JBC, Vol. 251, Issue 10, 2983-2986, May, 1976
The fructose 6-phosphate site of phosphofructokinase. Epimeric specificity
T. A. Koerner Jr, R. J. Voll, A. L. Ashour and E. S. Younathan
The epimeric specificity of the catalytic site of rabbit muscle
phosphofructokinase was investigated by testing three ketose phosphates as
alternate substrates. These (and their epimeric carbons) included:
D-psicose-6-P (C-3), D-tagatose-6-P (C-4), and L-sorbose-6-P (C-5). The
Michaelis constants (and relative maximal velocities) were: 3.0 mM (45%),
0.054 mM (104%), and 11 mM (15%), respectively. Under the same conditions,
D-fructose-6-P had a Km of 0.043 mM and an arbitrary Vmax of 100%. The low
affinity of the enzyme for D-psicose-6-P indicates that the L configuration
at C-3 is required for effective binding, a specificity similar to several
other fructose-metabolizing enzymes. The D configuration at C-5 is also
important for tight binding and the proper orientation of the phosphate
group of the substrate. The kinetic constants of D-tagatose-6-P were
identical with those of D-fructose-6-P, within experimental error. Thus,
the configuration at C-4 is not essential for activity; an indication that
D-tagatose may be utilized in mammalian tissues. A novel method for the
synthesis of D-psicose-6-P and an improved procedure for the synthesis of
D-tagatose-6-P are described. All products and intermediates were
characterized unequivocally by chemical and physical methods.
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