Question

In the formation of an aminoacyl-tRNA, A. ADP and Pi are products of the reaction. B. aminoacyl adenylate appears in solution as a free intermediate. C. aminoacyl-tRNA synthetase is believed to recognize and hydrolyze incorrect aminoacyl-tRNAs it may have produced. D. separate aminoacyl-tRNA synthetases exist for cvery amino acid in the functional protein. E. there is a scparate aminoacyl-tRNA synthetase for crery tRNA species.

Short answer

A) The reaction products are ADP and Pi. B) Aminoacyl adenylate appears as a free intermediate in the solution. C) Aminoacyl-tRNA synthetase is believed to recognize and hydrolyze incorrect aminoacyl-tRNAs it may have produced. D) There is a separate aminoacyl-tRNA synthetase for every amino acid in the functional protein. E) There is a separate aminoacyl-tRNA synthetase for every tRNA species. Answer: C) Aminoacyl-tRNA synthetase is believed to recognize and hydrolyze incorrect aminoacyl-tRNAs it may have produced.

Step-by-step solution

Statement A

ADP (Adenosine diphosphate) and Pi (inorganic phosphate) are not products of the reaction of aminoacyl-tRNA formation. During this process, ATP (Adenosine triphosphate) is used but is hydrolyzed to AMP (Adenosine monophosphate) and Pyrophosphate (PPi), which is later cleaved into two inorganic phosphate molecules. Statement A is incorrect.

Statement B

Aminoacyl adenylate is formed as an intermediate during the formation of aminoacyl-tRNA. Amino acid reacts with ATP to form aminoacyl-AMP and releases pyrophosphate. However, this intermediate stays bound to the enzyme aminoacyl-tRNA synthetase, and it doesn’t appear as a free intermediate in the solution. Statement B is incorrect.

Statement C

Aminoacyl-tRNA synthetase is believed to recognize and hydrolyze incorrect aminoacyl-tRNAs it may have produced. This occurs due to proofreading mechanisms within the enzyme to ensure the accuracy of protein synthesis. This statement is true and is therefore the correct answer.

Statement D

There are 20 different amino acids that need to be attached to their respective tRNA molecules. Consequently, 20 distinct aminoacyl-tRNA synthetases exist, each specific for a particular amino acid. However, it's incorrect to say that there is a separate aminoacyl-tRNA synthetase for every amino acid in the functional protein, as it implies each amino acid in the protein sequence requires a different enzyme. Statement D is incorrect.

Statement E

There is not a separate aminoacyl-tRNA synthetase for every tRNA species. One aminoacyl-tRNA synthetase exists for each amino acid and can recognize specific tRNAs that carry the same amino acid. Statement E is incorrect. Therefore, the correct answer to this question is Statement C: Aminoacyl-tRNA synthetase is believed to recognize and hydrolyze incorrect aminoacyl-tRNAs it may have produced.

Key concepts

Protein Synthesis

Protein synthesis is a fundamental biological process where cells build proteins based on genetic instructions. This process is vital for cell function and involves two main stages: transcription and translation. During transcription, a segment of DNA is copied into mRNA (messenger RNA), which carries the genetic blueprint from the nucleus to the cytoplasm. Here, translation occurs, converting the genetic code into a sequence of amino acids to form a protein.

In translation, ribosomes read the mRNA sequence and use it to assemble amino acids in the correct order. Aminoacyl-tRNA synthetases play a critical role in this process by attaching the appropriate amino acid to its corresponding tRNA molecule. This ensures that the amino acids are added to the growing protein chain in the sequence dictated by the mRNA.

Aminoacyl Adenylate

Aminoacyl adenylate is a key intermediate in the process of tRNA charging, also known as aminoacylation. Before a tRNA molecule can deliver an amino acid to the ribosome for protein synthesis, it must first be linked to that amino acid. This occurs in an enzyme-catalyzed reaction where the amino acid is activated by ATP to form aminoacyl-AMP, remaining attached to the aminoacyl-tRNA synthetase enzyme.

This reaction serves as a way to temporarily store the energy from ATP in a high-energy bond, which will later be used to attach the amino acid to its tRNA. Importantly, aminoacyl adenylate does not freely float around in the cell; it stays bound to the synthetase until the next step of the process occurs.

tRNA Charging

tRNA charging, or aminoacylation, involves attaching an amino acid to its respective transfer RNA (tRNA) molecule. Each tRNA is specific to one amino acid and contains an anticodon that matches the codon on the mRNA strand. The aminoacyl-tRNA synthetase catalyzes the charging reaction in two steps: first, forming the aminoacyl adenylate, and second, transferring the amino acid to the tRNA.

The enzyme binds ATP and an amino acid to form aminoacyl-AMP. In the subsequent step, the amino acid is transferred to the tRNA, resulting in a charged tRNA ready for protein synthesis. The tRNA can now transport the correct amino acid to the ribosome, where it is added to the growing polypeptide chain.

Proofreading Mechanisms

The accuracy of protein synthesis is critical for the proper functioning of cells. Mistakes in protein composition can have detrimental effects, so cells employ proofreading mechanisms to minimize errors.

Aminoacyl-tRNA synthetases have an essential 'proofreading' function, also known as 'editing', to ensure that the correct amino acid is attached to the correct tRNA. If a wrong amino acid is mistakenly attached, these enzymes can hydrolyze the incorrect aminoacyl-tRNA, freeing the tRNA to be charged again. This prevents misincorporation of amino acids in the protein sequence, thereby maintaining the fidelity of protein synthesis.