Trypanosoma brucei thymidine kinase is tandem protein consisting of two homologous parts, which together enable efficient substrate binding.

Department of Medical Biochemistry and Biophysics, Umeå University, SE-901 87 Umeå, Sweden.
Journal of Biological Chemistry (Impact Factor: 4.6). 03/2012; 287(21):17628-36. DOI: 10.1074/jbc.M112.340059
Source: PubMed

ABSTRACT Trypanosoma brucei causes African sleeping sickness, a disease for which existing chemotherapies are limited by their toxicity or lack of efficacy. We have found that four parasites, including T. brucei, contain genes where two or four thymidine kinase (TK) sequences are fused into a single open reading frame. The T. brucei full-length enzyme as well as its two constituent parts, domain 1 and domain 2, were separately expressed and characterized. Of potential interest for nucleoside analog development, T. brucei TK was less discriminative against purines than human TK1 with the following order of catalytic efficiencies: thymidine > deoxyuridine ≫ deoxyinosine > deoxyguanosine. Proteins from the TK1 family are generally dimers or tetramers, and the quaternary structure is linked to substrate affinity. T. brucei TK was primarily monomeric but can be considered a two-domain pseudodimer. Independent kinetic analysis of the two domains showed that only domain 2 was active. It had a similar turnover number (k(cat)) as the full-length enzyme but could not self-dimerize efficiently and had a 5-fold reduced thymidine/deoxyuridine affinity. Domain 1, which lacks three conserved active site residues, can therefore be considered a covalently attached structural partner that enhances substrate binding to domain 2. A consequence of the non-catalytic role of domain 1 is that its active site residues are released from evolutionary pressure, which can be advantageous for developing new catalytic functions. In addition, nearly identical 89-bp sequences present in both domains suggest that the exchange of genetic material between them can further promote evolution.

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    ABSTRACT: Thymidine kinase 1 (TK1) provides a crucial precursor, thymidine monophosphate (dTMP), for nucleic acid synthesis, and the activity of TK1 increases up to 200-fold during the S-phase of cell division in humans. An important part of the regulatory check-points is the ATP and enzyme concentration-dependent transition of TK1 from a dimer with low catalytic efficiency to a tetramer with high catalytic efficiency. This regulatory fine-tuning serves as an additional control to provide the balanced pool of nucleic acid precursors in the cell. We sub-cloned and over-expressed ten different TK1s, originating from widely different organisms, and characterized their kinetic and oligomerization properties. While bacteria, plants and Dictyostelium only exhibited dimeric TK1, we found that all animals had a tetrameric TK1. However, a clear ATP dependent switch between dimer and tetramer was found only in higher vertebrates, and was especially pronounced in mammalian and bird TK1s. We suggest that the dimer form is the original one and the tetramer originated in the animal lineage after the split of Dictyostelium and the lineages leading to invertebrates and vertebrates. The efficient switching mechanism was likely settled first in the warm-blooded animals when they separated from the rest of vertebrates. © 2013 The Authors Journal compilation © 2013 FEBS.
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