Protein kinases

Protein KinasesProtein kinases are activated by various molecules, and then go on to activate other proteins, forming a cascade reaction. There are many examples of various protein kinases.

Protein kinase A

Protein kinase A (PKA) is activated by the binding of cyclic AMP (cAMP), which causes it to undergo a conformational change. As previously mentioned, PKA then goes on to phosphoylate other proteins in a phosphorylation cascade (which required ATP hydrolysis).

An application in everyday life of PKA is in the 'fight or flight response'. For example, if you were to cross a road and a car was to accelerate as you were doing so, your immediate response would be to run across the road in order to avoid being hit. This response is caused by the following reactions: the nearby car acts as a stimulus, and the response is to run away, which requires energy. This energy is produced in the form of ATP, which is made by aerobic respiration which requires glucose. Glucose is generated by the break down of glycogen.

To begin with, epinephrine is secreted by the adrenal glands¹. It binds to a trimeric g-protein recepter on the plasma membrane, which induces a conformational change in the g-protein, causing the alpha subunit to let go of GDP and bind GTP, activating the alpha subunit and causing it to dissociate from the beta and gamma subunit. The alpha subunit then binds to adenyl cyclase, which converts ATP into cAMP. cAMP then binds to protein kinase A, which activates it. Protein kinase A then goes on to phosphorylise glycogen synthase a into glycogen synthase b, which inactivates it (this is done to prevent glycogen synthesis whilst it is being broken down into glucose). PKA also phosphorylates phosphorylase kinase, which then phosphorylates phosphorylase b to phosphorylase a, which causes the breaking down on glycogen into glucose², which can be used in aerobic respiration to generate ATP, so you would now have the energy surge to run across the road.

1.April Cashin Garbutt, What is Epinephrine (adrenaline)? 2013; Available from: http://www.news-medical.net/health/What-is-Epinephrine-(Adrenaline).aspx

2. W.H.Freeman and Company. Biochemistry. Sixth Edition. 2007. Figure 21-17.