The N terminal of a protein is the first segment (or residue) in the protein.

The "N" refers to the nitrogen atom in the "amine group" of an amino acid.

Proteins are built in a specific order. The N terminal residue is always one that was added before any others.

Cartoon with Rockets

"Rockets" cartoon view of protein in Jmol. The arrowheads on the helixes and sheets point toward the C terminal of the protein, and away from the N terminal.

Methionine is always the first amino acid added to a protein, but it's often removed by a post-translational modification (PTM).

The order of segments is important.

Some protein-viewing tools have "rockets", similar to the cartoon view in Foldit, but the arrowheads on sheets and helixes. The arrowheads point toward the end of the protein, the higher-numbered segments.

The last segment of a protein is called the C terminal.

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