Burchert HH; Medical Faculty, University of Basel, Basel, Switzerland.
Stringer WW; Dash RK

Journal of Applied Physiology. 141(2):401-416, 2026 Aug 01.

Oxygen (O2) binding and release by hemoglobin (Hb) are governed by
cooperative interactions among its four subunits. During incremental
workload exercise, femoral venous oxyhemoglobin (O2Hb) saturation exhibits
a reproducible, momentary increase at the gas exchange
threshold-coinciding with the inflection point of the in vivo O2
nonequilibrium curve (ONC). This suggests a transient shift in Hb’s
binding dynamics. We hypothesized that at this threshold, Hb tetramers
carrying <=1 bound O2 become predominant. In this state, the last bound O2
promotes further cooperative binding, but its release confers no
cooperative advantage for unloading, biasing toward O2 rebinding. Using
the O2 equilibrium curve models of Dash et al. (Eur J Appl Physiol 116:
97-113, 2016) and Adair, we computed the distribution of Hb’s O2 ligation
states across 12 pooled mean femoral venous blood samples from incremental
workload cardiopulmonary exercise testing of five healthy male
participants. At the gas exchange threshold-where the ONC inflects and
flattens-tetramers with <=1 O2 indeed dominated. This ligation-state
distribution is consistent with Perrella et al.’s (J Biol Chem 274:
2605-2608, 1999) cryogenic resolution of native human Hb, which shows that
carbon monoxide-ligated Hb tetramers peak at ~15%-20% saturation, matching
femoral venous ranges at the gas exchange threshold. Our results suggest
that, at sufficiently low O2Hb saturation, Hb may favor O2 rebinding over
cooperative unloading. We propose that glycolytic proton production and
other Bohr effectors may counter this predicted binding bias supporting
continued O2 unloading. If confirmed, this mechanism unifies long-standing
controversies in O2 transport physiology, framing the Hb-Bohr system as a
proportional-integral controller of tissue oxygenation. NEW & NOTEWORTHY
Anaerobic metabolism is usually viewed as a fallback when oxygen delivery
becomes insufficient. Our analysis suggests a different role: it may
preserve oxygen delivery by correcting a hemoglobin-binding bias that
emerges at low oxyhemoglobin saturation. When hemoglobin tetramers carry
one or no oxygen molecules, the remaining bound oxygen can promote
cooperative rebinding but not cooperative release. Glycolytic proton
production and other Bohr effectors may counter