Question

Which of the following is the amino acid that appears at the \(\mathrm{N}\) -terminus of all newly translated prokaryotic and eukaryotic polypeptides? a. tryptophan b. methionine c. selenocysteine d. glycine

Short answer

The amino acid that appears at the N-terminus of all newly translated prokaryotic and eukaryotic polypeptides is methionine.

Step-by-step solution

Understanding the Question

The question is asking to identify the amino acid that is initially added to the nascent polypeptide chain during protein synthesis in both prokaryotic and eukaryotic cells. This amino acid is universally found at the N-terminus, which is the beginning of the protein sequence.

Recalling Protein Synthesis Basics

In both prokaryotes and eukaryotes, the first amino acid during the initiation of translation is methionine (for eukaryotes) or a modified form of methionine (N-formylmethionine, fMet, in prokaryotes). This amino acid is encoded by the start codon AUG in the mRNA sequence.

Identifying the Correct Amino Acid

Based on the knowledge of protein synthesis, we can identify that methionine is the amino acid that fits the description and thus is the correct answer.

Key concepts

N-terminus of polypeptides

When learning about how proteins are made, a key concept is the 'N-terminus of polypeptides'. This term refers to the very start of a protein or polypeptide chain. Proteins are comprised of amino acids linked together, and the N-terminus is the end of the protein that features a free amine group (-NH2). In contrast, the other end of the protein, known as the 'C-terminus', has a free carboxyl group (-COOH).

Understanding the directionality of polypeptides is crucial because the sequence of amino acids from the N-terminus to the C-terminus determines the protein's unique structure and function. The N-terminus is significant not only for protein structure but also for post-translational modification, which can affect protein location, activity, and interactions within a cell. For instance, the N-terminal methionine can be removed or modified after protein synthesis, influencing protein stability and regulation.

Protein synthesis basics

Delving into 'protein synthesis basics', every student should grasp the process of transferring genetic information from DNA to functional proteins. This central biological mechanism consists of two main stages: transcription and translation. During transcription, a segment of DNA is copied into mRNA (messenger RNA), which serves as a temporary blueprint. The mRNA then travels from the nucleus to the ribosome, where translation occurs.

In translation, the mRNA sequence is decoded into a polypeptide chain, which folds into an active protein. The ribosome reads the mRNA in three-letter sequences called codons, each specifying a particular amino acid. The amino acids are brought to the ribosome by tRNA (transfer RNA) molecules.

Significance of Ribosomes and tRNA

The ribosome is the site where the mRNA code is translated into an amino acid sequence. Each tRNA carries a specific amino acid and an anticodon that pairs with the mRNA codon. The start codon marks the beginning of this process, and the stop codons signal its end, releasing the newly synthesized polypeptide for further folding and processing.

Start codon AUG

A vital piece of the protein synthesis puzzle is the 'start codon AUG'. This codon is universally recognized as the initiation signal for translation, marking the location on the mRNA where the ribosome should begin synthesizing the polypeptide chain. It's like the 'ready, set, go!' for protein production.

In the mRNA sequence, AUG is not only a start signal; it also codes for the amino acid methionine, which is the default first amino acid in new polypeptides. Interestingly, even though methionine is specified by AUG, not all proteins ultimately begin with methionine because it is sometimes cleaved off after the polypeptide chain is made.

The Role of tRNA and Initiation Factors

Dedicated tRNA molecules recognize the start codon and bring the appropriate methionine to initiate the protein chain. Eukaryotic cells use methionine to start the polypeptide chain, while prokaryotic cells use a modified version called N-formylmethionine (fMet). Additionally, special proteins called initiation factors help the ribosome assemble and locate the start codon, ensuring that translation begins correctly and efficiently. This highlights the meticulous orchestration required to commence the complex process of protein synthesis.