Influenza A virus hemagglutinin protein subtype 5 (HA (H5))

The **influenza A virus hemagglutinin protein subtype 5** (HA H5) is a homotrimeric glycoprotein and a class I fusion protein found on the viral envelope of influenza A viruses belonging to the H5 subtype[1][3][4]. Hemagglutinin is responsible for two critical steps in viral infection: it binds to sialic acid-containing receptors on host cells, allowing the virus to attach to and enter the cell, and, after endocytosis, facilitates the fusion of the viral envelope with the endosomal membrane, triggering release of the viral genome into the host cell cytosol[2][3][6][8]. H5 hemagglutinin is the principal antigen recognized by the host immune response, and its structural changes underpin antigenic drift and shift in influenza viruses, necessitating continual monitoring for vaccine development and pandemic preparedness[1][5][7]. The H5 subtype is best known for its association with highly pathogenic avian influenza A (H5N1) strains, which can infect humans and possess pandemic potential due to their zoonotic and lethal potential[1][3]. While most antiviral drugs in clinical use target the neuraminidase protein, hemagglutinin remains a major focus for vaccine design, serological diagnosis, and the development of potentially novel direct-acting antivirals and therapeutic antibodies[3][7][8]. The protein is composed of two subunits, HA1 and HA2, generated by proteolytic cleavage of the precursor HA0. The HA1 subunit forms the external globular head responsible for receptor binding and contains major antigenic sites, while HA2 forms the stem region, mediating membrane fusion[5][6][8]. Mutations in H5 hemagglutinin can alter the species specificity of receptor binding (from avian-type to human-type receptors), a key factor in zoonosis and pandemic risk[1][3]. The molecular structure of H5 hemagglutinin has been extensively characterized; its variability in the receptor-binding domain is a challenge for long-lasting vaccine efficacy, but certain conserved regions, especially in the stem, are targets for broadly neutralizing antibodies[7][8].

Other names

Hemagglutinin H5Influenza hemagglutinin H5HA H5H5 hemagglutinin

Neutralizing antibodies block receptor binding or fusion activity - Some small molecules or peptides under investigation aim to block the fusion conformational change