Protein kinase

A protein kinase is an enzyme that can transfer a [phosphate group]? from a donor molecule (usually ATP) to an amino acid residue of a protein. This mechanism is used in signal transduction for the regulation of enzymes; such a phosphorylation? can activate (or inhibit) the enzymatic activity of an enzyme. While most protein kinases are specialized on a single kind of amino acid residue, some of them have a dual kinase activity (they can phosphorylate two different kinds of amino acids). Protein kinases can be regulated by

• Activator proteins
• Inhibitor proteins
  • Pseudosubstrate?s
  • Autoinhibition (a part of the protein kinase mimics a pseudosubstrate)
• Ligands binding to regulatory subunits
• Cofactor?s / second messenger
• Phosphorylation in the active center (intrasterical regulation) by
  • other protein kinases (trans-phosphorylation)
  • itself (cis-phosphorylation/autophosphorylation)
• Localization in the cell

Serine?/threonine?-specific protein kinases

Serine/threonine protein kinases phosphorylate the OH group of serine or threonine (which have quite similar residues). These protein kinases can be regulated by:

• cAMP/cGMP
• Diacylglycerole
• Ca²⁺/calmodulin?

These kinases do not have a similar [consensus sequence]?; there is no common "target sequence" to be phosphorylated. As the substrate to be phosphorylated will align to the kinase by several key amino acids (usually by hydrophopic forces and [ion bond]?s), a kinase is usually spefic not to a single substrate, but to a whole "substrate family" with common properties. The kinases are usually inactivated by a pseudosubstrate? that binds to the kinase similar to a real substrate, but lacks the amino acid to phosphorylated. Its removal will activate the kinase.
The catalytic domain of the kinases is [highly conserved]?.

Phosphorylase kinase

Phosphorylase kinase was the first Ser/Thr? protein kinase to be discovered (in 1959 by Krebs et al.).

Protein kinase A

The protein kinase A consists of two domains, a small domain with several [β sheet]? structures, and a larger domain containing several [α helices]?. In the catalytic cleft between the domains (or lobes), the the substrate and ATP binding sites are located. On ATP and substrate binding, the two lobes rotate so the phosphate group of the ATP and the target amino acid of the substrate move into the right position for the catalytic reaction to take place.

Regulation

Protein kinase A has several function within the cell, including regulation of glycogen?-, sugar- and lipid metabolism. It is controlled by cAMP; in the absence of cAMP, the kinase is a tetramer of each two regulatory and two catalytic subunits (R₂C₂), with the regulatory subints blocking the catalytic center of the catalytic subunits. Binding of cAMP to the regulatory subunit leads to the dissociation of active RC-dimers. Also, the catalytic subunit itself can be regulated by phosphorylation.
Downregulation of protein kinase A is done by a feedback mechanism. One of the substrates that is activated by the kinase is a phosphodiestrase?, which converts cAMP to AMP?, thus reducing the amount of cAMP that can activate protein kinase A.

Protein kinase C

Protein kinase C actually describes a family of protein kinases that need Ca²⁺, diacylglycerole, and a phospholipid like phosphatidylcholine? for activation. This shows that protein kinase C is activated through the same signal transduction pathway as phospholipase C. There have been at least twelve members of the proteine kinase C family identified in mammals by their high [sequence homology]?. The protein kinase C usually means the protein kinase Cα enzyme.

Structure and regulation

Protein kinase C enzymes consist of a N-terminal regulatory domain, and a C-terminal catalytic domain. The kinases are inactive in the absence of activating agents by autoinhibition? of the regulatory domain. They can be activated tumor promotors like tetradecanoyle-phorbole-acetate? (or TPA), or by the cofactors Ca²⁺, diacylglycerole and a phospholipid. The common linear structure of protein kinase C enzymes is :

 N - pseudosubstrate - TPA-binding - (Ca2+-binding) - ATP-binding - substrate-binding - C 

Upon activation, protein kinase C enzymes are translocated to the plasma membrane by [RACK protein]?s (membrane-bound receptor for activated protein kinase C proteins). The protein kinase C enzymes are known for their long-term activation; they are still activated after the original activation signal or the Ca²⁺-wave are already gone. This is presumably achieved by the protuction of diacylglycerole from phosphatidylcholine? by a phospholipase; fatty acids may also play a role in long-term activation.

Function

The consensus sequence of protein kinase C enzymes is similar to that of protein kinase A, as it contains basic amino acids close to the Ser/Thr? to be phosphorylated. Their substrates are MARCKS proteins, [MAP kinase]?, transcription factor inhibitor I_XB, the vitamin D₃ receptor VDR?, [Raf kinase]?, calpaine?, and the [EGF receptor]?.

Ca²⁺/calmodulin-dependent protein kinases

Also called CaM? kinases, these kinases are primarily regulated by the Ca²⁺/calmodulin? complex. A remakably property of these kinases is a memory effect on activation. There are two types of CaM? kinases:

1. Specialized CaM? kinases. An example is the Myosine? Light Chain Kinase (MLCK) that phosphorylates myosine, causing muscles to contract.
2. Multifunctional CaM? kinases. Also collectively called CaM? kinase II, they are engaged in a multitude of processes, like neurotransmitter secretion?, [transcription factor]? regulation, and glycogen? metabolism.

Structure and autoregulation

The Cam kinases consist of a N-terminal catalytic domain, a regulatory domain, and an associative domain. In the absence of Ca²⁺/calmodulin?, the catalytic domain is autoinhibited by the regulatory domain, which conteins a pseudosubstrate? sequence. Several Cam kinases aggregate to a homooligomer or heterooligomer. On activation by Ca²⁺/calmodulin?, the activated CaM? kinases autophosphorylate each other in an intermolecular reaction. This has two effects:

1. It increases the affinity for the calmodulin complex, prolonging the time the kinase is active.
2. The phosphorylated kinase complex remains active for a while even after the calmodulin complex has dissociated from the kinase complex, prolonging the active state even further.

MAP kinases

Mos/Raf? kinases

Tyrosine-specific protein kinases

Histidine?-specific protein kinases

[Aspartic acid]?/[glutamic acid]?-specific protein kinases