: Naturally-occurring, unbranched polymer in which the monomer units are amino acids
• Most abundant substance in cells after water – Account for about 15% of a cell’s overall mass
• Elemental composition - Carbon (C), hydrogen (H), nitrogen (N), oxygen (O), and sulfur (S) – Average nitrogen content is 15.4% by mass
• Contain iron (Fe), phosphorus (P), and other metals in certain specialized proteins
