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A difference between a Class I and a Class II tRNA synthetase is (select all that apply):

a. the Class I synthetase reaction requires a transesterification reaction not necessary in the Class II reaction.
b. the Class I synthetase uses the 2' OH of the 3' end of the tRNA as a nucleophile and the Class II uses the 3' OH.
c. the Class I synthetase requires ATP as a cofactor and the Class II does not.
d. the Class II synthetases are typically monomeric, whereas the Class I enzymes frequently function as multimeric protein complexes.

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Answer:

Option b is the correct answer.

Explanation:

The 2' OH of each 3' end of the tRNA is used as a nucleophile by Class I synthetase, whereas the 3' OH is used by Class II.

The Class I aminoacyl-tRNA synthetase attaches amino acids to the 2'-OH group of the tRNA's 3'-end ribose residue, whilst the Class II synthetase attaches amino acids only to the 3'-OH group of the tRNA's 3'-end ribose residue. Both Class I and Class II enzymes conduct the transesterification process and the need of ATP.

What we have is option d is incorrect because, Class I enzyme is typically monomeric, while the Class II enzyme is dimeric or multimeric in nature.