Proteins perform many essential functions in cells including structural support, storage, transport, cell communication, movement, and defense. They are made up of polymers of amino acids called polypeptides that fold into unique 3D shapes determined by their amino acid sequence. A change in a protein's primary structure can affect its structure and function, as seen in sickle-cell disease where a single amino acid change causes hemoglobin to form rigid fibers that deform red blood cells. Enzymes are protein catalysts that speed up biochemical reactions and can perform their functions repeatedly; they require cofactors and coenzymes to regulate their activity.