In eukaryotic cells progression through cell cycle

In eukaryotic cells, progression through leonurine sale is dependent on the activity of Cdks (Pavletich, 1999; Malumbres and Barbacid, 2005; Li et al., 2015), a group of proline directed proteins that are inactive as monomers (Peyressatre et al., 2015; Li et al., 2015), and are activated upon association to a cyclin, a regulatory subunit through the serine/threonine catalytic domain (Nigg, 1995; Honda et al., 2005; Shapiro, 2006). All kinases that have been characterized to date present a conserved catalytic core, known as S_TKc domain that contains an ATP binding site, a PSTAIRE cyclin binding domain and an activating T-loop motif. The predicted structure of the shrimp Cdk-2 herein reported shows the characteristic bi-lobed structure of all kinases (Malumbres, 2014). This structure presents the N-terminal lobe composed of several β-sheet segments, the C-helix also known as the PSTAIRE helix, a glycine rich region that contains the Thr16 and Tyr17 and its phosphorylation causes the inhibition of the kinase. The C-terminal lobe of the Cdk-2 contains the T-loop motif which includes the activation segment that spans from the DFG to the APE motives where the Thr161 is present, a sensitive to phosphorylation residue. Phosphorylation of this residue results in activation of the Cdk/Cyclin complex (Brown et al., 1999; Li et al., 2015). In addition, the ATP binding site was found between the two lobes (Fig. 4). Activation of Cdks occurs in two ways, first Cdks binds to the cyclin subunit gaining partial kinase activity; then, the formed Cdk-Cyclin complex is phosphorylated (Pavletich, 1999; Malumbres, 2014). This activation processes result in conformational modifications of the Cdk upon cyclin binding (Harper and Adams, 2001). After the binding occurs, the C-helix located in the N-terminal lobe packs against a helix located in the cyclin through hydrophobic interactions and adjusts the ATP binding site (Peyressatre et al., 2015). Also, the cyclin removes the activation segment of the C-terminal lobe out of the catalytic site, thus, the threonine residue is prone to phosphorylation and stabilization of the activated form of the Cdk-Cyclin complex (Honda et al., 2005; Malumbres, 2014). The predicted structure of the active conformation of the Cdk-2 showed changes in the T-loop and C-helix, in which both elements were separated, exposed the active site and presented the conserved phospho-threonine (Thr161). Activation of Cdk proteins is regulated by the Cdk-activating kinases (CAKs). These proteins phosphorylate the conserved phospho-threonine residue present in the T-loop, this improves the stability of the complex as well as substrate binding, thus causing Cdk activation (Harper and Adams, 2001; Lim and Kaldis, 2013). Cdks are grouped into two distinctive groups that are based on their functions, one related to cell cycle progression and one related to transcriptional regulation, Cdk-2 is grouped in the first category which is thought to be essential in cell cycle progression (Ortega et al., 2003; Shapiro, 2006). The Cdk-2/Cyclin-E complex is necessary in the transition of the G1/S phase and later the formation of the Cdk-2/Cyclin-A complex permits DNA replication at the beginning of the S phase (Matsumoto et al., 1999; Shapiro, 2006). Some cancer cells exposed to severe hypoxia suffer cell cycle arrest in the G1 phase or at the beginning of the S phase, while cells that are in the late S phase, G2 phase or M phase will finish division and arrest occurs in the G1 phase (Åmellem and Pettersen, 1991; Box and Demetrick, 2004; Muz et al., 2015). Under moderate hypoxia (2.5–5% oxygen), the cell cycle arrest in the G1 phase is associated with the decrease in the activity of the Cdk-cyclin complex (Krtolica et al., 1998).