Ziyang Qiao, Chau An Tran, Hannah Whipple

1. PKA Phosphorylation of CREB

Protein Kinase A is an essential signal regulator in human metabolism. As mentioned in the  Phosphorylation of cAMP Response Element Binding Protein (CREB) pathway, its activation is induced by the body’s circadian rhythm via the SCN, as well as glucagon production during time of fasting. Through a signaling cascade, glucagon activates both G-coupled protein receptors to produce ATP and thus allows adenylate cyclase to convert ATP to cAMP. The production of four cAMP molecules is sufficient to activate one PKA.

PKA is a tetramer made up of four subunits: two regulatory (R-subunits) and two catalytic (C-subunits). The R-subunits are responsible for ensuring that PKA is only activated when it is needed. These are bound to the C-subunits’ regulatory activation site, and will only release the C-subunits when cAMP is bound to their active sites (Figure 1) [20]. The key residues in the binding domain of PKA’s R-subunits are threonine and arginine, both of which stabilize and attach themselves to cAMP (Figure 2).

Figure 1. Activation of PKA via cAMP binding to R-subunits. Four cAMP signaling molecules bind to the two R-subunits. This causes the R-subunits to activate and release the C-subunits for further signaling.
Figure 2. Close-up of the interaction between cAMP and the binding site of the R-subunits of PKA. A water molecule is used to form interactions between Thr207 and an oxygen on the phosphate of cAMP. Arg241 stabilizes the negative charge on ribose’s hydroxyl group. Adapted from Fig. 1C in [21]

Once the C-subunits are released, they cross from the cytosol into the nucleus to phosphorylate various proteins. However, before it can phosphorylate CREB, the C-subunit must acquire a phosphate group in its active site by converting an ATP to an ADP (Figure 3).

Figure 3. PKA’s C-subunit’s active site removes a phosphate group from ATP, releasing an ADP. Key residues within the active site consist of threonine, which attacks the phosphate group, and arginine, which stabilizes the negative charges within the active site. Once the active site is phosphorylated, ADP leaves.

Once PKA is activated via phosphorylation, CREB enters PKA’s active site. The Ser133 residue on CREB attacks and removes the phosphate group from PKA (Figure 4), therefore activating CREB [9].

Figure 4. Phosphorylation of CREB through PKA. Ser133 residue on CREB attacks and removes the phosphate attached to threonine on the PKA active site.

Intermittent fasting has the potential to restore glucose metabolism modulated by the circadian clock: by eating earlier and fasting throughout the night, CREB phosphorylation is limited to occurring only in the fasting state during the day.  

Phosphorylated CREB will eventually function to induce gluconeogenesis and glycogenolysis and release glucose into the bloodstream. Individuals with Type II diabetes are more likely to have this signaling pathway induced in both fasting and fed-states, while those that practice intermittent fasting have tighter regulation on glucagon production, phosphorylating CREB only in the fasted state.

7 Comments

  1. Ryan P. Hayes

    All of the information on this page is awesome! Some small things that I noticed in your diagrams: the Arginine side chain aesthetically looks a bit funky without that third Nitrogen atom at the end of the chain to me, and in Figure 4 the arginine gets protonated without reason (there probably is a good reason not obvious to me). The information all looks great!

  2. Pun Sangruji

    As usual, the diagrams are beautiful and the information is presented in a concise and clear manner. Through reading through the page, I am to follow this entire reaction cascade step by step, which speaks volumes about how nicely the information is presented. The final two paragraphs also provide a strong conclusion to the page, reinforcing the significance of the pathway. It effectively emphasizes why intermittent fasting serves as a solution to the downregulation of CREB phosphorylation, particularly in its limitation to occur only in the morning. These paragraphs also build seamlessly on the information initially presented in the first chemical pathway page.

  3. Andy Z. Wu

    Nice work on this mechanism! Each step of the phosphorylation of CREB was very methodical, and I especially liked how you related Figure 1 and Figure 2 (1 describing the interactions of the R and C subunits on a more macro level, and 2 highlighting the more specific biochemical interactions of PKA’s active site residues with cAMP). Furthermore, you connected this mechanism’s relative importance tactfully by your initial mentioning of the pathway page, which was advantageous in maintaining cohesiveness. Not really much to add other than potentially adding lone pairs to the attacking hydroxyl in Figure 4. Otherwise it looks great to me!

  4. Ezra A. Rivera

    Really nice explanations, especially in the second paragraph. You said that four cAMP molecules are needed to activate one PKA, but it seems like the activated C domains are separate from each other; it’s a little bit confusing, so it could be addressed. Additionally, the arginine switches from charged to uncharged several times without explanation. Once again, I liked the visuals a lot.

    • Chau An C. Tran

      The caption on Figure 1 has been updated. Hopefully this will help clarify the mechanism.

  5. Sam B. Saint Pre

    The information in this page can be easily followed. I really liked the final paragraph which answers the “so what? Why is this important?” question. It provides clear detail of the ultimate result of the mechanism depicted. My only feedback would be in figure 3 where ADP is still incorporated in the enzyme, I think it would be useful to have a leaving arrow showing ADP released and have the active site component being the phosphorylated residue.

    • Chau An C. Tran

      This has been added in Figure 3 and the caption is updated.

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