The Monod-Wyman-Changeux model for hemoglobin (MWC) (1) is based on the existance of two types: Tense and Relaxed. The model is general for enzyme kinetics; for hemoglobin (Hb), it was first proposed by the German physiologist Niesel (2), also in his dissertation (3).
In the model, the two types of hemoglobin have different oxygen (O2) affinities and can bind up to four O2 molecules. Indeed, this is true for Hb. If each tensed form is indicated by Tk and each relaxed form by Rk the k range is 0–4 (no O2 up to 4 molecules bound). Then, you need three parameters to describe the saturation curve, saturation S against oxygen pressure P: the binding affinity to T, KT, the binding affinity to R, KR, and the equilibrium between T4 and R4, L4:
| S = | L4{1+1/(KTP)3} + {1+1/(KRP)3} |
| L4{1+1/(KTP)4} + {1+1/(KRP)4} |
For blood, the MWC model is less suitable since in the red cell more things happen with changing O2 amount, such as shift in pH. Therefore, for full blood the Hill model is more appropriate – and easier. Even for stripped Hb, the description is not fully correct (4).
The rate at which oxygen is bound or released also can be incorporated in the MWC model. Click the topic ‘Kinetics’ to see how.
(2) Niesel W: Ein Beitrag zur Deutung des S-förmigen Verlaufes der Sauerstoffbindungskurve. Pflügers Arch 274: 16-17 (1961).
(3) Niesel W: Die Umlagerungshypothese. Ein Beitrag zur Klärung der Sauerstoff-Hämoglobin-Reaktion. Inauguraldissertation, Kiel (1961).
(4) Hoofd LJC Kreuzer FJA: Incompatibility of the MWC model with the oxygen dissociation curve of stripped human hemoglobin. 19th Dutch Fed Meeting, Rotterdam: 273 (1978).