Structural basis for cooperative oxygen binding and bracelet-assisted assembly of Lumbricus terrestris hemoglobin.

The iron-containing hemoglobins (Hbs) are essential proteins to serve as oxygen transporters in the blood. Among various kinds of Hbs, the earthworm Hbs are the champions in carrying oxygen due to not only their large size but also the unusually high cooperativity of ligand binding. However, the cooperative oxygen binding mechanisms are still mostly unknown. Here we report the cryo-electron microscopy structure of Lumbricus terrestris Hb in its native, oxygenated state at 9.1 Å resolution, showing remarkable differences from the carbon monoxide-binding X-ray structure. Our structural analysis first indicates that the cooperative ligand binding of L. terrestris Hb requires tertiary and quaternary transitions in the heme pocket and a global subunit movement facilitated by intra-ring and inter-ring contacts. Moreover, the additional sinusoidal bracelet provides the confirmation for the long-standing debate about the additional electron densities absent in the X-ray crystal structure.

Adaptive responses to glucose restriction enhance cell survival, antioxidant capability, and autophagy of the protozoan parasite Trichomonas vaginalis.

BACKGROUND: To establish an infection in the vagina, Trichomonas vaginalis must adapt to various environmental cues for survival and further replication. Nutrient competition by lactobacilli, the major normal vaginal flora, is one of the mechanisms to limit the growth of other microorganisms. Additionally, lactobacilli produce H2O2 that can reduce the genital infections caused by other pathogens. Thus, the ability to overcome the metabolic stresses, such as glucose restriction (GR), as well as the oxidative stresses, is critical for T. vaginalis to establish an infection. METHODS: To gain insights into the molecular mechanisms of adaptation to GR, we utilized next-generation RNA sequencing (RNA-seq) to quantify the gene expression changes upon GR. Autophagy, a cytoprotective response to starvation, was monitored by using autophagy-specific staining, autophagy inhibition assay, and co-localization of autophagosomes with lysosomes. RESULTS: We demonstrated that GR promotes the survival of T. vaginalis. Besides, GR-cultivated cells exhibit higher H2O2 resistance. Our RNA-seq data revealed that genes involved in general energy metabolism were downregulated, whereas genes encoding glutamate metabolism-related aminotransferases were strikingly upregulated under GR. Furthermore, autophagy was first identified and characterized in T. vaginalis under GR. CONCLUSIONS: These data suggest that GR induces a metabolic reprogramming, enhancing antioxidant ability and autophagy for cellular homeostasis to maintain survival. GENERAL SIGNIFICANCE: Our work not only led to significant advances in understanding the transcriptional changes in response to GR but also provided possible strategies elicited by GR for T. vaginalis to adapt to the vaginal microenvironment.