Acute Intermittent Porphyria: An Overview of Therapy Developments and Future Perspectives Focusing on Stabilisation of HMBS and Proteostasis Regulators.

Acute intermittent porphyria (AIP) is an autosomal dominant inherited disease with low clinical penetrance, caused by mutations in the hydroxymethylbilane synthase (HMBS) gene, which encodes the third enzyme in the haem biosynthesis pathway. In susceptible HMBS mutation carriers, triggering factors such as hormonal changes and commonly used drugs induce an overproduction and accumulation of toxic haem precursors in the liver. Clinically, this presents as acute attacks characterised by severe abdominal pain and a wide array of neurological and psychiatric symptoms, and, in the long-term setting, the development of primary liver cancer, hypertension and kidney failure. Treatment options are few, and therapies preventing the development of symptomatic disease and long-term complications are non-existent. Here, we provide an overview of the disorder and treatments already in use in clinical practice, in addition to other therapies under development or in the pipeline. We also introduce the pathomechanistic effects of HMBS mutations, and present and discuss emerging therapeutic options based on HMBS stabilisation and the regulation of proteostasis. These are novel mechanistic therapeutic approaches with the potential of prophylactic correction of the disease by totally or partially recovering the enzyme functionality. The present scenario appears promising for upcoming patient-tailored interventions in AIP.

Towards a characterization of the structural determinants of specificity in the macrocyclizing thioesterase for deoxyerythronolide B biosynthesis.

Type I polyketide synthases (PKSs) are giant multidomain proteins that synthesize many therapeutics and other natural products. The synthesis proceeds by a thiotemplate mechanism whereby intermediates are covalently attached to the PKS. The release of the final polyketide is catalyzed by the terminal thioesterase (TE) domain through hydrolysis, transesterification, or macrocyclization. The PKS 6-deoxyerythronolide B synthase (DEBS) produces the 14-membered macrolide core of the clinically important antibiotic erythromycin. The TE domain of DEBS (DEBS TE) has well-established, empirically-defined specificities for hydrolysis or macrocyclization of native and modified substrates. We present efforts towards understanding the structural basis for the specificity of the thioesterase reaction in DEBS TE using a set of novel diphenyl alkylphosphonates, which mimic substrates that are specifically cyclized or hydrolyzed by DEBS TE. We have determined structures of a new construct of DEBS TE alone at 1.7Å, and DEBS TE bound with a simple allylphosphonate at 2.1Å resolution. Other, more complex diphenyl alkylphosphonates inhibit DEBS TE, but we were unable to visualize these faithful cyclization analogs in complex with DEBS TE. This work represents a first step towards using DEBS TE complexed with sophisticated substrate analogs to decipher the specificity determinants in this important reaction.