Mechanism and Theory in Food Chemistry, Second Edition by Dominic W.S. Wong

Author:Dominic W.S. Wong
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

4.12.4 Oxygenation of Myoglobin

It is in the ferrous form that myoglobin, Mb(II), binds molecular oxygen reversibly as the primary physiological function (Eq. 4.21). The oxygen dissociation constant K d is 1.15 × 10−6 M calculated from k f = 1.64 × 107 s−1 M−1 and k r = 19 s−1 (sperm whale, pH 7.0, 25 °C) [40].

(4.21)

4.12.5 Autoxidation of Oxymyoglobin

Oxymyoglobin is known to undergo slow oxidation to ferrimyoglobin (Eq. 4.22). Under air-saturated conditions, the primary step for the autoxidation reaction is first order with respect to the unoxidized Mb(II)O2, with k ob = 8 × 10−3 h−1 (sperm whale, pH 7.0, 25 °C) [40]. The rate is different from protein to protein depending on the source.

Several factors affect the rate of myoglobin autoxidation: (1) The rate increases markedly with increasing H + concentration (lowering pH). The pH effect is attributed to the promotion of proton-assisted catalysis (see below). (2) The rate is also markedly affected by temperature, with increased rate at higher temperatures. (3) The rate is dependent on the partial pressure of oxygen, proceeding maximally at the half-saturating oxygen (pO2 of 1–2 torr) pressure where Mb(II) and Mb(II)O2 are equal in concentration.

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