Practice question

Diphtheria toxin produced Corynebacterium diphtheriae lysogenized temperate corynephage beta carrying tox gene regulated iron-dependent repressor DtxR provides ideal warhead immunotoxin because catalytic mechanism well characterized potency extreme one molecule sufficient kill cell enzymatic turnover inactivating millions ribosomes extensive clinical experience vaccine toxoid. AB architecture A domain catalytic 21 kDa active site Glu148 critical nucleophile performing NAD-dependent ADP-ribosylation EF2 B domain receptor-binding translocation B composed receptor-binding subdomain C-terminal 482-535 binding HBEGF ubiquitously expressed mediating endocytosis plus translocation domain hydrophobic helices forming pore acidic endosomal membrane pH 5.5 facilitating cytosolic entry. Wild-type B domain causes nonspecific toxicity liver peripheral nerves limiting therapeutic window. Immunotoxin construction receptor-binding subdomain deleted generating DT388 variant 1-388 retaining translocation helices TH8 TH9 catalytic preserving ability translocate abrogating native tropism reducing off-target 1000-fold. Fusion anti-CD22 scFv flexible linker produces BL22 CAT-3888 moxetumomab pasudotox improved affinity 14-fold mutagenesis heavy chain CDR3. Upon endocytosis furin cleaves arginine-rich loop disulfide reduction releasing A chain cytosol. Clinical immunotoxins denileukin diftitox Ontak IL-2 DT389 CD25 cutaneous T-cell lymphoma Tagraxofusp IL3 DT blastic plasmacytoid dendritic neoplasm illustrate successful retargeting achieving FDA approvals objective response 70 percent.