of cAMP-Dependent Protein Kinase A Catalytic Subunit Structure , Function and Regulation

Word Count: 172 Main Body Word Count: 2137 Abstract Protein Kinase A (PKA) catalyzes phosphoryl group transfer by orienting a protein substrate and ATP molecule in the active site cleft of the catalytic subunit, making it an integral part of many cell signaling pathways. Homology studies have identified PKA as a template for all kinases because it represents the conserved core, although specifically, it is part of the AGC kinase group in the human kinome. These conservations focus on residues in the active site and an internal architecture called the C-spine and R-spine. X-ray crystallography studies have been used to investigate the details of the catalytic mechanism Magnesium ions in the active site have emerged as important players in the positioning of the gamma-phosphate of ATP for transfer. PKA is regulated by both phosphorylation and myristylation; these regulation strategies contribute to the molecular switch functioning of PKA rather than efficacious catalyst. Furthermore, the important role of phosphorous in biology as a regulator for other proteins and the integral role PKA has in signaling pathways makes it a target of drug design.Protein Kinase A (PKA) catalyzes phosphoryl group transfer by orienting a protein substrate and ATP molecule in the active site cleft of the catalytic subunit, making it an integral part of many cell signaling pathways. Homology studies have identified PKA as a template for all kinases because it represents the conserved core, although specifically, it is part of the AGC kinase group in the human kinome. These conservations focus on residues in the active site and an internal architecture called the C-spine and R-spine. X-ray crystallography studies have been used to investigate the details of the catalytic mechanism Magnesium ions in the active site have emerged as important players in the positioning of the gamma-phosphate of ATP for transfer. PKA is regulated by both phosphorylation and myristylation; these regulation strategies contribute to the molecular switch functioning of PKA rather than efficacious catalyst. Furthermore, the important role of phosphorous in biology as a regulator for other proteins and the integral role PKA has in signaling pathways makes it a target of drug design. Introduction Phosphorylation plays a regulatory role in many biological processes. Phosphate is negatively charged, and thus, by transferring the gammaphosphate from ATP to the hydroxyl groups of serine, threonine, and tyrosine by kinases, conformational adjustments cause enzyme to be active or inactive. Protein Kinase A (PKA), for instance, only achieves an active catalytic state after being phosphorylated at Thr197 and Ser338. Phosphorylation can also indicate a binding site or alter the conformation of a domain to limit interactions of a protein with a ligand. Despite the ability of cAMP-dependent protein kinase A to catalyze the transfer of phosphate groups, it mainly acts as a molecular switch by regulating the activation of other enzymes (Taylor et al., 2013). PKA is tetrameric with two regulatory units and two catalytic subunits. PKA is downstream of the signal cascade initiated when a hormone binds to a GProtein Coupled Receptor (GPCR). Because GPCRs are very common transmembrane proteins, PKA is not specific to a pathway. However, there is some evidence that binding of regulatory units near certain GPCRs can lead to specialization. Upon binding a hormone, GPCRs undergo a conformational change that release guanine diphospate (GDP) in the s-alpha subunit of the Gprotein attached to the receptor. As guanine triphosphate (GTP) replaces GDP, the activated G s-alpha subunit separates and binds to adenylyl cylclase, an enzyme that catalyzes the cyclization of ATP to cyclic adenosine monophosphate (cAMP). cAMP is a secondary messenger that binds regulatory units, releasing PKAc—catalytic PKA (Nelson and Cox, 2013). Regulatory subunits bind in the substrate-binding cleft (Figure 1), inhibiting catalytic activity (see Figure 2). Key structural conservations between PKA and related kinases contribute to three primary functions of catalysis: binding and orienting ATP and substrate, transferring the gamma-phosphate of ATP to the substrate. Among these are twelve conserved subdomains (Hanks et al., 1988). Domains one through four make up the N-lobe of the catalytic subunit, also called the small lobe. Named for its proximity to the N-terminus, the N-lobe has one alpha helix and many betasheets and turns; it is malleable and binds ATP. The C-lobe, named for its proximity to the carboxyl-terminus, is mostly of alpha helices; thus, it is much less flexible than the N-lobe (see Figure 4). The interface between the Nand C-lobe positions the substrate and ATP molecule for group transfer. This review of the PKAc attempts to connect the conservation of key residues with specific structural and catalytic functions.