Globin X is a six-coordinate globin that reduces nitrite to nitric oxide in fish red blood cells.

The discovery of novel globins in diverse organisms has stimulated intense interest in their evolved function, beyond oxygen binding. Globin X (GbX) is a protein found in fish, amphibians, and reptiles that diverged from a common ancestor of mammalian hemoglobins and myoglobins. Like mammalian neuroglobin, GbX was first designated as a neuronal globin in fish and exhibits six-coordinate heme geometry, suggesting a role in intracellular electron transfer reactions rather than oxygen binding. Here, we report that GbX to our knowledge is the first six-coordinate globin and the first globin protein apart from hemoglobin, found in vertebrate RBCs. GbX is present in fish erythrocytes and exhibits a nitrite reduction rate up to 200-fold faster than human hemoglobin and up to 50-fold higher than neuroglobin or cytoglobin. Deoxygenated GbX reduces nitrite to form nitric oxide (NO) and potently inhibits platelet activation in vitro, to a greater extent than hemoglobin. Fish RBCs also reduce nitrite to NO and inhibit platelet activation to a greater extent than human RBCs, whereas GbX knockdown inhibits this nitrite-dependent NO signaling. The description of a novel, six-coordinate globin in RBCs with dominant electron transfer and nitrite reduction functionality provides new insights into the evolved signaling properties of ancestral heme-globins.