About Protein Tyrosine Phosphatase / PTP:
Protein Tyrosine Phosphatases (PTPs) are a group of enzymes responsible for the removal of phosphate groups from phosphorylated tyrosine residues on proteins. Protein tyrosine (pTyr) phosphorylation is a very common post-translational modification. It is used to create recognition motifs for protein interactions and cellular localisation. It directly affects the stability of protein stability, and plays a role in regulating enzyme activity.
This makes the maintenance of PTP levels essential for a wide range of cellular functions. Here we’ll take a look into their functions and interactions as well as detailing their mechanisms and the role they play in their classifications as well as their structures.
Protein Tyrosine Phosphatase Functions and Interactions
Tyrosine-specific protein phosphatases work by catalysing the removal of a phosphate group attached to a tyrosine residue. This is done using a cysteinyl-phosphate enzyme intermediate. These enzymes are key regulatory components with a pivotal role in signal transduction pathways and cell cycle control. The MAP kinase pathway is a commonly referenced example. and cell cycle control, and are important in the control of the growth, differentiation and proliferation of cells as well as the regulation of synaptic plasticity.
PTPs (in tandem with tyrosine kinases) regulate the phosphorylation state of many important signalling molecules. Although we know less about them than tyrosine kinases it’s widely held that they are an integral part of signal transduction cascades.
As such, PTPs have been implicated in regulation of many cellular processes, some of the most important such as cell growth, cellular differentiation, cancer formation (oncogenic transformation), mitotic cycles / cell proliferation and receptor endocytosis.
In the case of most PTPs classification is intrinsically linked to their mechanism although some are classified by their localization.
Class I, by far the largest group, Class I PTPs have 99 members, of which 38 are “classical” PTPs (21 receptor tyrosine phosphatases and 17 nonreceptor-type PTPs) while 61 are VH-1-like or Dual-Specific Phosphatases (DSPs).
These Dual-Specificity Phosphatases are a group of enzymes have both Ser/Thr and tyrosine-specific protein phosphatase activity making them able to remove the serine/threonine or the tyrosine-bound phosphate group from a wide range of phosphoproteins. DSPs have been found to regulate mitogenic signal transduction as well as controlling cell cycles.
Elevated levels of an activated PTP (PTPN5) has been demonstrated to play a role in Alzheimer’s disease and schizophrenia by affecting the stability of synapses.
Class II PTP contains only one member, low-molecular-weight phosphotyrosine phosphatase, while Class III PTPs contains three members, CDC25 A, B, and C.
Class IV PTPs are members of the HAD superfamily, including pTyr and pSer/Thr specific phosphatases as well as small molecule phosphatases. The subfamily EYA (eyes absent) is believed to be pTyr-specific, and has four members in human physiology- EYA 1-4 and is characterized by its own distinct catalytic mechanism from the other three classes.
All PTPs employ the common catalytic mechanism of protein tyrosine phosphorylation and this is a fundamentally important regulatory mechanism for all kinds of lifeforms.
Protein Tyrosine Phosphatase Structure
Virtually all PTPs (other than Class IV PTPs in the EYA family) carry the highly conserved active site motif C(X)5R. This PTP signature motif is a core structure comprised of a central parallel beta-sheet with flanking alpha-helices containing a beta-loop-alpha-loop.