From Proteopediaproteopedia link
Crystal Structure of PTP1B-inhibitor Complex
[PTN1_HUMAN] Tyrosine-protein phosphatase which acts as a regulator of endoplasmic reticulum unfolded protein response. Mediates dephosphorylation of EIF2AK3/PERK; inactivating the protein kinase activity of EIF2AK3/PERK. May play an important role in CKII- and p60c-src-induced signal transduction cascades. May regulate the EFNA5-EPHA3 signaling pathway which modulates cell reorganization and cell-cell repulsion. 
Publication Abstract from PubMed
The following account describes our systematic effort to replace one of the carboxylate groups of our diacid thiophene PTP1B inhibitors. Active hits were validated using enzymatic assays before pursuing efforts to improve the potency. Only when the C2 carboxylic acid was replaced with another ionizable functional group was reversible and competitive inhibition retained. Use of a tetrazole ring or 1,2,5-thiadiazolidine-3-one-1,1-dioxide as a carboxylate mimetic led to the discovery of two unique starting series that showed improved permeability (PAMPA) and potency of the order of 300nM. The SAR from these efforts underscores some of the major challenges in developing small molecule inhibitors for PTP1B.
Probing acid replacements of thiophene PTP1B inhibitors.,Wan ZK, Follows B, Kirincich S, Wilson D, Binnun E, Xu W, Joseph-McCarthy D, Wu J, Smith M, Zhang YL, Tam M, Erbe D, Tam S, Saiah E, Lee J Bioorg Med Chem Lett. 2007 May 15;17(10):2913-20. Epub 2007 Feb 20. PMID:17336064
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.