Kinetic mechanism of Toxoplasma gondii adenosine kinase and the highly efficient utilization of adenosine

Comp Biochem Physiol B Biochem Mol Biol. 2015 Jun 22. pii: S1096-4959(15)00119-0. doi: 10.1016/j.cbpb.2015.06.006. [Epub ahead of print]

Naguib FN1, Rais RH1, Al Safarjalani ON1, El Kouni MH2.

Author information

Abstract

Toxoplasma gondii has an extraordinarily ability to utilize adenosine (Ado) as the primary source of all necessary purines in this parasite which lacks de novo purine biosynthesis. The activity of T. gondii adenosine kinase (TgAK, EC 2.7.1.20) is responsible for this efficient salvage of Ado in T. gondii. To fully understand this remarkable efficiency of TgAK in the utilization of Ado, complete kinetic parameters of this enzyme are necessary. Initial velocity and product inhibition studies of TgAK demonstrated that the basic mechanism of this enzyme is a hybrid random bi-uni ping-pong uni-bi. Initial velocity studies showed an intersecting pattern, consistent with substrate-enzyme-co-substrate complex formation and a binding pattern indicating that binding of the substrate interferes with the binding of the co-substrate and vice versa. Estimated kinetic parameters were KAdo = 0.002 ■ 0.0002 mM, KATP = 0.05 ■ 0.008 mM, and Vmax = 920 ■ 35 μmol/min/mg protein. Ado exhibited substrate inhibition suggesting the presence of more than one binding site for Ado on the enzyme. ATP relieved substrate inhibition by Ado. Thus, Ado also binds to the ATP binding site. AMP was competitive with ATP, inferring that AMP binds to the same site as ATP. AMP, ADP and ATP were non-competitive with Ado, therefore, none of these nucleotides binds to the Ado binding site. Combining ATP with ADP was additive. Therefore, the binding of either ATP or ADP does not interfere with the binding of the other. It is concluded that for every ATP consumed, TgAK generates three new AMPs. These findings along with the fact that a wide range of nucleoside 5'-mono, di, and triphosphates could substitute for ATP as phosphate donors in this reaction may explain the efficient and central role played by TgAK in the utilization of Ado as the major source from which all other purines can be synthesized in T. gondii.

Copyright © 2015. Published by Elsevier Inc.

KEYWORDS: 

Adenosine kinase; Enzyme kinetics; Parasite; Purine metabolism; Toxoplasma gondii

PMID:

 

26112826

 

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