The blue S is the serine to be phosphorylated. Consider, for example, the following amino acid sequence: This gives kinases their ability to have more than one substrate. Recall even from my previous article on tools that phospho-specific antibodies- another biochemical interface like a kinase binding site- will also recognize not only a phosphorylated amino acid, but also an “epitope” of adjacent amino acids.Ĭonsensus sequences depend on certain characteristics of select amino acids- polar, nonpolar, acidic, basic- while allowing for other positions to be flexible. The presence or absence of these consensus sequences determine whether a certain protein will be phosphorylated by a given kinase. In the process of phosphorylating an amino acid, kinases also recognize and bind to adjacent amino acids towards both the N- and C-termini of the phosphorylation site, known as a consensus sequence. Just as all enzymes have a discrimination and recognition for their appropriate substrate, so too do kinases have recognition for which sites they should be phosphorylating. Does that mean that a PSTK will phosphorylate every single serine and threonine in every single protein? Clearly, they won’t. Realistically, they have many, many more than one. The code (and no… not Da Vinci’s)Īll proteins have at least one serine or threonine present in their primary amino acid sequence. That said, each passing year allows more and more histidine kinase discoveries to creep into mammalian systems. Histidine kinases, for instance, are more prominent in bacteria and barely seen in humans. While these 3 phosphorylated amino acids exist in eukaryotic proteins, other amino acids can, less commonly, be phosphorylated. Those that are well-known to do so are termed dual-specificity kinases, or mixed kinases. Some kinases have the ability to phosphorylate both serine/threonine and tyrosine. As easy as it would be to say that these are strict barriers- especially in a “Basics” article- nothing in biosciences is ever so rigid. They are thus appropriately named the protein serine/threonine kinases (PSTKs) and protein tyrosine kinases (PTKs). Kinases generally fall into two categories: they will either phosphorylate the smaller serine and/or threonine- usually interchangeably- or they will phosphorylate the bulkier aromatic tyrosine. Even so, only three amino acids are most commonly recognized as phosphorylatable: serine, threonine, and tyrosine. Out of those 20, several have the electron-rich functional groups containing oxygen, nitrogen, and even sulphur to which an inorganic phosphate group could be added. Three’s a crowdĢ0 amino acids are shared amongst all forms of life, used to build the proteins that allow the biochemistry to take place. We’ll talk about the amino acids phosphorylated by kinases, the families of protein kinases, and how they choose their targets. Now, we’ll return to the protein kinases to take a deeper look- though our “deeper” look will barely scratch the surface, as there are currently over 500 known kinases with several thousand targets. So far in our “Basics” series, we’ve taken an overview of the major players involved in protein phosphorylation, and some of the tools that one might use to study them.
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