Proteins, similar to carbohydrates and lipids, are made up of such elements as carbon, hydrogen and oxygen. They are amino acid chains, made up from 20 different L-alpha-amino acids, also referred to as residues, that fold into unique three-dimensional protein structures. The shape in which a protein naturally folds is known as its native state, which is determined by its sequence of amino acids. Under 40 residues the term peptide is frequently used. A certain number of residues is necessary to perform a particular biochemical function, and around 40-50 residues appears to be the lower limit for a functional domain size. Protein sizes range from this lower limit to several thousand residues in multi-functional or structural proteins. However, the current estimate for the average protein length is around 300 residues. Very large aggregates can be formed from protein subunits, for example many thousand actin molecules assemble into an actin filament. Large protein complexes with RNA are found in the ribosome particles, which are in fact 'ribozymes'.