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PDBsum entry 2q7d
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References listed in PDB file
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Key reference
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Title
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Integration of inositol phosphate signaling pathways via human itpk1.
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Authors
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P.P.Chamberlain,
X.Qian,
A.R.Stiles,
J.Cho,
D.H.Jones,
S.A.Lesley,
E.A.Grabau,
S.B.Shears,
G.Spraggon.
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Ref.
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J Biol Chem, 2007,
282,
28117-28125.
[DOI no: ]
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PubMed id
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Abstract
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Inositol 1,3,4-trisphosphate 5/6-kinase (ITPK1) is a reversible, poly-specific
inositol phosphate kinase that has been implicated as a modifier gene in cystic
fibrosis. Upon activation of phospholipase C at the plasma membrane, inositol
1,4,5-trisphosphate enters the cytosol and is inter-converted by an array of
kinases and phosphatases into other inositol phosphates with diverse and
critical cellular activities. In mammals it has been established that inositol
1,3,4-trisphosphate, produced from inositol 1,4,5-trisphosphate, lies in a
branch of the metabolic pathway that is separate from inositol
3,4,5,6-tetrakisphosphate, which inhibits plasma membrane chloride channels. We
have determined the molecular mechanism for communication between these two
pathways, showing that phosphate is transferred between inositol phosphates via
ITPK1-bound nucleotide. Intersubstrate phosphate transfer explains how competing
substrates are able to stimulate each others' catalysis by ITPK1. We further
show that these features occur in the human protein, but not in plant or
protozoan homologues. The high resolution structure of human ITPK1 identifies
novel secondary structural features able to impart substrate selectivity and
enhance nucleotide binding, thereby promoting intersubstrate phosphate transfer.
Our work describes a novel mode of substrate regulation and provides insight
into the enzyme evolution of a signaling mechanism from a metabolic role.
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Figure 2.
FIGURE 2. Sequential intersubstrate phosphate transfer
hypotheses for hITPK1. This graphic shows hypothetical
intersubstrate phosphate transfer mechanisms by which levels of
Ins(1,3,4)P[3] could regulate the synthesis of Ins(3,4,5,6)P[4].
Panel A shows the two candidate phosphate carriers with enzyme
represented as "E" with the transferred phosphate highlighted in
red. Panel B expands the nucleotide-mediated phosphate transfer
hypothesis in which enzyme-bound nucleotide acts as the
phosphate carrier. Unliganded ITPK1 is represented by E;
phosphotransferase reactions are indicated by red graphics. The
position of the [^32P] group that is transferred between
inositol phosphates is shown in red.An asterisk indicates a
reaction for which Ins(1,3,4,5)P[4] is also produced at a
reduced rate. We do not rule out the possibility that a
phosphoenzyme intermediate might also participate in these
reactions.
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Figure 5.
FIGURE 5. Detail of the hITPK1 inositol phosphate binding
pocket. Identical projections for eITPK1 (A) and hITPK1 (B),
showing the reduction in inositol phosphate binding site volume
and ATP solvent accessibility for hITPK1. Ins(1,3,4)P[3] is
shown modeled into the hITPK1 based on structural alignment with
eITPK1 and exhibits a steric clash with His-162. A
solvent-accessible surface is shown for each molecule. The
position of the ATP  -phosphate is indicated
by " ". Metal ions are shown
as green spheres. Panel C is a stereodiagram showing structural
superposition of hITPK1 with eITPK1. hITPK1 is shown with blue
carbons atoms, with a bound sulfate shown in gold and red.
Residues capable of carrying a phosphate that are proximal to
the ATP -phosphate are shown in
yellow. eITPK1 in complex with Ins(1,3,4)P[3] is shown with
orange carbon atoms for protein, magenta carbons for the
inositol.
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(2007,
282,
28117-28125)
copyright 2007.
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