PAH (phenylalanine hydroxylase), also known as PH, belongs to the biopterin-dependent aromatic amino acid hydroxylase family. It contains 1 ACT domain, N-terminal region of PAH is thought to contain allosteric binding sites for phenylalanine and to constitute an "inhibitory" domain that regulates the activity of a catalytic domain in the C-terminal portion of the molecule. In humans, PAH is expressed both in the liver and the kidney, and there is some indication that it may be differentially regulated in these tissues. PAH catalyzes the hydroxylation of the aromatic side-chain of phenylalanine to generate tyrosine. It is one of three members of the pterin-dependent amino acid hydroxylases, a class of monooxygenase that uses tetrahydrobiopterin and a non-heme iron for catalysis. Defects in PAH are the cause of phenylketonuria (PKU). PKU is an autosomal recessive inborn error of phenylalanine metabolism, due to severe phenylalanine hydroxylase deficiency. It is characterized by blood concentrations of phenylalanine persistently above 1200 mumol.
Serpins are the largest and most diverse family of serine protease inhibitors which are involved in a number of fundamental biological processes such as blood coagulation, complement activation, fibrinolysis, angiogenesis, inflammation and tumor suppression and are expressed in a cell-specific manner. SerpinB2, also known as Plasminogen activator inhibitor 2, Placental plasminogen activator inhibitor, Monocyte Arg-serpin, Urokinase inhibitor and PAI2, is a cytoplasm protein which belongs to the ,serpin family and Ov-serpin subfamily. SerpinB2 is a major product of activated monocytes and macrophages and is substantially induced during most inflammatory processes. Distinct from its widely described extracellular role as an inhibitor of urokinase plasminogen activator. SerpinB2 has been shown to have an intracellular role as a retinoblastoma protein (Rb)-binding protein that inhibits Rb degradation. SerpinB2 is widely described as an inhibitor of urokinase plasminogen activator. SerpinB2 inhibits urokinase-type plasminogen activator. The monocyte derived SerpinB2 is distinct from the endothelial cell-derived PAI-1. SerpinB2 is a potentially important inducible host factor that significantly promotes HIV-1 replication.
PAK3 is a member of PAK proteins, a family of serine/threonine p21-activating kinases, serve as effectors of small Rho GTPases Cdc42 and RAC and have been implicated in a wide range of biological activities. There are six mammalian PAKs which can be divided into two groups: group I PAKs (PAK1-3) and group II PAKs (PAK4-6). Although the two PAK groups are architecturally similar there are differences in their mode of regulation suggesting their cellular functions are likely to be different. Group I p21-activated kinases (PAK1/2/3) is demonstrated as ERK3/ERK4 activation loop kinases. It has been shown that group I PAKs phosphorylate ERK3 and ERK4 on Ser-189 and Ser-186, respectively, both in vitro and in vivo, and that expression of activated Rac1 augments this response. Besides regulation enzymatic activation of ERK3/ERK4, PAKs can also play roles in downstream activation of MAP kinase-activated protein kinase 5 (MK5) in vivo. Thus, the group I PAKs act as upstream activators of ERK3 and ERK4 and unravel a novel PAK-ERK3/ERK4-MK5 signaling pathway. In clinical, PAK has been proposed as a potential therapeutic target in schwannomas.
As downstream targets of the Rho GTPases, the p21-activated kinase (PAK) family of serine/threonine kinases regulates the organization of the actin cytoskeleton in mammalian cells. The PAK family is structurally categorized in two groups, each with three members: group I PAK1-3, and group II PAK4-6. PAK6 is expressed most highly in brain and testes, with lower levels in multiple tissues. Both MKK6 and p38 MAPK activate PAK6, suggesting a role for this kinase in the cellular stress response.
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