Proteins are high molecular weight organic compounds composed of a unique sequence of amino acids joined together by peptide bonds and folded into complex three-dimensional structures. They are responsible for many important structural, functional and regulatory roles in the human body and in the cells of all living organisms and viruses. These include forming the contracting components of muscle cells. Most hormones, enzymes and antibodies are also proteins. Their structure is coded in the sequence of DNA bases. Proteins comprise nearly 50% of the dry weight of cells. Peptides are molecules consisting of two or more amino acids bonded together in a defined order. They are subunits of proteins.