We construct a peptide-conjugated metal cluster as an enzyme-like catalytic bioprobe to enhance quantitative analysis of a membrane protein biomarker and detect epithelial-to-mesenchymal transition of tumor cells. This bioprobe with atomically precise formula, termed clusterzyme, possesses selective recognition and intrinsic enzyme-like activity. These favorable features facilitate sensitive quantitative analysis of the membrane protein in situ through on-cell catalytic signal amplification. This clusterzyme-based analytical method exhibits excellent compatibility with a traditional enzyme-linked immunosorbent assay and improved detection sensitivity with accuracy and robustness. Further, the expression level of the membrane protein reflects the ability of migration and invasion of model tumor cells, revealing epithelial-to-mesenchymal transition process. This work offers a facile and sensitive approach to monitor tumor cell type evolution at the molecular level, demonstrating a potential application of early cancer diagnosis and therapy assessment.