Jw. Gross et al., KINETIC MECHANISM OF NICOTINIC-ACID PHOSPHORIBOSYLTRANSFERASE - IMPLICATIONS FOR ENERGY COUPLING, Biochemistry, 37(12), 1998, pp. 4189-4199
Nicotinic acid phosphoribosyltransferase (NAPRTase; EC 2.4.2.11) is a
facultative ATPase that uses the energy of ATP hydrolysis to drive the
synthesis of nicotinate mononucleotide and pyrophosphate from nicotin
ic acid (NA) and phosphoribosyl pyrophosphate (PRPP). To learn how NAP
RTase uses this hydrolytic energy, we have further delineated the kine
tic mechanism using steady-state and pre-steady-state kinetics, equili
brium binding, and isotope trapping. NAPRTase undergoes covalent phosp
horylation by bound ATP at a rate of 30 s(-1). The phosphoenzyme (E-P)
binds PRPP with a K-D of 0.6 mu M, a value 2000-fold lower than that
measured for the nonphosphorylated enzyme. The minimal rate constant f
or PRPP binding to E-P is 0.72 x 10(5) M-1 s(-1). Isotope trapping sho
ws that greater than 90% of bound PRPP partitions toward product upon
addition of NA. Binding of NA to E-P.PRPP is rapid, k(on) greater than
or equal to 7.0 x 10(6) M-1 s(-1), and is followed by rapid formation
of NAMN and PPi, k greater than or equal to 500 s(-1). After product
formation, E-P undergoes hydrolytic cleavage, k = 6.3 s(-1), and produ
cts NAMN, PPi, and P-i are released. Quenching from the steady state u
nder V-max conditions indicates that slightly less than half the enzym
e is in phosphorylated forms. To account for this finding, we propose
that one step in the release of products is as slow as 5.2 s(-1) and,
together with the E-P cleavage step, codetermines the overall k(cat) o
f 2.3 s(-1) at 22 degrees C. Energy coupling by NAPRTase involves two
strategies frequently proposed for ATPases of macromolecular recogniti
on and processing. First, E-P has a 10(3)-fold higher affinity for sub
strates than does nonphosphorylated enzyme, allowing the E-P to bind s
ubstrate from low concentration and nonphosphorylated enzyme to expel
products against a high concentration. Second, the kinetic pathway fol
lows ''rules'' [Jencks, W. P. (1989) J. Biol. Chem. 264, 18855-18858]
that minimize unproductive alternative reaction pathways. However, an
analysis of reaction schemes based on these strategies suggests that s
uch nonvectorial reactions an intrinsically inefficient in ATP use.