149

Quantum Compass of Migratory Birds

FIGURE 4.1 Ethanolamine ammonia-lyase (Harkins and Grissom, 1994). Magnetic feld dependence of

(a) Vmax with unlabeled ethanolamine, (b) Vmax/Km with unlabeled ethanolamine, and (c) Vmax/Km with l, l,

2,2-D4-ethanolamine. Each assay contained 100 mM N-(2-hydroxyethyl)piperazine-N'-2-ethane sulfonic acid

(Hepes) pH 7.48, 5 µM adenosylcob(III)alamin, and ethanolamine ammonia-lyase at 25°C. Each data point repre­

sents the kinetic parameter derived by ftting observed d[P]/dt vs [ethanolamine] data to d[P]/dt =Vmax − S]ⁿ/Km + [S]

n by nonlinear methods. Te Hill number, n, varied only slightly between 0.75 and 0.85. In order to keep the mea­

sured rates with deuterated and unlabeled substrates similar, 8.59-fold more EAL enzyme was used in assays with

deuterated ethanolamine than in assays with unlabeled ethanolamine. Tis yields an observed kinetic isotope efect

of ^DVmax = 6.8 ±0.2 and ^DVmax/Km = 5.4 ±0.4 at 0 T (Reproduced with permission from Harkins and Grissom, 1994,

Copyright 1994, AAAS.)

feld sensitive in 1977 (Blankenship et al., 1977; Hof et al., 1977). Only one other biological system,

an enzyme with radical pair intermediates, has been shown to exhibit any magnetic feld-dependent

parameters (Harkins and Grissom, 1994). Harkins and Grissom (1994) presented the magnetic feld

dependence of Vmax and Vmax/Km for ethanolamine ammonia-lyase (Figure 4.1). Te kinetic parameter

Vmax is independent of applied magnetic feld up to 250 mT, whereas Vmax/Km exhibits a decrease of 25%

at 100 mT (Harkins and Grissom, 1994). Under conditions of saturating substrate (expressed by the

kinetic parameter Vmax), product dissociation is followed immediately by the binding of another sub­

strate molecule that is poised for the next round of catalysis (Harkins and Grissom, 1994). Te net efect

is no dependence of Vmax on the magnetic feld (Harkins and Grissom, 1994). In contrast, under condi­

tions of a less-than-saturating substrate (expressed by the kinetic parameter Vmax/Km), product dissocia­

tion occurs and the “resting” state of the enzyme-cofactor complex is restored (Harkins and Grissom,

1994). Deuteration of ethanolamine produces a larger 60% decrease in Vmax/Km at 150 mT (Harkins and

Grissom, 1994).

Grissom (1995) presented the rate of Cbl^II formation vs magnetic feld (Figure 4.2, modifed from

Harkins and Grissom, 1995). Tis result unambiguously identifes {AdoCH2 •Cbl^II} recombination as

the magnetic feld-sensitive step in ethanolamine ammonia-lyase (Harkins and Grissom, 1995). No deu­

terium isotope efect on the rate of Cbl^II formation is observed, and the magnetic feld dependence of

Cbl^II formation is independent of the isotopic composition of ethanolamine (Harkins and Grissom,

1995).

More recently, Chen and Ke (2018) reported that application of an external magnetic feld in the range

of 650–850 mT triggers intersystem crossing to the singlet {cob(II)alamin – substrate} radical-pair state.

Spin-conserved H back-transfer from deoxyadenosine to the substrate radical yields a singlet {cob(II)