A phosphate group consists of a phosphorus atom bonded to four oxygen atoms. It is usually ionized and attached to the carbon skeleton by one of its oxygen atoms. Compounds with phosphate groups are called organic phosphates and they are frequently involved in energy transfer reactions. Phosphates can also be found in the backbone of DNA forming phosphodiester bonds (two esters with a common posphonyl).
Phosphate can exist in four different forms. In acidic conditions, it can take the form of phosphoric acid while in the more basic conditions it can be fully deprotonated to the phosphate ion. The forms phosphoric acid, dihydrogen phosphate ion, hydrogen phosphate ion, and phosphate ion all act and behave as individual weak acids. Each pK value differs greater than 4. This creates three pH regions of very acidic, moderately acidic to moderately basic, and very basic, depending on which deprotonation stage. These characteristics give the phosphate group a functionality of being a good buffer. Polymeric ions such as pyrophosphate and metaphosphates can be formed. When a pyrophosphate binds to a calcium ion, calcium phosphate is formed. Calcium phosphate is and important component to the strength and solidity of animal teeth and bones.
Phosphorylation of proteins is important for several reasons. Phosphate has a charge of negative two, which is important for disrupting or forming electrostatic interactions that alter the structure and function of proteins. Phosphate is also capable of forming three hydrogen bondshydrogen bonds. The free energy of phosphorylation is large allowing it to change the equilibrium between conformational states. Phosphorylation can also take place very quickly or very slowly making it very flexible and versatile in meeting the needs of the body. Phosphorylation also results in highly amplified effects where hundreds of target proteins are phosphorylated by one activated kinase in a short time span.