Biomedical subjects

Birgitte Munch-Petersen

Publications and source records attributed to Birgitte Munch-Petersen.

10 recordsLinked to original sources

PubMed2005

Structural basis for the changed substrate specificity of Drosophila melanogaster deoxyribonucleoside kinase mutant N64D.

The Drosophila melanogaster deoxyribonucleoside kinase (Dm-dNK) double mutant N45D/N64D was identified during a previous directed evolution study. This mutant enzyme had a decreased activity towards the natural substrates and decreased feedback inhibition with dTTP, whereas the activity with 3'-modified nucleoside analogs like 3'-azidothymidine (AZT) was nearly unchanged. Here, we identify the mutation N64D as being responsible for these changes. Furthermore, we crystallized the mutant enzyme in the presence of one of its substrates, thymidine, and the feedback inhibitor, dTTP. The introduction of the charged Asp residue appears to destabilize the LID region (residues 167-176) of the enzyme by electrostatic repulsion and no hydrogen bond to the 3'-OH is made in the substrate complex by Glu172 of the LID region. This provides a binding space for more bulky 3'-substituents like the azido group in AZT but influences negatively the interactions between Dm-dNK, substrates and feedback inhibitors based on deoxyribose. The detailed picture of the structure-function relationship provides an improved background for future development of novel mutant suicide genes for Dm-dNK-mediated gene therapy.

Animals↗

PubMed2004

Structures of thymidine kinase 1 of human and mycoplasmic origin.

Cytosolic thymidine kinase 1, TK1, is a well known cell-cycle-regulated enzyme of importance in nucleotide metabolism as well as an activator of antiviral and anticancer drugs such as 3'-azido-3'-deoxythymidine (AZT). We have now determined the structures of the TK1 family, the human and Ureaplasma urealyticum enzymes, in complex with the feedback inhibitor dTTP. The TK1s have a tetrameric structure in which each subunit contains an alpha/beta-domain that is similar to ATPase domains of members of the RecA structural family and a domain containing a structural zinc. The zinc ion connects beta-structures at the root of a beta-ribbon that forms a stem that widens to a lasso-type loop. The thymidine of dTTP is hydrogen-bonded to main-chain atoms predominantly coming from the lasso loop. This binding is in contrast to other deoxyribonucleoside kinases where specific interactions occur with side chains. The TK1 structure differs fundamentally from the structures of the other deoxyribonucleoside kinases, indicating a different evolutionary origin.

Amino Acid Sequence↗

PubMed2004

Animal deoxyribonucleoside kinases: 'forward' and 'retrograde' evolution of their substrate specificity.

Deoxyribonucleoside kinases, which catalyse the phosphorylation of deoxyribonucleosides, are present in several copies in most multicellular organisms and therefore represent an excellent model to study gene duplication and specialisation of the duplicated copies through partitioning of substrate specificity. Recent studies suggest that in the animal lineage one of the progenitor kinases, the so-called dCK/dGK/TK2-like gene, was duplicated prior to separation of the insect and mammalian lineages. Thereafter, insects lost all but one kinase, dNK (EC 2.7.1.145), which subsequently, through remodelling of a limited number of amino acid residues, gained a broad substrate specificity.

Amino Acid Sequence↗

PubMed2004

Effect of valine 106 on structure-function relation of cytosolic human thymidine kinase. Kinetic properties and oligomerization pattern of nine substitution mutants of V106.

Information on the regulation and structure-function relation of enzymes involved in DNA precursor synthesis is pivotal, as defects in several of these enzymes have been found to cause depletion or deletion of mitochondrial DNA resulting in severe diseases. Here, the effect of amino acid 106 on the enzymatic properties of the cell-cycle-regulated human cytosolic thymidine kinase 1 (TK1) is investigated. On the basis of the previously observed profound differences between recombinant TK1 with Val106 (V106WT) and Met106 (V106M) in catalytic activity and oligomerization pattern, we designed and characterized nine mutants of amino acid 106 differing in size, conformation and polarity. According to their oligomerization pattern and thymidine kinetics, the TK1 mutants can be divided into two groups. Group I (V106A, V106I and V106T) behaves like V106WT, in that pre-assay exposure to ATP induces reversible transition from a dimer with low catalytic activity to a tetramer with high catalytic activity. Group II (V106G, V106H, V106K, V106L and V106Q) behaves like V106M in that they are permanently high activity tetramers, irrespective of ATP exposure. We conclude that size and conformation of amino acid 106 are more important than polarity for the catalytic activity and oligomerization of TK1. The role of amino acid 106 and the sequence surrounding it for dimer-tetramer transition was confirmed by cloning the putative interface fragment of human TK1 and investigating its oligomerization pattern.

Amino Acid Sequence↗

PubMed2003

Structural basis for feedback inhibition of the deoxyribonucleoside salvage pathway: studies of the Drosophila deoxyribonucleoside kinase.

Deoxyribonucleoside kinases are feedback inhibited by the final products of the salvage pathway, the deoxyribonucleoside triphosphates. In the present study, the mechanism of feedback inhibition is presented based on the crystal structure of a complex between the fruit fly deoxyribonucleoside kinase and its feedback inhibitor deoxythymidine triphosphate. The inhibitor was found to be bound as a bisubstrate inhibitor with its nucleoside part in the nucleoside binding site and with its phosphate groups partially occupying the phosphate donor site. The overall structure of the enzyme--inhibitor complex is very similar to the enzyme--substrate complexes with deoxythymidine and deoxycytidine, except for a conformational change within a region otherwise directly involved in catalysis. This conformational change involves a magnesium ion, which is coordinated in the inhibitor complex to the phosphates and to the primary base, Glu52, that normally is positioned close to the 5'-OH of the substrate deoxyribose.

Adenosine Triphosphate↗

PubMed2003

Mosquito has a single multisubstrate deoxyribonucleoside kinase characterized by unique substrate specificity.

In mammals four deoxyribonucleoside kinases, with a relatively restricted specificity, catalyze the phosphorylation of the four natural deoxyribonucleosides. When cultured mosquito cells, originating from the malaria vector Anopheles gambiae, were examined for deoxyribonucleoside kinase activities, only a single enzyme was isolated. Subsequently, the corresponding gene was cloned and over-expressed. While the mosquito kinase (Ag-dNK) phosphorylated all four natural deoxyribonucleosides, it displayed an unexpectedly higher relative efficiency for the phosphorylation of purine versus pyrimidine deoxyribonucleosides than the fruit fly multisubstrate deoxyribonucleoside kinase (EC 2.7.1.145). In addition, Ag-dNK could also phosphorylate some medically interesting nucleoside analogs, like stavudine (D4T), 2-chloro-deoxyadenosine (CdA) and 5-bromo-vinyl-deoxyuridine (BVDU). Although the biological significance of multisubstrate deoxyribonucleoside kinases and their diversity among insects remains unclear, the observed variation provides a whole range of applications, as species specific and highly selective targets for insecticides, they have a potential to be used in the enzymatic production of various (di-)(deoxy-)ribonucleoside monophosphates, and as suicide genes in gene therapy.

Amino Acid Sequence↗

PubMed2003

Striking ability of adenosine-2'(3')-deoxy-3'(2')-triphosphates and related analogues to replace ATP as phosphate donor for all four human, and the Drosophila melanogaster, deoxyribonucleoside kinases.

In extension of an earlier report, six non-conventional analogues of ATP, three adenosine-2'-triphosphates (3'-deoxy, 3'-deoxy-3'-fluoro- and 3'-deoxy-3'-fluoroxylo-), and three adenosine-3'-triphosphates (2'-deoxy-, 2'-deoxy-2'-fluoro- and 2'-deoxy-2'-fluoroara-), were compared with ATP as potential phosphate donors for human deoxycytidine kinase (dCK), cytosolic thymidine kinase (TK1), mitochondrial TK2, deoxyguanosine kinase (dGK), and the deoxyribonucleoside kinase (dNK) from Drosophila melanogaster. With one group of enzymes, comprising TK1, TK2, dNK and dCK (with dAdo as acceptor), only 3'-deoxyadenosine-2'-triphosphate was an effective donor (5-60% that for ATP), and the other five analogues much less so, or inactive. With a second set, including dCK (dCyd, but not dAdo, as acceptor) and dGK (dGuo as acceptor), known to share high sequence similarity (approximately 45% sequence identity), all six analogues were good to excellent donors (13-119% that for ATP). With dCK and ATP1, products were shown to be 5'-phosphates. With dCK, donor properties of the analogues were dependent on the nature of the acceptor, as with natural 5'-triphosphate donors. With dCK (dCyd as acceptor), Km and Vmax for the two 2'(3')-deoxyadenosine-3'(2')-triphosphates are similar to those for ATP. With dGK, Km values are higher than for ATP, while Vmax values are comparable. Kinetic studies further demonstrated Michaelis-Menten (non-cooperative) or cooperative kinetics, dependent on the enzyme employed and the nature of the donor. The physiological significance, if any, of the foregoing remains to be elucidated. The overall results are, on the other hand, highly relevant to studies on the modes of interaction of nucleoside kinases with donors and acceptors; and, in particular, to interpretations of the recently reported crystal structures of dGK with bound ATP, of dNK with bound dCyd, and associated modeling studies.

Animals↗

PubMed2002

A few amino acid substitutions can convert deoxyribonucleoside kinase specificity from pyrimidines to purines.

In mammals, the four native deoxyribonucleosides are phosphorylated to the corresponding monophosphates by four deoxyribonucleoside kinases, which have specialized substrate specificities. These four enzymes are likely to originate from a common progenitor kinase. Insects appear to have only one multisubstrate deoxyribonucleoside kinase (dNK, EC 2.7.1.145), which prefers pyrimidine nucleosides, but can also phosphorylate purine substrates. When the structures of the human deoxyguanosine kinase (dGK, EC 2.7.1.113) and the dNK from Drosophila melanogaster were compared, a limited number of amino acid residues were identified and proposed to be responsible for the substrate specificity. Three of these key residues in Drosophila dNK were then mutagenized and the mutant enzymes were characterized regarding their ability to phosphorylate native deoxyribonucleosides and nucleoside analogs. The mutations converted the dNK substrate specificity from predominantly pyrimidine specific into purine specific. A similar scenario could have been followed during the evolution of kinases. Upon gene duplication of the progenitor kinase, only a limited number of single amino acid changes has taken place in each copy and resulted in substrate-specialized enzymes.

Amino Acid Sequence↗

PubMed2002

Deoxyribonucleoside kinases belonging to the thymidine kinase 2 (TK2)-like group vary significantly in substrate specificity, kinetics and feed-back regulation.

In eukaryotic cells deoxyribonucleoside kinases belonging to three phylogenetic sub-families have been found: (i) thymidine kinase 1 (TK1)-like enzymes, which are strictly pyrimidine deoxyribonucleoside-specific kinases; (ii) TK2-like enzymes, which include pyrimidine deoxyribonucleoside kinases and a single multisubstrate kinase from Drosophila melanogaster (Dm-dNK); and (iii) deoxycytidine/deoxyguanosine kinase (dCK/dGK)-like enzymes, which are deoxycytidine and/or purine deoxyribonucleoside-specific kinases. We cloned and characterized two new deoxyribonucleoside kinases belonging to the TK2-like group from the insect Bombyx mori and the amphibian Xenopus laevis. The deoxyribonucleoside kinase from B. mori (Bm-dNK) turned out to be a multisubstrate kinase like Dm-dNK. But uniquely for a deoxyribonucleoside kinase, Bm-dNK displayed positive cooperativity with all four natural deoxyribonucleoside substrates. The deoxyribonucleoside kinase from X. laevis (Xen-PyK) resembled closely the human and mouse TK2 enzymes displaying their characteristic Michaelis-Menten kinetic with deoxycytidine and negative cooperativity with its second natural substrate thymidine. Bm-dNK, Dm-dNK and Xen-PyK were shown to be homodimers. Significant differences in the feedback inhibition by deoxyribonucleoside triphosphates between these three enzymes were found. The insect multisubstrate deoxyribonucleoside kinases Bm-dNK and Dm-dNK were only inhibited by thymidine triphosphate, while Xen-PyK was inhibited by thymidine and deoxycytidine triphosphate in a complex pattern depending on the deoxyribonucleoside substrate. The broad substrate specificity and different feedback regulation of the multisubstrate insect deoxyribonucleoside kinases may indicate that these enzymes have a different functional role than the other members of the TK2-like group.

Amino Acid Sequence↗

PubMedDate not supplied

Ability of adenosine-2'(3')-deoxy-3'(2')-triphosphates and related analogues to replace ATP as phosphate donor for all human and Drosphila melanogaster deoxyribonucleoside kinases.

Six non-conventional adenosine-2'- and 3'-triphosphate analogues of ATP were tested as potential phosphate donors for all four human, and D. melanogaster, deoxyribonucleoside kinases. With dCK (only dAdo as acceptor), TK1, TK2 and dNK only 3'-deoxyadenosine-2'-triphosphate was an effective donor (5-60% that for ATP). With dCK (dCyd as acceptor) and dGK (dGuo as acceptor), sharing 45% sequence identity, donor activities ranged from 13 to 119% that for ATP. Products were 5'-phosphates. In some instances, kinetics are dependent on the nature of the acceptor, and donor and acceptors properties are mutually interdependent. Results are highly relevant to studies on the modes of interaction with the enzymes, and to interpretations of reported crystal structures of dCK and dNK with bound ligands.

Adenosine Triphosphate↗