<P> Proteins have two types of well - classified, frequently occurring elements of local structure defined by a particular pattern of hydrogen bonds along the backbone: alpha helix and beta sheet . Their number and arrangement is called the secondary structure of the protein . Alpha helices are regular spirals stabilized by hydrogen bonds between the backbone CO group (carbonyl) of one amino acid residue and the backbone NH group (amide) of the i + 4 residue . The spiral has about 3.6 amino acids per turn, and the amino acid side chains stick out from the cylinder of the helix . Beta pleated sheets are formed by backbone hydrogen bonds between individual beta strands each of which is in an "extended", or fully stretched - out, conformation . The strands may lie parallel or antiparallel to each other, and the side - chain direction alternates above and below the sheet . Hemoglobin contains only helices, natural silk is formed of beta pleated sheets, and many enzymes have a pattern of alternating helices and beta - strands . The secondary - structure elements are connected by "loop" or "coil" regions of non-repetitive conformation, which are sometimes quite mobile or disordered but usually adopt a well - defined, stable arrangement . </P> <P> The overall, compact, 3D structure of a protein is termed its tertiary structure or its "fold". It is formed as result of various attractive forces like hydrogen bonding, disulfide bridges, hydrophobic interactions, hydrophilic interactions, van der Waals force etc . </P> <P> When two or more polypeptide chains (either of identical or of different sequence) cluster to form a protein, quaternary structure of protein is formed . Quaternary structure is an attribute of polymeric (same - sequence chains) or heteromeric (different - sequence chains) proteins like hemoglobin, which consists of two "alpha" and two "beta" polypeptide chains . </P> <P> An apoenzyme (or, generally, an apoprotein) is the protein without any small - molecule cofactors, substrates, or inhibitors bound . It is often important as an inactive storage, transport, or secretory form of a protein . This is required, for instance, to protect the secretory cell from the activity of that protein . Apoenzymes becomes active enzymes on addition of a cofactor . Cofactors can be either inorganic (e.g., metal ions and iron - sulfur clusters) or organic compounds, (e.g., flavin and heme). Organic cofactors can be either prosthetic groups, which are tightly bound to an enzyme, or coenzymes, which are released from the enzyme's active site during the reaction . </P>

Two kinds of biomolecules that can work as enzymes are