Table 4.
Comparison of the characteristics of acetone carboxylases of A. aromaticum with acetone carboxylases of various organisms and acetophenone carboxylase of A. aromaticuma
| Characteristic | Acetone carboxylase of: |
Hydantoinase (ATP hydrolyzing) of a Pseudomonas sp. (16) | Acetophenone carboxylase of A. aromaticum (10) | ||
|---|---|---|---|---|---|
| A. aromaticum | X. autotrophicus (22) | R. capsulatus (22) | |||
| Subunit composition | (αβγ)2 (85/75/20)b | (αβγ)2 (85/78/20) | (αβγ)2 (85/78/20) | (αβ)2 (80/70) | (αββ′γ)2 + ε2 (87/75/70/15 + 34) |
| Sequence similarityc | 70/70/60 | 68/69/59 | 28/23 | 31/24/26/31 | |
| Nucleotide hydrolysis | 2ATP → 2AMP + 4Pi | ATP → ADP + Pi → AMP + 2Pi | ATP → AMP + 2Pi | ATP → ADP + Pi | 2ATP → 2ADP + 2Pi |
| Uncoupled ATPase activity with: | Ketones without HCO3 | Ketones without HCO3 | NDd | Ketones without HCO3 and with HCO3 without ketones | |
| Cofactor | 1 Zn/(αβγ)2, 2.2 Fe/(αβγ)2 | 1 Zn/(αβγ)2, 0.7 Fe/(αβγ)2, 1.3 Mn/(αβγ)2 | 0.31 Zn/(αβγ)2, 0.14 Fe/(αβγ)2, 1.5 Mn/(αβγ)2 | ND | 2 Zn/(αββ′γ)2 |
| Substrate specificity | Aliphatic ketones (acetone, butanone) | Aliphatic ketones (acetone, butanone) | ND | Hydantoin and N-methylhydantoin, e.g. | Acetophenone, some aromatic and alicyclic ketones |
| Carboxylation site | Methyl groups | Methyl groups | Methyl groups | No carboxylation | Methyl and methylene groups |
| Apparent Km value | Acetone, 7.5 ± 0.8 μM; butanone, 5.1 ± 0.6 μM; ATP, 1.0 ± 0.1 mM; HCO3, 6.9 ± 0.4 mM | Acetone, 7.8 ± 0.8 μM; ATP, 122 ± 14 μM; HCO3, 4.2 ± 0.7 mM | Acetone, 8.2 ± 0.5 μM; ATP, 98 ± 15 μM; HCO3, 2.3 ± 0.2 mMe | N-Methylhydantoin, 32 μM | Acetophenone, 9 ± 1 μM; ATP, 700 ± 200 μM; HCO3, 0.5 ± 0.15 mM |
a
Besides high sequence similarity, obvious differences in subunit composition, cofactor content, and ATP hydrolysis are observed. The numbers after the organism names are reference numbers.
b
Values in parentheses are molecular masses (in kilodaltons).
c
Sequence similarity is given as the percentage of identical amino acids of AcxA (β-subunit)/B (α subunit)/C (γ subunit) compared with the respective subunits of acetone carboxylase of A. aromaticum.
d
ND, not determined.
e
The original paper states a Km of 2.3 ± 0.2 nM, which is most probably a typing error.