Aminopeptidase N

Aminopeptidase N. It is a zinc-dependent metalloprotease enzyme that is widely expressed on the surface of many cell types. This ectoenzyme plays crucial roles in the final stages of protein digestion and the processing of various bioactive peptides. Its discovery and characterization have linked it to important physiological processes and pathological conditions, including roles in tumor progression and viral entry.

Structure and function

The enzyme is a type II transmembrane protein with a short N-terminal cytoplasmic tail, a single transmembrane domain, and a large extracellular domain containing the catalytic site. It functions as a homodimer on the cell surface, with each subunit possessing a conserved zinc-binding motif characteristic of the M1 family of aminopeptidases. Its primary biochemical function is the cleavage of neutral amino acids, particularly alanine, from the N-terminus of peptides and proteins. This activity is integral to the terminal digestion of dietary proteins in the small intestine and the processing of regulatory peptides like enkephalins and angiotensin III in various tissues.

Biological role and clinical significance

In the gastrointestinal tract, it is essential for nutrient absorption, working in concert with other brush border enzymes. Beyond digestion, it modulates peptide hormone activity and influences immune response by regulating processes on antigen-presenting cells and leukocytes. Its overexpression is a documented feature in cells from several malignancies, including acute myeloid leukemia and prostate carcinoma, where it contributes to angiogenesis and metastasis. Furthermore, it serves as a receptor for certain coronaviruses, such as human coronavirus 229E, facilitating viral attachment and cellular entry.

Genetics and expression

The gene encoding this enzyme, ANPEP, is located on human chromosome 15 in the q25-q26 region. Expression of the gene is regulated by various transcription factors and can be induced by inflammatory cytokines like interferon-gamma. It is constitutively expressed on epithelial cells of the kidney and intestine, on myeloid lineage cells, and on synovial fibroblasts. Its expression pattern is utilized in clinical diagnostics; for instance, it is a marker for identifying certain leukemias via flow cytometry using antibodies against CD13.

Inhibitors and therapeutic applications

Research into specific inhibitors has been driven by its roles in cancer and inflammation. Early inhibitors included broad-spectrum chelating agents like bestatin, which is used as an immunomodulator in Japan. More selective, potent inhibitors have been developed, such as ubenimex and protease inhibitor compounds designed to mimic transition state analogs. Therapeutic strategies aim to block its enzymatic activity to suppress tumor growth or inhibit its function as a viral receptor. These approaches are being explored in clinical trials for conditions ranging from solid tumors to COVID-19.

Research history and discovery

The enzyme was first identified in the mid-20th century through biochemical studies of intestinal mucosa and renal brush border membranes. Its classification as EC 3.4.11.2 was established by the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology. The independent discovery of its identity with the myeloid differentiation antigen CD13 in the 1980s, led by researchers like Kurt D. Look, unified the fields of enzymology and immunology. Its role as a coronavirus receptor was elucidated following work on human coronavirus 229E by scientists including Hubert Laude and François Delmas.

Category:Enzymes Category:EC 3.4.11