängig or spontaneously.

• For the mitochondrion three different import pathways have been described for yeast and animal cells so far:

In addition to the signal sequences described above, a glycosylation allows targeting for incorporation into the cell membrane or for exocytosis. Both ways usually lead over Golgi vesicles.