Mechanistic Effects of Autophosphorylation on Receptor Tyrosine Kinase Catalysis: Enzymatic Characterization of Tie2 and Phospho-Tie2
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- 作者：Brion W. Murray ; Ellen S. Padrique ; Chris Pinko ; and Michele A. McTigue
- 刊名：Biochemistry
- 出版年：2001
- 出版时间：August 28, 2001
- 年：2001
- 卷：40
- 期：34
- 页码：10243 - 10253
- 全文大小：155K
- 年卷期：v.40,no.34(August 28, 2001)
- ISSN：1520-4995
文摘
Activation of receptor tyrosine kinases by autophosphorylation is one of the most commonand critical transformations in signal transduction, yet its role in catalysis remains controversial.Autophosphorylation of the angiogenic receptor tyrosine kinase Tie2 was studied in terms of theautophosphorylation sites, sequence of phosphorylation at these sites, kinetic effects, and mechanisticconsequences. Isoelectric focusing electrophoresis and mass spectrometric analysis of a Tie2 autophosphorylation time course showed that Tyr992 on the putative activation loop was phosphorylated firstfollowed by Tyr1108 in the C-terminal tail (previously unidentified autophosphorylation site). Autophosphorylation of Tie2 to produce pTie2 resulted in a 100-fold increase in kcat and a 460-fold increase inkcat/Km. Viscosity studies showed that the unphosphorylated Tie2 was partially limited by product diffusion((kcat)
= 0.67 ± 0.06), while product release was more rate-limiting ((kcat) = 0.94 ± 0.08) forautophosphorylated Tie2 (pTie2). Furthermore, autophosphorylation did not significantly affect thephosphoacceptor dissociation constants. There was a significant (kcat)H/(kcat)D solvent isotope effect (SIE)for unphosphorylated Tie2 (2.42 ± 0.12) and modest SIE (1.28 ± 0.04) for pTie2, which is consistentwith the chemistry step being more rate-limiting for Tie2 as compared to pTie2. The pH-rate profiles ofTie2 and pTie2 revealed a >0.5 unit shift in the pKa values of catalytically relevant ionizable residuesupon autophosphorylation. The shift in rate-limiting step will result in a different distribution of enzymepools (e.g., E, E
S, EP, etc.) which may modulate the susceptibility to inhibition. Tie2 and pTie2 wereprofiled with a panel of known ATP-competitive kinase inhibitors. Tie2 activation perturbs catalytic residueionizations, shifts the rate-limiting step to almost exclusive diffusion-control, and transforms the kinaseinto a more perfect catalyst.
= 0.67 ± 0.06), while product release was more rate-limiting ((kcat) = 0.94 ± 0.08) forautophosphorylated Tie2 (pTie2). Furthermore, autophosphorylation did not significantly affect thephosphoacceptor dissociation constants. There was a significant (kcat)H/(kcat)D solvent isotope effect (SIE)for unphosphorylated Tie2 (2.42 ± 0.12) and modest SIE (1.28 ± 0.04) for pTie2, which is consistentwith the chemistry step being more rate-limiting for Tie2 as compared to pTie2. The pH-rate profiles ofTie2 and pTie2 revealed a >0.5 unit shift in the pKa values of catalytically relevant ionizable residuesupon autophosphorylation. The shift in rate-limiting step will result in a different distribution of enzymepools (e.g., E, ES, EP, etc.) which may modulate the susceptibility to inhibition. Tie2 and pTie2 wereprofiled with a panel of known ATP-competitive kinase inhibitors. Tie2 activation perturbs catalytic residueionizations, shifts the rate-limiting step to almost exclusive diffusion-control, and transforms the kinaseinto a more perfect catalyst.