Any of a large class of natural polymers composed of chains of amino acids connected by peptide linkages. Proteins occur in the cells of all living organisms and in biological fluids. Their primary structure is determined by the order of their amino acids. Examples of soluble proteins include enzymes and antibodies. Insoluble proteins include keratin and collagen. Examples of protein containing compounds are leather, gelatin, albumen, animal glue, silk, ivory, egg, and casein.