Structural Biochemistry/Organic Chemistry/Organic Functional Group/Phosphate

Introduction

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). The negative charges on phosphates are part of the reason why the backbone of DNA is on the outside. A good example of transferring energy is the three phosphate group which is found in the energy compound ATP(adenosine triphosphate).

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 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. Phosphorylation is prevalently seen with the application of ATP, which aids in the regulation of metabolism.

Properties

A phosphate group is a hypervalent molecule containing five bonds which makes ten electrons. Phosphate salts can form through ionic bonds between a cation and one of the oxygen anions. Molecules with a phosphate group are usually not very soluble in water. 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 the 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. Phosphate is and important component to the strength and solidity of animal teeth and bones. The exoskeleton of crustaceans and insects contain calcium phosphate as well.

Phosphate Applications

Phosphate can be used chemically as polish metal alloys. Phosphoric acid polishers work by reacting with various metal ions and generating hydrogen gas thus releasing some of the metal ions. By doing so, the metal is "phosphatized" with a phosphate coating that prevents corrosion and increases paint adhesion. This also plays a role in producing and maintaining a system of drinkable water because water usually runs through pipes and other water distribution sources that can easily erode. Because phosphate acts as a good buffer, it ensures that the pH of drinking water is stable and resists dramatic changes.