HCaA – Kinetic Mechanisms of Enzyme Inhibition and Activation

Hyperbolic catalytic activation

**Fingerprints of HCaA: Dependence of the parameters on [X]**

**Fingerprints of HCaA: Specific velocity plot and replot**

Featured examples

V/K_m ⫫ (∴ k_cat/K_m ⫫) The apparent specificity constant is independent of [X]
K_m/V ⫫ (∴ K_m/k_cat ⫫) The apparent reciprocal specificity constant is independent of [X]
K_m↑ The apparent Michaelis constant increases with increasing [X]
V ↑ (∴ k_cat↑) The apparent limiting rate, and therefore the catalytic constant, increase with increasing [X]

These symbols are shown only when the featured dependencies of the parameters on modifier concentration have been demonstrated by the authors.

#	Enzyme Species	EC no.	Modifier	Substrate⁽¹⁾	Name given by authors ⁽²⁾	Reference⁽³⁾
1	Glutamate dehydrogenase (NAD(P)⁺) Not specified	1.4.1.3	ADP	NADP_red	Uncompetitive activating behavior. Clear diagnosis, parameters imprecise.	Frieden (1963)
2	Thioglucosidase Raphanus sativus	3.2.1.147	Ascorbate	Sinigrin	Linear uncompetitive activation ⁽⁴⁾ K_m↑, V↑	Shikita (1999)
3	Amidase Pseudomonas aeruginosa	3.5.1.4	Hydroxylamine	Acetamide	Uncompetitive activation K_m↑, V↑, α = β = 9.2, K_X = 16.8 mM	Woods (1979)
4	Peroxidase Armoracia rusticana	1.11.1.7	Melamine	o-Phenylenediamine pH = 7.4	Noncompetitive activation K_m↑, V↑, V/K_m ⫫, K_m/V ⫫ ⁽⁵⁾	Karaseva (2003)

⁽¹⁾ Always the varied substrate. In two- or more-substrate reactions the concentration(s) of the non varied substrate(s) is/are kept constant.

⁽²⁾ Name of the mechanism given by the authors in the quoted reference. α, β and the inhibition/activation constants for the modifier (X), uniformly denoted K_X, are the values specified by the authors. In some cases, missing parameters have been calculated from graphical or tabular data provided in the papers. In two- or more-substrate reactions, K_X represents an apparent constant at given concentrations of the fixed substrates and no calculations of the intrinsic values have been attempted.

⁽³⁾ Full references at the end of the page provide also the digital object identifier (doi), if available. Clicking the authors (highlighted) opens the reference in PubMed.

⁽⁴⁾ From the graphics in Figure 4 (Shikita, 1999) the mechanism is clearly nonessential activation, which cannot be linear, and the correct name is hyperbolic catalytic activation. The allosteric coupling constant and the activation constant K_X have not been calculated. The confusion about uncompetitive interactions in this and other publications is due to misinterpretation of the basic mechanisms as discussed elsewhere (pp. 150-153 and 273-276)

⁽⁵⁾ The last two properties deduced from Fig. 7 in the paper. At pH = 4.3 the mechanism changes from activation to inhibition with mechanism HMx(Sp<Ca)I.

References

Frieden C (1963) Glutamate dehydrogenase. V. The relation of enzyme structure to the catalytic function. J Biol Chem 238: 3286-3299.
Karaseva EI, Naumchik IV, Metelitza DI (2003) Noncompetitive activation of the peroxidase-catalyzed oxidation of o-phenylenediamine by melamine (in Russian). Bioorganicheskaya Khimiya 29: 49-56. doi:10.1023/A:1022278402568
Shikita M, Fahey JW, Golden TR, Holtzclaw WD, Talalay P (1999) An unusual case of ‘uncompetitive activation’ by ascorbic acid: purification and kinetic properties of a myrosinase from Raphanus sativus seedlings. Biochem J 341: 725-732. doi:10.1042/bj3410725
Woods MJ, Findlater JD, Orsi BA (1979) Kinetic mechanism of the aliphatic amidase from Pseudomonas aeruginosa. Biochim Biophys Acta 567: 225-237. doi:10.1016/0005-2744(79)90189-X