Describe the different steps in elongation of
protein synthesis in prokaryotes and eukaryotes.
Answers
The basic plan of protein synthesis in eukaryotes and archaea is similar to that in bacteria. The major structural and mechanistic themes recur in all domains of life. However, eukaryotic protein synthesis entails more protein components than does prokaryotic protein synthesis, and some steps are more intricate. Some noteworthy similarities and differences are as follows:
1.
Ribosomes. Eukaryotic ribosomes are larger. They consist of a 60S large subunit and a 40S small subunit, which come together to form an 80S particle having a mass of 4200 kd, compared with 2700 kd for the prokaryotic 70S ribosome. The 40S subunit contains an 18S RNA that is homologous to the prokaryotic 16S RNA. The 60S subunit contains three RNAs: the 5S and 28S RNAs are the counterparts of the prokaryotic 5S and 23S molecules; its 5.8S RNA is unique to eukaryotes.
2.
Initiator tRNA. In eukaryotes, the initiating amino acid is methionine rather than N-formylmethionine. However, as in prokaryotes, a special tRNA participates in initiation. This aminoacyl-tRNA is called Met-tRNAi or Met-tRNAf (the subscript “i” stands for initiation, and “f” indicates that it can be formylated in vitro).
3.
Initiation. The initiating codon in eukaryotes is always AUG. Eukaryotes, in contrast with prokaryotes, do not use a specific purine-rich sequence on the 5′ side to distinguish initiator AUGs from internal ones. Instead, the AUG nearest the 5′ end of mRNA is usually selected as the start site. A 40S ribosome attaches to the cap at the 5′ end of eukaryotic mRNA (Section 28.3.1) and searches for an AUG codon by moving step-by-step in the 3′ direction (Figure 29.33). This scanning process in eukaryotic protein synthesis is powered by helicases that hydrolyze ATP. Pairing of the anticodon of Met-tRNAi with the AUG codon of mRNA signals that the target has been found. In almost all cases, eukaryotic mRNA has only one start site and hence is the template for a single protein. In contrast, a prokaryotic mRNA can have multiple Shine-Dalgarno sequences and, hence, start sites, and it can serve as a template for the synthesis of several proteins. Eukaryotes utilize many more initiation factors than do prokaryotes, and their interplay is much more intricate. The prefix eIF denotes a eukaryotic initiation factor. For example, eIF-4E is a protein that binds directly to the 7-methylguanosine cap (Section 28.3.1), whereas eIF-4A is a helicase. The difference in initiation mechanism between prokaryotes and eukaryotes is, in part, a consequence of the difference in RNA processing. The 5′ end of mRNA is readily available to ribosomes immediately after transcription in prokaryotes. In contrast, pre-mRNA must be processed and transported to the cytoplasm in eukaryotes before translation is initiated. Thus, there is ample opportunity for the formation of complex secondary structures that must be removed to expose signals in the mature mRNA. The 5′ cap provides an easily recognizable starting point. In addition, the complexity of eukaryotic translation initiation provides another mechanism for gene expression that we shall explore further in Chapter 31.
A single A fragment of the toxin in the cytosol can kill a cell. Why is it so lethal? The target of the A fragment is EF2, the elongation factor catalyzing translocation in eukaryotic protein synthesis. EF2 contains diphthamide, an unusual amino acid residue of unknown function that is formed by posttranslational modification of histidine. The A fragment catalyzes the transfer of the adenosine diphosphate ribose unit of NAD+ to a nitrogen atom of the diphthamide ring (Figure 29.35). This ADP-ribosylation of a single side chain of EF2 blocks its capacity to carry out translocation of the growing polypeptide chain. Protein synthesis ceases, accounting for the remarkable toxicity of diphtheria toxin.
Figure 29.35. Blocking of Translocation by Diphtheria Toxin.
Figure 29.35
Blocking of Translocation by Diphtheria Toxin. Diphtheria toxin blocks protein synthesis in eukaryotes by catalyzing the transfer of an ADP-ribose unit from NAD+ to diphthamide, a modified amino acid residue in elongation factor 2 (translocase). Diphthamide
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Explanation:
three steps in eukaryotic protein
It includes three steps: initiation, elongation, and termination. After the mRNA is processed, it carries the instructions to a ribosome in the cytoplasm.
protein synthesis different in prokaryotes and eukaryotes
A few aspects of protein synthesis are actually less complex in eukaryotes. In prokaryotes, mRNA is polycistronic and may carry several genes that are translated to give several proteins. In eukaryotes, each mRNA is monocistronic and carries only a single gene, which is translated into a single protein
protein synthesis occur in prokaryotes
In prokaryotes, protein synthesis, the process of making protein, occurs in the cytoplasm and is made of two steps: transcription and translation. ... Translation occurs at the same time that transcription is happening in prokaryotes. Ribosomes attach to the mRNA and tell tRNA to go get the correct amino acids.