Proteinkinase
Protein kinases are enzymes that (usually ATP) catalyze the transfer of a phosphate group from a donor to the side chain hydroxyl (OH ) group of one amino acid. Kinases are therefore Phosphoryltransferasen.
Evolution of the kinases
According to Manning about 2% of all genes encoding the eukaryotic genome for kinases. The approximately 518 kinases can phosphorylate 30% of all proteins in a cell. The sum of all kinases of a cell are also called kinome (based on omics ). A classification, based on sequence similarity of the kinase domains of known biological functions, shows 10 groups, 143 families and 212 subfamilies in the Kinomen of people, flies, worms and yeast.
Functions of the kinases
Protein phosphorylation is a very important post-translational control mechanism in the signal transduction of the cell. These include, for example, the regulation of the activity of enzymes or transcription factors. Phosphorylation can by charge or conformational changes activate / inhibit or determine its subcellular localization of the target protein. The physiological effects of this protein phosphorylation depend on the substrate of the kinase.
Dysfunctions of protein kinases are the cause of numerous diseases. Thus, protein kinases attractive molecular targets drug intervention and intensive research by the pharmaceutical industry. " Specific " inhibitors of protein kinases have been successfully used in cancer therapy ( see, eg, imatinib in the treatment of chronic myeloid leukemia).
Regulation of the kinase activity
Due to the far-reaching influences of the protein kinases they have to be even strictly regulated. Regulating factors include:
- Cofactors / Secondary messengers ( Ca2 , IP3, cAMP, et cetera )
- Activator and inhibitor proteins
- Pseudosubstrates Autoinhibition ( a part of the peptide chain of protein kinase acts as a pseudo- substrate)
- Ligand binding to regulatory subunits
- Other protein kinases (trans- phosphorylation )
- Itself ( Cis-phosphorylierung/Auto-phosphorylierung )
Types of protein kinases
Most protein kinases phosphorylate either the amino acids serine / threonine or tyrosine aromatic. However, there are bi-specific protein kinases ( EC 2.7.12, for example MEK or MAP2K ), which can phosphorylate both serine / threonine and tyrosine residues.
Serine / threonine kinases
These protein kinases ( EC 2.7.11 ) phosphorylate the hydroxyl groups ( OH groups) of the amino acids serine and threonine. These kinases are regulated by:
- 1,2- diacylglycerols (DAG )
- Ca2 and calmodulin
- PIP3 and other phospholipid derivatives
That serine / threonine kinases phosphorylate not all threonine and serine residues of other proteins, is determined by the interaction with the surrounding peptide sequence. These sequences are called consensus sequences. Due to the low specificity of any individual proteins are phosphorylated by these kinases, but whole families of proteins. These enzymes are inhibited by a pseudo- substrate binding to the active site, by mimicking the target sequence of the corresponding kinase, but may not serine or threonine.
Muscle phosphorylase kinase A
This enzyme (EC 2.7.11.19 ) is the first Ser / Thr kinase, which was discovered in 1956. This is a key enzyme of glycogen metabolism.
Protein kinase A
This enzyme has several functions in the cell. These include the regulation of glycogen, glucose and lipid metabolism. Protein kinase A ( EC 2.7.11.11 ) is composed of two domains. The smaller is made a large number of β -strands, while the large subunit has a large number of α - helices. The catalytic site is located between the two subunits. ATP binds to a substrate, the subunits rotate against each other, so that the γ -phosphate of ATP is near the to be phosphorylated amino acid and the transfer reaction can take place.
They themselves are regulated by cAMP. By binding of cAMP to the inactive tetramer made up of two regulatory and two catalytic subunits ( R2C2 ), the regulatory subunits are separated from the catalyst, whereby the phosphorylation of other proteins is possible. A protein kinase is in turn regulated by phosphorylation.
In addition, the available amount of cAMP is reduced by conversion to AMP through the activation of a phosphodiesterase. This means that the protein kinase A causes inhibition of its own and it could lead to no "permanent activation" of the kinase.
Protein kinase B
In the English literature often particularly called Akt kinase is an enzyme of the PI3K/Akt-Signalweges which has numerous effects on the homeostasis of the cell, and survival, apoptosis, proliferation, and metabolic regulation. Act comes in three closely related isoforms ( Akt1, Akt2, Akt3 ) whose sequences on chromosome 14q32, 19q13 1q43 respectively are coded. No substrate specificity differences were found between the different isoforms.
The activation is carried out in several steps. First phosphatidylinositol 3-kinases (PI3K ) are recruited by activated receptor to the membrane, and thus activated. Activated PI3K catalyze the phosphorylation of phosphatidylinositol at phosphatidylinositol -3 - phosphate, or phosphatidylinositol -4- phosphate to phosphatidyl -3 ,4- bisphosphate and phosphatidylinositol -4 ,5- bisphosphate to phosphatidylinositol -3 ,4,5- trisphosphate. The substrates thus produced now recruiting proteins with pleckstrin homology (PH ) domains to the membrane. The so- docked act is now exhibiting over another PH- domain signaling molecule that phosphorylates PDK1 ( phosphoinositide - dependent kinase 1 ) are capitalized.
As mentioned above regulated protein kinase B including the survival, proliferation and death of the affected cell cycle. One example is the increased incidence of glucose receptor Glut4 in insulin response to an incoming signal. The transcription of this Carrier is cheered cascade and the installation thereof by vesicle transport and constriction causes. Can be self-regulated the activity of the enzyme, for example, by the tumor suppressor protein PTEN, which ,4,5- trisphosphate for the dephosphorylation of phosphatidylinositol -3, for example, phosphatidylinositol -4 ,5- bisphosphate (or other 3'- phosphorylated phosphoinositides ) is responsible. Thus, the substrate of the protein kinase is disabled, since it is now no longer able to dock with the PH domain.
The function and regulation mechanisms are diverse and are examined in detail today, because it is assumed that a mutation in certain areas of the signal path (eg: PTEN ) is a basis for the formation of tumors.
Protein kinase C
The term protein kinase C ( EC 2.7.11.13 ) meets a family of proteins with 12 members in mammals that require Ca2 , diacylglycerol, and a phospholipid such as phosphatidylcholine for activation. In most cases the protein kinase Cα is meant when these enzymes is the speech.
Protein kinases are highly conserved proteins that consist of an N-terminal regulatory domain and a C-terminal catalytic domain. As long as the enzyme is not a tumor promoter such as tetradecanoyl phorbolacetat (TPA ), or one of the abovementioned co-factors is activated, it is inactive. The general linear structure:
N - pseudosubstrate - TPA / DAG - binding - Ca2 binding - ATP binding - substrate binding - C
During activation shifted the protein kinase C to the cell membrane by means of receptors for activated protein kinase C ( RACK proteins). After activation, these kinases remain active for a long time, although the Ca2 concentration has fallen again. This is attributed to the action of diacylglycerol, which is formed of phosphatidylinositol with a phospholipase. It is activated by the same signal, such as protein kinase itself
The target sequence of the protein kinase C is the protein kinase A similar since it contains many basic amino acid residues in the vicinity of the phosphorylated Ser / Thr residues. Substrates of the protein kinase C are MARCKS protein, MAP kinases, transcription factor inhibitor IkB, vitamin D3 receptor, Raf kinase, calpain and the EGF receptor.
Tyrosine kinases
Among the tyrosine kinases ( EC 2.7.10 ), there are so-called receptor tyrosine kinases ( RTKs ) and tyrosine kinases without receptor function. It summarizes the 50 different RTKs in 18 receptor families. After the ligand these RTKs are grouped into the following families:
- EGF
- PDGF
- Insulin
- VEGF
- FGF
- Ephrin
- Angiopoietins
- And others.