Signal Transduction

FIG 21.4  The arrangement at the active site of PTP1B. (a) The sides of the catalytic cleft of tyrosine phosphatases are characterized by three motifs: the WPD loop, containing an invariant Asp, the Q-loop, containing an invariant Gln and the phosphotyrosine-binding loop (Ptyr), containing an invariant Tyr (1ptt10).

(b) The invariant tyrosine determines the depth of the catalytic cleft, 10 Å, and allows the target phosphotyrosine to make contact with the catalytic cysteine residue (yellow). (c) The pocket is too deep for phosphoserine or phosphothreonine to contact the catalytic cysteine residue (2hnp11).

Unlike the kinases (see page 782), the catalytic domains of tyrosine protein phosphatases do not require post-translational modifications in order

to become catalytically competent. The binding of substrate induces a conformational change in the protein in which the WPD loop closes around the substrate to create the recognition pocket, generating the catalytically active form of the enzyme (Figure 21.4a). Protein tyrosine phosphatases

are, however, subject to regulation, by either intramolecular interactions, phosphorylation of non-catalytic domains, association of regulatory subunits, or spatial separation from substrate.

The catalytic mechanism of the tyrosine phosphatases is quite different from that of the serine/threonine phosphatases. These are metalloenzymes that dephosphorylate their substrates in a single reaction step involving a metal-activated nucleophilic water molecule. In contrast, the PTPs catalyse dephosphorylation through a cysteinyl-phosphate enzyme intermediate (reviewed in Tonks12) (Figure 21.5).

PTPs in signal transduction

At first, interest in protein tyrosine phosphatases was driven by the perception that they might be antitumorigenic. They appeared to offer the possibility

646