Final answer:
N-terminal and C-terminal refer to the ends of proteins where amino and carboxyl groups are free, respectively. The N-terminal is used to determine sequence and is typically written first, on the left, while the C-terminal is on the right. Transmembrane proteins may have varied locations for these termini, based on how they insert into the membrane.
Step-by-step explanation:
Understanding N-Terminus and C-terminus Tags on Proteins
The N-terminal end of a protein or polypeptide is where the amino group is free and is known as the N-terminal amino acid. Methods like Sanger's, Edmann's, and Dansyl chloride reagents are used for determining the N-terminal amino acids. Conversely, the C-terminal end of a protein is where the carboxylic group is free, known as C-terminal amino acid, and can be detected using reagents like hydrazine.
During the representation on paper, the sequence of amino acids in a peptide or protein begins with the N-terminus on the left and ends with the C-terminus on the right. The N-terminus typically has a free ammonium group (–NH3), whereas the C-terminus has a free carboxyl group (-COOH).
Transmembrane proteins' N- and C-termini locations can vary depending on how these proteins insert into the membrane during synthesis. For example, with a single-pass transmembrane protein like glycophorin A, the N-terminal end is often exposed to the outside of the cell.
The N-end rule suggests that certain amino acids at the N-terminus can either stabilize or destabilize the protein, affecting its half-life. This 'punctuation system' of stabilizing and destabilizing amino acids is essential for the protein's function and degradation.