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==Phosphofructokinase (PFK)==
==Phosphofructokinase (PFK)==
<applet load='4pfk' scene='Sandbox/Judy_Voet/PFK/Pfk-r_state/1' size='300' frame='true' align='right' caption='PFK: R-state; from 4pfk' />
<applet load='4pfk' scene='Sandbox/Judy_Voet/PFK/4pfk_biol/1' size='300' frame='true' align='right' caption='PFK: R-state Biological tetramer; generated from 4pfk by QPS' />
 




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In the high-activity R state, the positively charged side chain of Arg 162 forms a hydrogen bonded salt bridge with the negatively charged 6-phosphate group of F6P (white dashed lines), an interaction which presumably stabilizes the R state relative to the T state and is therefore in part responsible for F6P's homotropic effect.
In the high-activity R state, the positively charged side chain of Arg 162 forms a hydrogen bonded salt bridge with the negatively charged 6-phosphate group of F6P (white dashed lines), an interaction which presumably stabilizes the R state relative to the T state and is therefore in part responsible for F6P's homotropic effect.


==KINEMAGE 2: The Major Conformational Changes in a Subunit of PFK.==
== 2: The Major Conformational Changes in a Subunit of PFK.==


KINEMAGE 2 shows those segments near the allosteric site (residues 53-60 are not shown here). As in KINEMAGE 1, the polypeptide is represented by its Ca chain with R state Subunits 1 and 2 in redtint and pink, and T state Subunits 1 and 2 in bluetint and skyblue. KINEMAGE 2 comes up in the R state showing the phosphate group of F6P (hotpink) bound in the enzyme's active site in a hydrogen bonded salt bridge (dashed white lines) with the side chain of Arg 162 (cyan). An ADP (yellow; "ADP-allo") occupies the adjacent allosteric site. Click once on "ANIMATE" to switch to the T state. This replaces the ADP in the R state allosteric site with the inhibitor and PEP analog PGC (gold). F6P no longer occupies the active site but its position in the R state is indicated by the "ghost" F6P (gray; "F6P site").  
This KINEMAGE shows those segments near the allosteric site (residues 53-60 are not shown here). As in KINEMAGE 1, the polypeptide is represented by its Ca chain with R state Subunits 1 and 2 in redtint and pink, and T state Subunits 1 and 2 in bluetint and skyblue.  
<kinemage align="right" width="450" height= "450" file="PFK2.kin" />
KINEMAGE 2 comes up in the R state showing the phosphate group of F6P (hotpink) bound in the enzyme's active site in a hydrogen bonded salt bridge (dashed white lines) with the side chain of Arg 162 (cyan). An ADP (yellow; "ADP-allo") occupies the adjacent allosteric site. Click once on "ANIMATE" to switch to the T state. This replaces the ADP in the R state allosteric site with the inhibitor and PEP analog PGC (gold). F6P no longer occupies the active site but its position in the R state is indicated by the "ghost" F6P (gray; "F6P site").  


What happens to the central polypeptide helical segment (residues 149-164) in the R to T transition? What does this do to the relative positions of the negatively charged Glu 161 and the positively charged Arg 162? Click on "F6P site". What influence would the presence of the carboxylate group of Glu 161 have on the phosphate group of F6P were it to bind in the active site? Does this explain, at least in part, why T state PFK has low affinity for F6P? Go to View2 for a closeup of the F6P-sidechain interactions.
What happens to the central polypeptide helical segment (residues 149-164) in the R to T transition? What does this do to the relative positions of the negatively charged Glu 161 and the positively charged Arg 162? Click on "F6P site". What influence would the presence of the carboxylate group of Glu 161 have on the phosphate group of F6P were it to bind in the active site? Does this explain, at least in part, why T state PFK has low affinity for F6P? Go to View2 for a closeup of the F6P-sidechain interactions.


{{clear}}
== Atomic Coordinates==
The atomic coordinates for R state PFK were obtained from 4PFK; those for T state PFK were obtained from Philip Evans, MRC Laboratory of Molecular Biology, Cambridge, U. K.
The atomic coordinates for R state PFK were obtained from 4PFK; those for T state PFK were obtained from Philip Evans, MRC Laboratory of Molecular Biology, Cambridge, U. K.
CAPTION for Kinemage #1:
    R and T state phosphofructokinase PFK). R state subunits are pinktint and pink, and T state subunits are bluetint and skyblue. In the R state, F6P (hotpink) is bound to the active site, and ADP is bound both to the active site (green) and the allosteric site (yellow). In the T state, the allosteric inhibitor 2-phosphoglycolate (PGC; a PEP analog) is bound to the allosteric site (gold). The F6P-binding site in the T state, which does not bind PFK, is marked by the position of the F6P in the R state (gray). The side chains of Glu 161 and Arg 162 are red and cyan.
CAPTION for Kinemage #2:
    The allosteric site of phosphofructokinase (PFK) with residues 53-60 not shown.  Residues of R-state Subunits 1 and 2 are pinktint and pink, whereas T state residues of Subunits 1 and 2 are bluetint and skyblue. In the R state, active site-bound F6P is hotpink and allosteric site-bound ADP is yellow. In the T state, allosteric site-bound 2-phosphoglycolate (PGC) is gold. The T state active site, which does not contain F6P, is marked by the position of R state-bound F6P (gray; "F6P site"). The side chains of Glu 161 and Arg 162 are red and cyan.

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Judy Voet
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