An enveloped, negative-sense, single-stranded RNA virus. The genome is segmented into eight distinct structural and non-structural coding regions, allowing for rapid reassortment (antigenic shift) when multiple strains coinfect a single host cell.
Viral entry is mediated by the Hemagglutinin (HA) surface glycoprotein, which binds specifically to sialic acid residues on the host respiratory epithelium. Human-adapted strains predominantly exhibit binding affinity for $\alpha$-2,6-linked sialic acids, whereas avian strains prefer $\alpha$-2,3 linkages.
Drugs like Oseltamivir (Tamiflu) act as transition-state analogues. Upon host cell exit, the viral Neuraminidase (NA) enzyme must cleave terminal sialic acid to release the virion. Oseltamivir competitively inhibits the active site of NA, heavily stabilizing the complex through strong electrostatic interactions with highly conserved arginine residues (Arg118, Arg292, Arg371). This traps the newly formed virions at the host cell membrane.
An obligate pathogenic bacterium characterized by an exceptionally thick, waxy cell envelope rich in mycolic acids. This structure confers profound resistance to standard Gram stains, osmotic stress, and many broad-spectrum antibiotics, necessitating highly specialized pharmacological interventions.
Isoniazid is a prodrug requiring activation by the mycobacterial catalase-peroxidase enzyme, KatG. Once oxidized, the INH radical forms a covalent adduct with NAD+. This INH-NAD adduct is a potent, tight-binding competitive inhibitor of InhA (enoyl-acyl carrier protein reductase), a critical enzyme in the FAS-II pathway essential for mycolic acid biosynthesis.
The Ser315Thr mutation in the katG gene drastically reduces the enzyme's ability to activate Isoniazid while retaining enough catalase-peroxidase function for bacterial survival, representing the primary mechanism of clinical INH resistance.
A positive-sense, single-stranded RNA virus. Upon entry via the ACE2 receptor, the viral genome is immediately translated by host ribosomes to produce polyproteins pp1a and pp1ab, which are subsequently cleaved by viral proteases (Mpro and PLpro) into functional non-structural proteins (nsps).
Replication is driven by the RNA-dependent RNA polymerase (RdRp), primarily composed of nsp12, stabilized by co-factors nsp7 and nsp8.
Remdesivir (GS-5734) is an adenosine analogue prodrug. Once metabolized into its active triphosphate form within the host cell, it is incorporated into the nascent viral RNA chain by the RdRp. Because of a 1'-cyano group steric clash with the RdRp structure, the polymerase stalls exactly three nucleotides downstream of incorporation. This mechanism is known as delayed chain termination, successfully evading the virus's built-in exonuclease proofreading activity (nsp14).