Protein.

A large amino-acid chain that folds into a functional 3-D structure.

Proteins are long amino-acid chains that fold into specific three-dimensional shapes in order to carry out structural, enzymatic, transport, or signalling roles. The distinction from peptides is mainly one of length and folded complexity: a peptide is usually short enough that its activity depends on its sequence and a fairly simple shape, while a protein's function typically depends on a precise fold that can be destroyed by heat, pH, or rough handling. This distinction matters in peptide research for practical reasons. Small peptides can often be built by chemical synthesis, sequence by sequence, at reasonable cost. Larger proteins such as antibodies and many growth factors generally have to be produced in living cells, which is slower and more expensive, and they are far more sensitive to storage and shipping conditions. It also affects analysis, since confirming that a short peptide has the right mass is a much simpler problem than confirming that a large protein has folded correctly. A common point of confusion is that some compounds discussed in peptide communities are really proteins or protein fragments rather than peptides in the strict sense. The label used in marketing tends to be looser than the label used in biochemistry, so the word peptide on a product page does not reliably tell you what size of molecule is inside.