Phosphoryl groups are everywhere in biochemical pathways. When you eat a candy bar, one of the very first thing that happens to the ingested sugar molecules is that one of the hydroxyl groups is transformed into a phosphate ester. A functional group known as a 'phosphate diester' forms the molecular ‘tape’ that holds DNA and RNA together, and the biological activity of many proteins is switched ‘on’ or ‘off’ depending one whether certain amino acids with hydroxyl groups – serines, threonines, and tyrosines – have been converted to phosphate esters. Adenosine triphosphate (ATP) is the principle form of energy currency of living things - almost like a dollar bill for cellular energy - and ATP plays this role by acting as a phosphate group donor. As we learned in chapter 8, alcohols are frequently converted into phosphates, diphosphates, or nucleotide phosphates in order to create a good leaving group. And as we will see in chapter 13, the transfer of a phosphoryl group to a carboxylate is a critical step in the metabolism of fats and oils.
Clearly, in order to understand the chemistry that occurs in living things, you must understand the chemistry of phosphoryl groups and how they are transferred from one molecule to another. And as we shall soon see, this chemistry is will be somewhat familiar to you, as it shares many features with the nucleophilic substitution reactions we studied in the previous two chapters.