Tn. Luong et Jf. Kirsch, A general method for the quantitative analysis of functional chimeras: Applications from site-directed mutagenesis and macromolecular association, PROTEIN SCI, 10(3), 2001, pp. 581-591
Two new parameters, I and C, are introduced for the quantitative evaluation
of functional chimeras: I (impact) and C (context dependence) are the foe
energy difference and sum, respectively, of the effects on a given property
measured in forward and retro chimeras. The forward chimera is made by sub
stitution of a part "a" from ensemble A into the analogous position of homo
logous ensemble B (SB -->A). The C value is a measure of the interaction of
the interrogated position with its surroundings, whereas I is an expressio
n of the quantitative importance of the probed position. Both I and C vary
with the evaluated property, for example, kinetics, binding, thermostabilit
y, and so forth. The retro chimera is the reverse substitution of the analo
gous part "b" from B into A, SA -->B. The I and C values derived from origi
nal data for forward and retro mutations in aspartate and tyrosine aminotra
nsferase, from literature data for quasi domain exchange in oncomodulin and
for the interaction of Tar with bovine and human TAR are evaluated. The mo
st salient derived conclusions are, first, that Thr 109 (AATase) or Ser 109
(TATase) is an important discriminator for dicarboxylic acid selectivity b
y these two enzymes (I < -2.9 kcal/mol). The T109S mutation in AATase produ
ces a nearly equal and opposite effect to S109T in TATase (C < 0.4 kcal/mol
). Second, an I value of 5.5 kcal/mol describes the effects of mirror mutat
ions D94S (site 1) and S55D (site 2) in the Ca2+ binding sites of oncomodul
in on Ca2+ affinity. The second mirror set, G98D (site 1) and D59G (site 2)
, yields a smaller impact(l = -3.4 kcal/mol) on Ca2+ binding; however, the
effect is significantly more nearly context independent (C = -0.6 versus C
= -2.7 kcaymol). Third, the stem and loop regions of HIV and BIV TAR are pr
edominantly responsible for the species specific interaction with BIV Tat(6
5-81) (I = -1.5 to -1.6 kcal/mol), whereas I = 0.1 kcal/mol for bulge TAR c
himeras. The C values are from -0.3 to -1.2 kcal/mol. The analysis describe
d should have important applications to protein design.