Cascades of signals are transduced when, for example, a hormone meets its receptor, when one cell touches another, or when a lymphocyte contacts its cognate antigen. Many steps in these pathways involve protein phosphorylation. (See related story, Monitoring Protein Phosphorylation)
What are kinases and phosphatases? Kinases are enzymes that catalyze the addition of phosphate groups. Most large cellular molecules, including proteins, carbohydrates, and lipids, can be phosphorylated. Enzymes called phosphatases remove phosphates.
Who are the major players? Two protein kinase groups have been widely studied in eukaryotes: those that phosphorylate tyrosine side chains, and those that phosphorylate serine or threonine side chains. Recently, histidine kinases also have begun to be noticed. The functional distinctions between these groups are becoming blurry. (Aspartate kinases have so far been reported only in prokaryotes.)
How does phosphorylation allow for signal transduction? Adding phosphate to a protein may cause it to change its shape, perhaps ...
