Comparative analysis of the catalytic components in the archaeal dye-linked L-proline dehydrogenase complexes

Two types of hetero-oligomeric dye-linked L-proline dehydrogenases (α4β4 and αβγδ types) are expressed in the hyperthermophilic archaea belonging to Thermococcales. In both enzymes, the β subunit (PDHβ) is responsible for catalyzing L-proline dehydrogenation. The genes encoding the two enzyme types form respective clusters that are completely conserved among Pyrococcus and Thermococcus strains. To compare the enzymatic properties of PDHβs from α4β4- and αβγδ-type enzyme complexes, eight PDHβs (four of each type) from Pyrococcus furiosus DSM3638, Pyrococcus horikoshii OT-3, Thermococcus kodakaraensis KOD1 JCM12380 and Thermococcus profundus DSM9503 were expressed in Escherichia coli cells and purified to homogeneity using one-step Ni-chelating chromatography. The α4β4-type PDHβs showed greater thermostability than most of the αβγδ-type PDHβs: the former retained more than 80 % of their activity after heating at 70 °C for 20 min, while the latter showed different thermostabilities under the same conditions. In addition, the α4β4-type PDHβs utilized ferricyanide as the most preferable electron acceptor, whereas αβγδ-type PDHβs preferred 2, 6-dichloroindophenol, with one exception. These results indicate that the differences in the enzymatic properties of the PDHβs likely reflect whether they were from an αβγδ- or α4β4-type complex, though the wider divergence observed within αβγδ-type PDHβs based on the phylogenetic analysis may also be responsible for their inconsistent enzymatic properties. By contrast, differences in the kinetic parameters among the PDHβs did not reflect the complex type. Interestingly, the k cat value for free α4β4-type PDHβ from P. horikoshii was much larger than the value for the same subunit within the α4β4-complex. This indicates that the isolated PDHβ could be a useful element for an electrochemical system for detection of L-proline.