Binds oxygen cooperatively with very high affinity (P(50) = 0.013 mmHg at 20 degrees Celsius) because of a fast combination (25 microM(-1).sec(-1)) and a slow dissociation (0.2 sec(-1)) rate. Binds 1 heme group per subunit. Homodimer. Belongs to the truncated hemoglobin family. Group I subfamily.