Sassi, G. | Search bioRxiv

bioRxiv2026

IsdH binding to hemoglobin:haptoglobin is decoupled from iron acquisition in Staphylococcus aureus

Staphylococcus aureus requires iron for proliferation during infection and acquires it mainly from host hemoglobin (Hb) through the iron-regulated surface determinant (Isd) system. The hemophores IsdB and IsdH mediate the initial steps of Hb recognition. Notably, IsdH also recognizes the hemoglobin:haptoglobin (HbHp) complex, although the molecular determinants and physiological relevance of this interaction remain unclear. Here, combining cryo-electron microscopy and biochemical and cellular assays, we define the basis of HbHp recognition by IsdH. The structure reveals how IsdH engages HbHp and captures the intrinsic conformational flexibility of the HbHp assembly. Functional analyses demonstrate that, despite retaining the ability to extract heme from HbHp, IsdH does not support S. aureus growth under iron-restricted conditions when HbHp is the sole iron source. These findings suggest that HbHp recognition by IsdH may serve functions beyond nutrient iron acquisition, contributing to modulation of host-pathogen interactions.

biochemistry↗

bioRxiv2026

Human ASPDH is a 2-aminomuconate reductase that produces L-2-aminohex-3-enedioic acid in tryptophan catabolism

Most tryptophan catabolism in animals occurs through the kynurenine pathway, which generates the essential NAD cofactor and multiple bioactive metabolites. Knowledge of this pathway in eukaryotes ends at the unstable intermediate 2-aminomuconate (2-AM). Here, by leveraging evolutionary information from more than 5,000 eukaryotes, we identify two distinct genes acting downstream of 2-AM in fungi and metazoa. The fungal gene is homologous to bacterial 2-AM deaminase, whereas the metazoan gene is homologous to aspartate dehydrogenase (ASPDH), which in prokaryotes catalyses the first reaction of NAD biosynthesis. Biochemical and structural analyses show that human ASPDH has evolved an unprecedented function as an NAD(P)H-dependent 2-AM reductase (AMR) in tryptophan catabolism. The reaction forms L-2-aminohex-3-enedioic acid, an unsaturated -amino acid absent from current biological databases. Isotope-labeling NMR experiments and structural modelling support a mechanism in which hydride transfer is coupled to double-bond rearrangement of the conjugated system. These findings reveal a previously unknown metazoan branch of the kynurenine pathway, expand the repertoire of endogenous amino acids, and illustrate how comparative genomics can uncover hidden reactions in human metabolism.

biochemistry↗