Chromogranins from Cell Biology to Physiology and Biomedicine

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the intact CGA and the L3-2 loop peptide is −6.0 ~ −6.9 kcal/mol (Kd, 11–17 μM)
at pH 5.5 whereas that between the near N-terminal peptide of CGA and the L3-2
loop peptide is −3.57 kcal/mol (Kd, 3.04 mM) (Yoo and Lewis 1995 , 1998 ), indicat-
ing approximately two orders of magnitude stronger interaction by intact CGA. This
result further suggests that the interaction strength between intact CGA and intact
IP 3 Rs in secretory granules will be several orders of magnitude higher than that
shown between intact CGA and the L3-2 loop at pH 5.5.
Moreover, in line with the more prominent role of CGB in activating the IP 3 R/
Ca2+ channels compared to CGA, the interaction of CGB with the IP 3 Rs is substan-
tially stronger than that of CGA. Highlighting such strong binding, the interaction
strength between intact CGB and the L3-2 loop even at pH 7.5, which is known to
be significantly weaker than that at pH 5.5, is −8.10 kcal/mol (Kd, 1.9 μM) (Table  3 )
(Yoo and Lewis 2000 ). This value indicates that the interaction between intact CGB
and the L3-2 loop at pH 7.5 is still an order of magnitude stronger than that shown
between intact CGA and the L3-2 loop at pH 5.5, whereby amply underscoring the
extraordinarily strong binding of CGB for the IP 3 R. Yet, reflecting a weaker interac-
tion than that shown between intact CGB and the L3-2 loop of the IP 3 Rs, the inter-
action strength between the conserved near N-terminal region of CGB and the L3-2
loop at pH 7.5 is −4.70 kcal/mol (Kd, 0.48 mM). Nonetheless, this interaction is still
significantly stronger than that shown with the near N-terminal region of CGA and
the L3-2 loop at pH 5.5, i.e., −3.57 kcal/mol (Kd, 3.04 mM).
Given that the interaction of the near N-terminal region of chromogranins with
the IP 3 R loop is significantly weaker than that of intact chromogranins, the actual
interaction between intact CGB and the IP 3 R/Ca2+ channels in cells will be far stron-
ger than the estimated values. Furthermore, considering that chromogranin interac-
tion with the IP 3 R at pH 5.5 is substantially stronger than at pH 7.5, the interaction
strength of CGB for the IP 3 Rs at pH 5.5 is certain to be several orders of magnitude
higher than that between CGA and the IP 3 Rs.

Table 3 The interaction strength of the intraluminal loop (L3-2) of the IP 3 R with intact
chromogranins A (CGA) and B (CGB), and with the conserved near N-terminal regions of each
chromogranin at different pH as expressed in the changes of standard free energy (ΔGo) at 37 °C
(310.15 K)

Chromogranin A Chromogranin B

Intact CGA-IP 3 R

L3-2

N-terminal

CGA fragment-

IP 3 R L3-2

Intact

CGB-IP 3 R

L3-2

N-terminal CGB

fragment-IP 3 R L3-2

ΔGo (kcal/

mol), pH 5.5

−6.0 ~ −6.9

(Kd:

11 ~ 17 μM)

−3.57

(Kd: 3.04 mM)

CGB or the N-terminal fragment

strongly self-associates at pH 5.5,

leading to aggregation (not

measurable)

ΔGo (kcal/

mol), pH 7.5

CGA and the L3–2 dissociate at

pH 7.5 (no interaction).

−8.10

(Kd: 1.9 μM)

−4.70

(Kd: 0.48 mM)

Conserved Nature of the Inositol 1,4,5-Trisphosphate Receptor and Chromogranin...