Described herein is the finding that a mutant form of human neuroglobin (H64L) with a stable five-coordinate geometry reduces nitrite to nitric oxide approximately 2000-times faster than the wild type neuroglobin. Five-coordinate neuroglobin is also capable of binding and releasing oxygen. Based on these findings, the use of five-coordinate neuroglobin as a blood substitute is described herein. Particularly provided is a method of replacing blood and/or increasing oxygen delivery to tissues in a subject by administering to the subject a therapeutically effective amount of neuroglobin with a stable five-coordinate geometry. In some cases, five-coordinate neuroglobin is administered in combination with another therapeutic agent or composition, such as a second blood replacement product (for example, a hemoglobin-based oxygen carrier), a blood product (such as red blood cells, serum or plasma) or whole blood.
