Biotin energy conformation

Figure 11 Calculated lowest energy conformations for biotin in (a) gas phase and (b) aqueous phase. The conformation observed in the biotin-streptavidine ligand-protein complex (pdb-code lstp) is shown in (c).

The activation constant (A a) for citrate is in the range of 3 to 8 mM, depending upon the source of enzyme (37, 94), while the dissociation constants (ifd) obtained by direct binding studies are about 3-14 fiM 37,46). The almost 1000-fold difference between the values ofi Td and Ka has been explained as "the reflection of binding energy needed for structiu al constraint to maintain the active conformation of enzyme-biotin-C02 64). 

Recently molecular dynamics has been used to model the disruption of biotin-streptavidin as it occurs in force adhesion experiments and to relate these forces to molecular structure and conformation. The force is calculated from the steepest slope in the free energy profile along the unbinding pathway. Interestingly the calculated rupture forces show a similar spread in values as is found in experimental data, suggesting that this spread is due to heterogeneity in the reaction pathway of biotin-streptavidin. 

---