What charge does the N-terminal of a protein typically have?

• A. Positively charged
• B. Negatively charged
• C. Neutral
• D. Varies based on conditions

Answer: (A)

The N-terminal of a protein typically has a positively charged amine group. In physiological pH, which is around 7.4, the amino group is protonated, meaning it carries an extra hydrogen ion (H+). This gives the N-terminal a positive charge, contributing to the overall charge of the protein.

Understanding the basic structure of amino acids is essential here. Each amino acid consists of a central carbon atom attached to an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom, and a unique side chain (R group). At physiological pH, the amino group at the N-terminal is protonated and thus carries a positive charge. Conversely, the carboxyl group tends to be deprotonated, resulting in a negative charge at the C-terminus.

This characteristic of the N-terminal contributes to the protein's overall stability, folding, and interactions with other molecules, which are crucial for biological functions. The charge state of the N-terminal can vary with changes in pH, but under standard physiological conditions, the positive charge predominates.