4.10 : Protein Kinases and Phosphatases

Phosphorylation and dephosphorylation are chemical modifications where enzymes add or remove a phosphate group from an amino acid residue on a protein substrate. These chemical changes can regulate the function of the target protein through alterations in conformation or activity.

Protein kinases are enzymes that phosphorylate proteins and other substrates. Kinases catalyze the reversible addition of a phosphate from ATP to the hydroxyl side chains of serine, threonine, or tyrosine residues. 

Eukaryotic protein kinases belong to an extensive family of enzymes with conserved structures and catalytic sequences. 

When kinases transfer a phosphate group to the substrate, it forms a hydrogen-bonded network with the surrounding amino acid residues. This network of hydrogen bonds alters the three-dimensional structure of the target protein, modifying its function.

Such changes in function can include activating or deactivating the substrate's enzymatic activity or creating a new surface where other molecules can interact with the substrate.

Phosphatases catalyze the hydrolysis of the phosphate, using a water molecule to remove a phosphate group as phosphate ion and leaving a free hydroxyl group on the amino acid residue. This disrupts the hydrogen bonding, restoring the original conformation and function.

Phosphorylation is a substrate-specific process determined by several elements, including the unique kinase catalytic sequence, local and distal binding with the substrate, and adaptor proteins that mediate distinct kinase-substrate interactions.  
 

Some phosphatases also have high substrate specificity— they remove phosphate from only one or a few selected proteins. Other phosphatases can act on several different protein substrates and are directed to a particular target by the substrate’s regulatory subunits. 

Protein kinases and phosphatases work together to toggle a protein between the phosphorylated and dephosphorylated states. These changes play crucial roles in different signaling and metabolic pathways. 

When glucose is elevated in the blood, insulin leads to increased protein phosphatase-1 activity. The enzyme dephosphorylates target substrates leading to an organism storing glucose as glycogen. 

As blood glucose levels drop, protein kinase A is activated.  Phosphorylation of target proteins by protein kinase A stimulates glycogen breakdown, releasing glucose into the bloodstream.