Vibrio cholerae V-cGAP3 Is an HD-GYP Phosphodiesterase with a Metal Tunable Substrate Selectivity

Activation and Allostery in a Fungal SAMHD1 Hydrolase: An Evolutionary Blueprint for dNTP Catabolism

Sterile alpha motif and HD domain-containing protein 1 (SAMHD1) is a metal-dependent hydrolase that plays key roles in dNTP homeostasis, antiretroviral defense, and regulation of various cancers in humans. Beyond mammals, SAMHD1 is also present in a wide range of eukaryotes, including invertebrates, plants, and human parasites. Although the specific mechanisms and biological significance of SAMHD1 in these organisms are not well understood, its functions are linked to essential processes such as photosynthesis, genome maintenance, and immune response. In this study, we bioinformatically mined the SAMHD1 superfamily and selected the ortholog from the mycorrhizal fungus Rhizophagus irregularis as a model system for both fungal and biochemically intractable plant SAMHD1s. Ri SAMHD1 retains the substrate promiscuity of the human enzyme but bypasses the strict requirement for allosteric activation through tetramerization, positioning it as a prototypical enzyme in which hydrolysis and allosteric regulation can be uncoupled. Its activity is selectively dependent on transition metal ions such as Mn and Fe, while Mg serves as a poor activator. Although Ri SAMHD1 lacks several ancillary regulatory features present in human SAMHD1, its activity is differentially modulated by GTP, which acts as an allosteric activator at lower concentrations and an allosteric inhibitor at higher concentrations. These results demonstrate that metal dependence and allosteric regulation are adaptive traits that have evolved divergently among mammals, fungi, and plants, invoking alternative molecular routes for fine-tuning dNTP levels. Our findings on Ri SAMHD1 provide a paradigm for the mechanistic diversification of SAMHD1 enzymes and offer valuable insights for dissecting the complex mechanisms of nucleotide regulation in humans.

Gas-Selective Catalytic Regulation by a Newly Identified Globin-Coupled Sensor Phosphodiesterase Containing an HD-GYP Domain from the Human Pathogen Vibrio fluvialis

Globin-coupled sensors constitute an important family of heme-based gas sensors, an emerging class of heme proteins. In this study, we have identified and characterized a globin-coupled sensor phosphodiesterase containing an HD-GYP domain (GCS-HD-GYP) from the human pathogen Vibrio fluvialis, which is an emerging foodborne pathogen of increasing public health concern. The amino acid sequence encoded by the AL536_01530 gene from V. fluvialis indicated the presence of an N-terminal globin domain and a C-terminal HD-GYP domain, with HD-GYP domains shown previously to display phosphodiesterase activity toward bis(3',5')-cyclic dimeric guanosine monophosphate (c-di-GMP), a bacterial second messenger that regulates numerous important physiological functions in bacteria, including in bacterial pathogens. Optical absorption spectral properties of GCS-HD-GYP were found to be similar to those of myoglobin and hemoglobin and of other bacterial globin-coupled sensors. The binding of O2 to the Fe(II) heme iron complex of GCS-HD-GYP promoted the catalysis of the hydrolysis of c-di-GMP to its linearized product, 5'-phosphoguanylyl-(3',5')-guanosine (pGpG), whereas CO and NO binding did not enhance the catalysis, indicating a strict discrimination of these gaseous ligands. These results shed new light on the molecular mechanism of gas-selective catalytic regulation by globin-coupled sensors, with these advances apt to lead to a better understanding of the family of globin-coupled sensors, a still growing family of heme-based gas sensors. In addition, given the importance of c-di-GMP in infection and virulence, our results suggested that GCS-HD-GYP could play an important role in the ability of V. fluvialis to sense O2 and NO in the context of host-pathogen interactions.

The Fe and Zn cofactor dilemma

Fe and Zn ions are essential enzymatic cofactors across all domains of life. Fe is an electron donor/acceptor in redox enzymes, while Zn is typically a structural element or catalytic component in hydrolases. Interestingly, the presence of Zn in oxidoreductases and Fe in hydrolases challenge this apparent functional dichotomy. In hydrolases, Fe either substitutes for Zn or specifically catalyzes certain reactions. On the other hand, Zn can replace divalent Fe and substitute for more complex Fe assemblies, known as Fe-S clusters. Although many zinc-binding proteins interchangeably harbor Zn and Fe-S clusters, these cofactors are only sometimes functional proxies.