{
  "abstract": "Mycobacterium tuberculosis (Mtb) is a significant intracellular pathogen that causes tuberculosis (TB), a disease with considerable morbidity and mortality. Mtb evades host immunity by secreting effector proteins, including phosphatidylinositol-3-phosphatase SapM, which inhibits phagosomal maturation. However, the structure and functional mechanisms of SapM remain incompletely understood. Here, we successfully isolated monomeric SapM protein, with a measured hydrodynamic diameter of 5.1 ± 0.3 nm. We used p-nitrophenyl phosphate (pNPP), a small chromogenic phosphate monoester substrate, as a surrogate for PI3P (phosphatidylinositol-3-phosphate) in enzymatic assays. Molecular docking was performed using a refined model, and site-specific mutations were introduced. The R61A, T236A, and T250A mutants demonstrated a 5- to 60-fold reduction in activity. The segment 237–249 displayed greater flexibility compared to other regions of SapM. All enzymatic assays used pNPP without added metal; the reported kinetic parameters are therefore apparent values. The active-site metal was not identified, and metal occupancy was not controlled. The SapM gene is critical for Mtb survival within macrophages. In this study, p-nitrophenyl phosphate (pNPP) was used as a chromogenic surrogate for PI3P; therefore, the work reports active-site integrity and pNPP hydrolysis rather than physiological PI3P catalysis. The present data do not establish PI3P turnover in a membrane environment, and PI3P-liposome assays remain future work. Our findings provide a molecular framework for SapM function and identify 237–249 as a candidate regulatory element for future studies. Whether SapM can be targeted therapeutically remains to be established.",
  "abstract_shown": true,
  "affiliations": [
    "College of Biological Sciences and Technology, Yili Normal University, Yining 835000, China",
    "School of Life Sciences, Xiamen University, Xiamen 361102, China"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 2,
  "authors": [
    {
      "affs": [
        0,
        1
      ],
      "name": "Dafeng Liu"
    },
    {
      "affs": [
        1
      ],
      "name": "Wenshuang Yao"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/7rrvze37z44t1.bib",
  "cited_by": 0,
  "doi": "10.3390/biom16111595",
  "fulltext_url": null,
  "id": "7rrvze37z44t1",
  "issns": [
    "2218-273X"
  ],
  "landing_url": "https://www.mdpi.com/2218-273X/16/11/1595",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [],
  "openalex": null,
  "pdf_url": null,
  "pmcid": null,
  "pmid": null,
  "published": "2026-11-02",
  "publisher": "MDPI AG",
  "ris": "https://research.draconitic.com/paper/7rrvze37z44t1.ris",
  "sources": [
    {
      "first_seen": "2026-11-03T00:23:55Z",
      "native_id": "10.3390/biom16111595",
      "source": "crossref",
      "url": "https://www.mdpi.com/2218-273X/16/11/1595"
    }
  ],
  "subjects": [
    {
      "field": "biochemistry",
      "origin": "issn:2218-273X",
      "subfield": "general"
    }
  ],
  "title": "The Surrogate Substrate p-Nitrophenyl Phosphate (pNPP) for the Phosphatidylinositol-3-Phosphatase SapM from Mycobacterium tuberculosis: Mutational Analysis of pNPP Hydrolysis and Preliminary Single-Gating Observations",
  "title_text": "The Surrogate Substrate p-Nitrophenyl Phosphate (pNPP) for the Phosphatidylinositol-3-Phosphatase SapM from Mycobacterium tuberculosis: Mutational Analysis of pNPP Hydrolysis and Preliminary Single-Gating Observations",
  "type": "peer-reviewed",
  "updated_at": "2026-11-04T00:24:50Z",
  "url": "https://research.draconitic.com/paper/7rrvze37z44t1",
  "venue": "Biomolecules",
  "versions": []
}