COMPLEMENTARY-DNA CLONING AND KINETIC CHARACTERIZATION OF A NOVEL INTRACELLULAR SERINE PROTEINASE-INHIBITOR - MECHANISM OF ACTION WITH TRYPSIN AND FACTOR XA AS MODEL PROTEINASES
Ka. Morgenstern et al., COMPLEMENTARY-DNA CLONING AND KINETIC CHARACTERIZATION OF A NOVEL INTRACELLULAR SERINE PROTEINASE-INHIBITOR - MECHANISM OF ACTION WITH TRYPSIN AND FACTOR XA AS MODEL PROTEINASES, Biochemistry, 33(11), 1994, pp. 3432-3441
The full-length cDNA encoding a novel human intracellular serine prote
inase inhibitor has been sequenced and found to encode a 376 amino aci
d protein (M(r) approximate to 42.5K) that we designate as cytoplasmic
antiproteinase. Analysis of the primary structure revealed that the c
ytoplasmic antiproteinase has the majority of structural motifs conser
ved among the greater superfamily of serine proteinase inhibitors, or
serpins. On the basis of several criteria such as amino acid identity
and the absence of a classical N-terminal signal peptide, the cytoplas
mic antiproteinase represents a new member of the intracellular serpin
family. Further inspection of the cytoplasmic antiproteinase amino ac
id sequence identified three potential N-glycosylation sites and Arg(3
41)-Cys(342) as the reactive site P-1-P-1' residues, respectively. We
have also employed the slow binding kinetic approach to detail the mec
hanism of bovine trypsin and human factor Xa inhibition by the novel c
ytoplasmic antiproteinase. Inhibition of trypsin by the cytoplasmic an
tiproteinase was preceded by a two-step mechanism corresponding to the
formation of an initial loose complex, followed by an isomerization s
tep to a more stable, tight complex. The binding of the cytoplasmic an
tiproteinase to trypsin occurred with a second-order association rate
constant of 2.8 x 10(6) M(-1) s(-1) and an overall equilibrium constan
t of 22.5 pM, demonstrating that the factor is a potent inhibitor of t
his proteinase. Under the appropriate conditions, the tight complex be
tween trypsin and the cytoplasmic inhibitor was reversible, indicated
by an exponential regeneration of proteinase amidolytic activity from
the preformed complex. Therefore, the tight complex appears to be stab
ilized predominantly by reversible bonds that form between trypsin and
the cytoplasmic inhibitor. In contrast to the inhibition of trypsin,
the inhibition of factor Xa amidolytic activity by the cytoplasmic ant
iproteinase followed a single-step binding mechanism. The apparent fir
st-order rate constant for factor Xa inhibition was found to increase
as a linear function of the inhibitor concentration range studied. For
mation of the inhibitory complex between factor Xa and the cytoplasmic
antiproteinase occurred with a second-order association rate constant
of approximately 1.3 x 10(5) M(-1) s(-1) and a equilibrium constant o
f 3.7 nM. These findings suggests that the cytoplasmic inhibitor may i
nitially encounter significant energy barriers for proper alignment wi
th the substrate binding cleft of factor Xa. However, once aligned, th
e reaction proceeds rapidly to a tight factor Xa.inhibitor complex tha
t dissociates at a slow rate.