 Deoxyhemoglobin

Deoxyhemoglobin is the unoxygenated form of hemoglobin whose available haem sites can reversibly bind oxygen. Releasing oxygen produces a binding-state change, rather than the ferric iron state defining methemoglobin. Carboxyhemoglobin differs again because a competing carbon monoxide ligand occupies a binding site. These distinctions matter when interpreting the components of an optical measurement.

In the instrument convention for functional oxygen saturation, HHb denotes the unoxygenated, oxygen-capable component. Its concentration appears with oxyhemoglobin in the denominator:The HHb label is not an instruction to treat deoxygenation as addition of a hydrogen molecule. Functional saturation also uses a different denominator from fractional oxyhemoglobin referenced to total hemoglobin.^[ref]

An original independent-site model illustrates a further counting distinction. Let each of four sites have oxygen-occupancy probability $\theta$, independently. The probability that a tetramer has exactly $m$ occupied sites isAt $\theta =1/2$, half the sites are empty on average, while only $P_0=1/16$ of tetramers have all four sites empty. A haem-equivalent unoxygenated fraction therefore need not equal the fraction of wholly unliganded tetramers. Real hemoglobin binding is cooperative, so independence here is a stated teaching assumption rather than a physiological model.

Deoxyhemoglobin and oxyhemoglobin have distinguishable absorption spectra. A primary study measured the absorptivities of these and other human hemoglobin derivatives across 450–1000 nm, including fetal/adult differences relevant to multicomponent analysis.^[ref] A controlled oxygenation comparison can therefore follow changing spectral contributions without inferring oxygen amount from apparent colour alone. The measured path length, concentrations and calibrated component spectra remain necessary inputs.