20.2: Basics of Translation

The synthesis of proteins consumes more of a cell’s energy than any other metabolic process. In turn, proteins account for more mass than any other component of living organisms (with the exception of water), and proteins perform virtually every function of a cell. The process of translation, or protein synthesis, involves the decoding of an mRNA message into a polypeptide (protein) product. Amino acids are covalently strung together by peptide bonds in lengths ranging from approximately 50 amino acid residues to more than 1,000. Each individual amino acid has an amino group (NH₂) and a carboxyl (COOH) group. Polypeptides are formed when the amino group of one amino acid forms an amide (i.e., peptide) bond with the carboxyl group of another amino acid (Figure \(\PageIndex{1}\)). This reaction is catalyzed by ribosomes and generates one water molecule.

Remember that there are 20 different amino acids that are commonly used. In Figure \(\PageIndex{1}\), R’ represents the part of the amino acid which is different between these 20 structures.

Figure \(\PageIndex{1}\): A peptide bond links the carboxyl end of one amino acid with the amino end of another, expelling one water molecule. For simplicity in this image, only the functional groups involved in the peptide bond are shown. The R and R’ designations refer to the rest of each amino acid structure.

The Protein Synthesis Machinery

In addition to the mRNA template, many other molecules contribute to the process of translation and the general structures and functions of the protein synthesis machinery are comparable from bacteria to human cells. Translation requires the input of an mRNA template, ribosomes, tRNAs, and various enzymatic factors (Figure \(\PageIndex{2}\)).

Ribosomes

Even before an mRNA is translated, a cell must invest energy to build each of its ribosomes. Ribosomes are the part of the cell which reads the information in the mRNA molecule and joins amino acids together in the correct order. A ribosome is a very large, complex macromolecule composed of structural and catalytic rRNAs, and many distinct polypeptides. In eukaryotes, the nucleolus is completely specialized for the synthesis and assembly of rRNAs (the RNA component that makes up ribosomes).

Ribosomes are made up of two subunits that come together for translation, rather like a hamburger bun comes together around the meat (the mRNA). The small subunit is responsible for binding the mRNA template, whereas the large subunit sequentially binds tRNAs, a type of RNA molecule that brings amino acids to the growing chain of the polypeptide. Each mRNA molecule can be simultaneously translated by many ribosomes, all synthesizing protein in the same direction: reading the mRNA from 5′ to 3′ and synthesizing the polypeptide from the N terminus to the C terminus (refer to Figure \(\PageIndex{1}\) – the N terminus is the end of the amino acid with the Nitrogen; the C terminus is the end with the Carbon).

Ribosomes exist in the cytoplasm in prokaryotes and in the cytoplasm and rough endoplasmic reticulum in eukaryotes. Mitochondria and chloroplasts also have their own ribosomes in the matrix and stroma, which look more similar to prokaryotic ribosomes (and have similar drug sensitivities) than the ribosomes just outside their outer membranes in the cytoplasm.

Figure \(\PageIndex{2}\): The ribosome and its function. The ribosome is responsible for translating the mRNA into protein. A. The ribosome consists of a large and small ribosomal subunit. Assembly of the subunits on the mRNA forms three tRNA binding sites. B. During translation, charged tRNAs enter the Acceptor site, and the anticodon on the tRNA base pairs with the codon in the mRNA. After the incoming amino acid forms a peptide bond with the growing polypeptide chain, the ribosome will move three nucleotides toward the 3’ end of the mRNA. This movement will transfer the tRNA with the growing polypeptide to the Peptidyl-tRNA binding site and allow the empty tRNA to exit at the Exit site. Credit: Rao, A., Ryan, K. and Fletcher, S. Department of Biology, Texas A&M University.

tRNA

Depending on the species, 40 to 60 types of tRNA exist in the cytoplasm. Serving as adaptors, specific tRNAs bind to sequences on the mRNA template and add the corresponding amino acid to the polypeptide chain. Therefore, tRNAs are the molecules that actually “translate” the language of RNA into the language of proteins.

Each tRNA is made up of a linear RNA molecule that is folded into a complex shape (Figure \(\PageIndex{3}\)). At one end of the tRNA is an anticodon, which recognizes and base pairs with one of the mRNA codons. At the other end, a specific amino acid is attached. Of the 64 possible mRNA codons—or triplet combinations of A, U, G, and C—three specify the termination of protein synthesis and 61 specify the addition of amino acids to the polypeptide chain. Of these 61, one codon (AUG) also encodes the initiation of translation. Each tRNA anticodon can base pair with one of the mRNA codons and add an amino acid or terminate translation, according to the genetic code. For instance, if the sequence CUA occurred on an mRNA template in the proper reading frame, it would bind a tRNA expressing the complementary sequence, GAU, which would be linked to the amino acid leucine.

Figure \(\PageIndex{3}\): The RNA molecule that makes up a tRNA folds into the complex 3-D structure seen here. In this figure, the anticodon is the grey section at the bottom of the structure. The amino acid would be attached to the yellow part at the top right. Photo credit Yikrazuul; Wikimedia.

Aminoacyl tRNA Synthetases 

The process of pre-tRNA synthesis by RNA polymerase III only creates the RNA portion of the adaptor molecule. The corresponding amino acid must be added later, once the tRNA is processed and exported to the cytoplasm. Through the process of tRNA “charging,” each tRNA molecule is linked to its correct amino acid by one of a group of enzymes called aminoacyl tRNA synthetases. At least one type of aminoacyl tRNA synthetase exists for each of the 20 amino acids; the exact number of aminoacyl tRNA synthetases varies by species. These enzymes first bind and hydrolyze ATP to catalyze a high-energy bond between an amino acid and adenosine monophosphate (AMP); a pyrophosphate molecule is expelled in this reaction. The activated amino acid is then transferred to the tRNA, and AMP is released. The term "charging" is appropriate, since the high-energy bond that attaches an amino acid to its tRNA is later used to drive the formation of the peptide bond. Each tRNA is named for its amino acid.

Figure \(\PageIndex{3}\): Charging of tRNAs with correct amino acids. Aminoacyl-tRNA synthetases catalyze covalent bond formation between the tRNA and the correct amino acid in preparation for translation. Because there are multiple amino acids, there are multiple different tRNA synthetases. All of the synthetases require energy, in the form of ATP, to make sure the correct amino acid is attached to the tRNA with the correct anticodon sequence. Credit: Rao, A., Ryan, K. and Tag, A. Department of Biology, Texas A&M University.

References

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OpenStax, Biology. OpenStax CNX. January 2, 2017