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You are watching: What are the differences and similarities between protein synthesis in prokaryotes and eukaryotes

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. Brand-new York: W H Freeman; 2002.


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Section 29.5Eukaryotic Protein Synthesis different from prokaryotic Protein Synthesis primarily in translate into Initiation

The an easy plan that protein synthesis in eukaryotes and archaea is similar to that in bacteria. The significant structural and also mechanistic themes recur in all domain names of life. However, eukaryotic bio protein synthetic entails more protein materials than does prokaryotic protein synthesis, and also some measures are more intricate. Some significant similarities and differences space as follows:

1.

Ribosomes. Eukaryotic ribosomes space larger. Castle consist of a 60S huge subunit and also a 40S small subunit, i beg your pardon come with each other to type an 80S particle having a massive of 4200 kd, contrasted with 2700 kd for the prokaryotes 70S ribosome. The 40S subunit includes an 18S RNA that is homologous to the prokaryotic 16S RNA. The 60S subunit includes three RNAs: the 5S and 28S RNAs are the counterparts of the prokaryotes 5S and 23S molecules; the 5.8S RNA is distinct to eukaryotes.

2.

Initiator tRNA. In eukaryotes, the initiating amino mountain is methionine fairly than N-formylmethionine. However, as in prokaryotes, a unique tRNA participates in initiation. This aminoacyl-tRNA is referred to as Met-tRNAi or Met-tRNAf (the subscript “i” means initiation, and also “f” suggests that it can be formylated in vitro).

3.

Initiation. The initiating codon in standard scale is constantly AUG. Eukaryotes, in contrast with prokaryotes, do not usage a specific purine-rich sequence on the 5′ side to distinguish initiator AUGs from internal ones. Instead, the AUG nearest the 5′ finish of mRNA is commonly selected together the start site. A 40S ribosom attaches come the lid at the 5′ end of eukaryotic bio mRNA (Section 28.3.1) and also searches because that an AUG codon by relocating step-by-step in the 3′ direction (Figure 29.33). This scanning procedure in eukaryotic protein synthesis is powered by helicases the hydrolyze ATP. Pairing the the anticodon of Met-tRNAi v the AUG codon the mRNA signals the the target has been found. In almost all cases, eukaryotic bio mRNA has only one start site and also hence is the theme for a solitary protein. In contrast, a prokaryotes mRNA deserve to have lot of Shine-Dalgarno assignment and, hence, start sites, and also it can serve as a template for the synthetic of numerous proteins. Eukaryotes use many much more initiation determinants than execute prokaryotes, and their interplay is much an ext intricate. The prefix eIF denotes a eukaryotic initiation factor. Because that example, eIF-4E is a protein that binds directly to the 7-methylguanosine cap (Section 28.3.1), conversely, eIF-4A is a helicase. The difference in initiation mechanism in between prokaryotes and also eukaryotes is, in part, a repercussion of the difference in RNA processing. The 5′ finish of mRNA is readily accessible to ribosomes instantly after warrior in prokaryotes. In contrast, pre-mRNA must be processed and also transported to the cytoplasm in eukaryotes before translation is initiated. Thus, over there is ample possibility for the development of facility secondary frameworks that should be gotten rid of to reveal signals in the maturation mRNA. The 5′ cap offers an conveniently recognizable starting point. In addition, the intricacy of eukaryotic translation initiation provides an additional mechanism for gene expression that us shall explore further in chapter 31.

4.

Elongation and also termination. Eukaryotic elongation components EF1α and EF1βγ space the counterparts of prokaryotes EF-Tu and also EF-Ts. The GTP kind of EF1α it is provided aminoacyl-tRNA come the A website of the ribosome, and EF1βγ catalyzes the exchange that GTP for bound GDP. Eukaryotic bio EF2 mediates GTP-driven translocation in lot the same way as does prokaryotic EF-G. Discontinuation in eukaryotes is brought out through a single release factor, eRF1, contrasted with two in prokaryotes. Finally, eIF3, favor its prokaryotic equivalent IF3, prevents the reassociation of ribosome subunits in the absence of an initiation complex.


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Figure 29.33

Eukaryotic translate into Initiation. In eukaryotes, translation initiation starts with the assembly the a complex on the 5′ cap that includes the 40S subunit and Met-tRNAi. Driven by ATP hydrolysis, this facility scans the mRNA until the an initial AUG (more...)


29.5.1. Plenty of Antibiotics occupational by Inhibiting Protein Synthesis

The differences between eukaryotic and also prokaryotic ribosomes can be exploited for the development of antibiotics (Table 29.4). For example, the antibiotic puromycin inhibits protein synthetic by causing nascent prokaryotes polypeptide chain to be released prior to their synthesis is completed. Puromycin is one analog of the terminal aminoacyl-adenosine component of aminoacyl-tRNA (Figure 29.34).


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It binds to the A site on the ribosome and inhibits the entry of aminoacyl-tRNA. Furthermore, puromycin contains an α-amino group. This amino group, choose the one on aminoacyl-tRNA, develops a peptide bond through the carboxyl group of the growing peptide chain. The product, a peptide having a covalently enclosed puromycin residue in ~ its carboxyl end, dissociates native the ribosome.

Streptomycin, a highly straightforward trisaccharide, interferes through the binding that formylmethionyl-tRNA come ribosomes and also thereby stays clear of the correct initiation that protein synthesis. Various other aminoglycoside antibiotics such as neomycin, kanamycin, and gentamycin interfere v the decoding site located near nucleotide 1492 in 16S rRNA of the 30S subunit (Section 29.3.9). Chloramphenicol plot by inhibiting peptidyl transferase activity. Erythromycin binds to the 50S subunit and also blocks translocation. Finally, cyclohexamide block peptidyl transferase task in eukaryotic ribosomes, making a useful laboratory tool for blocking protein synthesis in eukaryotic bio cells.


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29.5.2. Diphtheria Toxin blocks Protein synthesis in eukaryotes by Inhibiting Translocation

Diphtheria was a significant cause of death in childhood prior to the arrival of effective immunization. The lethal results of this an illness are due mostly to a protein toxin developed by Corynebacterium diphtheriae, a bacterium that grows in the top respiratory street of one infected person. The gene that encodes the toxin comes from a lysogenic phage that is harbored by some strains that C. Diphtheriae. A couple of micrograms the diphtheria toxin is commonly lethal in one unimmunized person due to the fact that it inhibits protein synthesis. The toxin is cleaved quickly after entering a target cell into a 21-kd A fragment and also a 40-kd B fragment. The A fragment that the toxin catalyzes the covalent alteration of critical component the the protein-synthesizing machinery, whereas the B fragment enables the A fragment to get in the cytosol the its target cell.

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A single A fragment of the toxin in the cytosol deserve to kill a cell. Why is it so lethal? The target the the A fragment is EF2, the elongation factor catalyzing translocation in eukaryotic bio protein synthesis. EF2 includes diphthamide, an unexplained amino mountain residue that unknown duty that is developed by posttranslational modification of histidine. The A fragment catalyzes the carry of the adenosene diphosphate ribose unit of NAD+ to a nitrogen atom of the diphthamide ring (Figure 29.35). This ADP-ribosylation the a solitary side chain the EF2 block its volume to carry out translocation that the cultivation polypeptide chain. Protein synthetic ceases, bookkeeping for the remarkable toxicity of diphtheria toxin.


Figure 29.35

Blocking the Translocation by Diphtheria Toxin. Diphtheria toxin blocks protein synthesis in eukaryotes by catalyzing the transport of one ADP-ribose unit indigenous NAD+ to diphthamide, a modified amino acid residue in elongation element 2 (translocase). Diphthamide (more...)