An investigation of the co-operative functioning of the haemoglobin of the crocodile, Crocodylus porosus

Abstract

1. 1. Crocodile haemoglobin binds oxygen co-operatively. 2. 2. Bicarbonate slightly increases the co-operativity of oxygen binding whilst significantly lowering oxygen affinity and increasing the enthalpy of oxygenation. 3. 3. The rate of oxygen dissociation from the oxyhaemoglobin complex is increased at low pH and is further enhanced in the presence of bicarbonate. 4. 4. Oxygen binding to the R-state form of oxyhaemoglobin occurs at 8 × 10 6 M −1 s −1. In the presence of bicarbonate a second reaction with rate = 2 × 10 6 M −1 s −1 is also observed. 5. 5. Carbon monoxide binding is co-operative in the presence of bicarbonate (Hill coefficient = 2.6) and is well fitted to a two-state model with K R = 0.04 μM ; K T = 1.6 μM and L = 5 × 10 4. 6. 6. Carbon monoxide binding to deoxyhaemoglobin follows accelerating time courses. 7. 7. The deoxy T-state protein binds CO with a rate of 5–6 × 10 4 M −1 s −1 irrespective of the presence or absence of bicarbonate. 8. 8. The R-state protein binds CO with a rate of 8 × 10 5 M −1 s −1 in the absence of bicarbonate, but exhibits two combination rates of 8 × 10 5 M −1 s −1 in the presence of bicarbonate.