Section: 7 | Apparent Equilibrium Constants for Enzyme-Catalyzed Reactions |

How to Cite this Reference

APPARENT EQUILIBRIUM CONSTANTS FOR ENZYME-CATALYZED REACTIONS

Robert N. Goldberg

This table contains values of apparent equilibrium constants K′ for selected enzyme-catalyzed reactions at specified temperatures T and pHs. In those cases where the ionic strength I and/or the pMg (pMg = –log₁₀[Mg²⁺]) have been reported, the values of these quantities are given.

There are two fundamentally different types of equilibrium constants. This is illustrated by the following example for the hydrolysis of adenosine 5′-triphosphate (ATP) to adenosine 5′-diphosphate (ADP) and phosphate:

ATP + H₂O = ADP + phosphate (1)

The apparent equilibrium constant for the overall biochemical reaction (1) is

K′ = [ADP][phosphate]/([ATP]cº) (2)

The biochemical reactants ATP, ADP, and phosphate each exist in several different ionized and metal bound forms. For example, ATP is an equilibrium mixture of the species ATP^4–, HATP^3–, H₂ATP^2–, MgATP^2–, MgHATP^–, Mg₂ATP⁰. Additional species would also have to be considered if Ca²⁺ were present. Thus, ATP has often been denoted in the literature as ΣATP or as (ATP)_tot. When it is clear that one is dealing with total amounts of substances, it is not necessary to use either the Σ or “tot.” Thus, these designations are not used in this table. In the above equation, cº=1 mol dm^–3; it is included to make K′ dimensionless. The standard transformed Gibbs energy of reaction Δ_rG′º at specified conditions of temperature T, pressure P, ionic strength I, pH, and pMg can be calculated from K′:

Δ_rG′º = –RTln K′ (3)

The molar gas constant, R, is equal to 8.314462 J K^–1 mol^–1. Δ_rG′º and the apparent equilibrium constant, K′, can be used to calculate the position of equilibrium of overall biochemical reactions.

It is also possible to choose a chemical reference reaction that involves selected solute species:

ATP^4–+ H₂O = ADP^3–+ HPO₄^2–+ H⁺ (4)

The equilibrium constant for this reference reaction is

K = [ADP^3–][HPO₄^2–][H+]/{[ATP^4–](cº)²} (5)

Equations and algorithms that relate these two different types of equilibrium constants have been published in Refs. 2–4. To calculate the equilibrium constant K for the reference reaction from the apparent equilibrium constant K′, or vice versa, one needs the equilibrium constants for the binding of H⁺ and for the relevant metal ions to ATP^4–, ADP^3–, and HPO₄^2–.

To avoid confusion between the two different types of equilibrium constants (K′ and K) and to avoid ambiguity about whether specific species or sums of species are intended, the word “ammonia,” for example, rather than NH₃ or NH₄⁺, is used for total ammonia, and chemical formulas are used for specific chemical species. Other substances such as carbon dioxide (CO₂, HCO₃^–, and CO₃^2–), and phosphate (H₂PO₄^–, HPO₄^2–, and PO₄^3–) are treated in the same manner. Exceptions are made for water, which is always written as H₂O, and for gaseous hydrogen and oxygen, which are written as H₂(g) and O₂(g), respectively.

For symmetrical reactions, there is no concern about the units used to calculate the value of an equilibrium constant. However, care must be exercised for reactions that are not symmetrical. In such cases, the units “mol dm^–3” have been used for all concentrations. As stated above, a cº (1 mol dm^–3) is then used to make all equilibrium constants dimensionless.

All substances are assumed to be in aqueous solutions unless specified otherwise. Column definitions for the table are as follows.

Column heading	Definition
Reaction	Enzyme-catalyzed reaction; see abbreviation list below for some reactants
K'	Apparent equilibrium constant of the reaction (see text above)
Enzyme Comm. No.	Enzyme Commission Number (Ref. 1) of the enzyme used to catalyze the reaction
T_meas	Temperature at which K′ is measured, in K
pH	pH at which K′ is measured
I	Ionic strength equilibrium mixture
pMg	pMg = –log₁₀[Mg²⁺]

Values of Δ_rG′º and K′ can also be calculated for many biochemical reactions by using the table “Standard Transformed Gibbs Energies of Formation for Biochemical Reactants” in this section of this CRC Handbook.

Abbreviations

ADP	adenosine 5′-diphosphate
AMP	adenosine 5′-monophosphate
ATP	adenosine 5′-triphosphate
CoA	coenzyme A
GDP	guanosine 5′-diphosphate
GMP	guanosine 5′-monophosphate
GTP	guanosine 5′-triphosphate
IDP	inosine 5′-diphosphate
IMP	inosine 5′-monophosphate
ITP	inosine 5′-triphosphate
NAD_ox	β-nicotinamide-adenine dinucleotide, oxidized form
NAD_red	β-nicotinamide-adenine dinucleotide, reduced form
NADP_ox	β-nicotinamide-adenine dinucleotide phosphate, oxidized form
NADP_red	β-nicotinamide-adenine dinucleotide phosphate, reduced form
UDP	uridine 5′-diphosphate
UTP	uridine 5′-triphosphate

References

Webb, E.C., Enzyme Nomenclature 1992, Academic Press, New York, 1992. See also <http://www.chem.qmul.ac.uk/iubmb/enzyme/>.
Akers, D.L., and Goldberg, R.N., Mathematica J., 8, 86–113, 2001.
Alberty, R.A., J. Biol. Chem., 243, 1337–1343, 1969. [https://doi.org/10.1016/S0021-9258(18)93551-9]
Alberty, R.A., Thermodynamics of Biochemical Reactions, Wiley-Interscience, Hoboken, NJ, 2003. [https://doi.org/10.1002/0471332607]
Goldberg, R.N., Tewari, Y.B., Bell, D., Fazio, K., and Anderson, E., J. Phys. Chem. Ref. Data, 22, 515-582, 1993. [https://doi.org/10.1063/1.555939]
Goldberg, R.N., and Tewari, Y.B., J. Phys. Chem. Ref. Data, 23, 547-617, 1994. [https://doi.org/10.1063/1.555948]
Goldberg, R.N., and Tewari, Y.B., J. Phys. Chem. Ref. Data, 23, 1035-1103, 1994. [https://doi.org/10.1063/1.555957]
Goldberg, R.N., and Tewari, Y.B., J. Phys. Chem. Ref. Data, 24, 1669-1698, 1995. [https://doi.org/10.1063/1.555969]
Goldberg, R.N., and Tewari, Y.B., J. Phys. Chem. Ref. Data, 24, 1765-1801, 1995. [https://doi.org/10.1063/1.555970]
Goldberg, R.N., J. Phys. Chem. Ref. Data, 28, 931–965, 1999. [https://doi.org/10.1063/1.556041]
Goldberg, R.N., Tewari, Y.B., and Bhat, T.N., Bioinformatics, 20, 2874–2877, 2004; <http://xpdb.nist.gov/enzyme_thermodynamics/>. [https://doi.org/10.1093/bioinformatics/bth314]
Goldberg, R.N., Tewari, Y.B., and Bhat, T.N., J. Phys. Chem. Ref. Data, 36, 1347–1397, 2007. [https://doi.org/10.1063/1.2789450]

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Apparent Equilibrium Constants for Enzyme-Catalyzed Reactions

Reaction	K′	Enzyme Comm. No.	T_meas/K	pH	I/mol dm^–3	pMg
Continued on next page...
benzyl alcohol + NAD_ox = benzaldehyde + NAD_red	9.8∙10^–4	1.1.1.1	298.15	7.5
1-butanol + NAD_ox = butanal + NAD_red	1.8∙10^–3	1.1.1.1	298.15	8.3
cyclohexanol + NAD_ox = cyclohexanone + NADH_red	0.09	1.1.1.1	298.15	7.2
1-hexanol + NAD_ox = hexanal + NAD_red	2.87∙10^–3	1.1.1.1	298.15	8.3
1-octanol + NAD_ox = octanal + NAD_red	1.1∙10–3	1.1.1.1	298.15	8.3
L-homoserine + NADP_ox = L-aspartate 4-semialdehyde + NADP_red	6.3∙10^–4	1.1.1.3	298.15	7.9
xylitol + NAD_ox = L-xylulose + NAD_red	2.97∙10^–4	1.1.1.10	298.15	7.00
D-sorbitol + NAD_ox = D-fructose + NAD_red	0.032	1.1.1.14	298.15	7.0
quinate + NAD_ox = 5-dehydroquinate + NAD_red	4.61∙10^–3	1.1.1.24	305.15	7.2
shikimate + NADP_ox = 5-dehydroshikimate + NADP_red	0.036	1.1.1.25	303.15	7.0
2-hydroxybutanoate + NAD_ox = 2-oxobutanoate + NAD_red	3.0∙10^–3	1.1.1.27	298.65	8.0
(R)-3-hydroxybutanoate + NAD_ox = 3-oxobutanoate + NAD_red	1.9∙10^–3	1.1.1.30	298.15	7.0
D-glucose 6-phosphate + NADP_ox = D-glucono-1,5-lactone 6-phosphate + NADP_red	1.50	1.1.1.49	301.15	6.40
5α-androstane-3α-ol-17-one + NAD_ox = 5α-androstane-3,17-dione + NAD_red	0.058	1.1.1.50	298.15	7.0
5α-pregnane-3α,17α,21-triol-20-one + NAD_ox = 5α-pregnane-17α,21-diol-3,20-dione + NAD_red	0.0113	1.1.1.50	298.15	7.0
5α-androstane-3β,17α-diol + NAD_ox = 5α-androstane-17α-ol-3-one + NAD_red	0.0211	1.1.1.51	298.15	7.0
4-androstene-17β-ol-3-one + NAD_ox = 4-androstene-3,17-dione + NAD_red	0.378	1.1.1.51	298.15	7.0
1,2-propanediol + NADP_ox = L-lactaldehyde + NADP_red	6.0∙10^–5	1.1.1.55	298.15	8.4
ribitol + NAD_ox = D-ribulose + NAD_red	3.1∙10^–3	1.1.1.56	310.15	7.4

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