Reference "Barley alpha-amylase bound to its endogenous protein inhibitor BASI: crystal structure of the complex at 1.9 A resolution"

Reference ID

8589

Title

Barley alpha-amylase bound to its endogenous protein inhibitor BASI: crystal structure of the complex at 1.9 A resolution

Source

Structure (London, England), 1998, vol. 6, pp. 649-659

Authors (7)

Vallee-F	Kadziola-A
Bourne-Y	Juy-M
Rodenburg-K-W	Svensson-B
Haser-R

Abstract

BACKGROUND: Barley alpha-amylase is a 45 kDa enzyme which is involved in starch
degradation during barley seed germination. The released sugars provide the
plant embryo with energy for growth. The major barley alpha-amylase isozyme
(AMY2) binds with high affinity to the endogenous inhibitor BASI (barley alpha-
amylase/subtilisin inhibitor) whereas the minor isozyme (AMY1) is not inhibited.
BASI is a 19.6 kDa bifunctional protein that can simultaneously inhibit AMY2 and
serine proteases of the subtilisin family. This inhibitor may therefore prevent
degradation of the endosperm starch during premature sprouting and protect the
seed from attack by pathogens secreting proteases. RESULTS: The crystal
structure of AMY2 in complex with BASI was determined and refined at 1.9 A
resolution. BASI consists of a 12-stranded beta-barrel structure which belongs
to the beta-trefoil fold family and inhibits AMY2 by sterically occluding access
of the substrate to the active site of the enzyme. The AMY2-BASI complex is
characterized by an unusual completely solvated calcium ion located at the protein-
protein interface. CONCLUSIONS: The AMY2-BASI complex represents the first
reported structure of an endogenous protein-protein complex from a higher plant.
The structure of the complex throws light on the strict specificity of BASI for
AMY2, and shows that domain B of AMY2 contributes greatly to the specificity of
enzyme-inhibitor recognition. In contrast to the three-dimensional structures of
porcine pancreatic alpha-amylase in complex with proteinaceous inhibitors, the
AMY2-BASI structure reveals that the catalytically essential amino acid residues
of the enzyme are not directly bound to the inhibitor. Binding of BASI to AMY2
creates a cavity, exposed to the external medium, that is ideally shaped to
accommodate an extra calcium ion. This feature may contribute to the inhibitory
effect, as the key amino acid sidechains of the active site are in direct
contact with water molecules which are in turn ligated to the calcium ion.