The insulin linked polymorphism region (ILPR) is known to regulate transcription of the gene coding for insulin. The ILPR has guanine rich segments, suggesting that G quadruplexes may be responsible for this regulatory role. Using mechanical unfolding in a laser tweezers instrument and circular dichroism (CD) spectroscopy, we provide compelling evidence that highly stable parallel and antiparallel G quadruplex structures coexist in the predominant ILPR sequence of (ACAGGGGTGTGGGG)2 at a physiologically relevant concentration of 100 mM KCl. Experiments at the single molecular level have shown that unfolding forces for parallel and antiparallel structures (Funfold: 22.6 vs 36.9 pN, respectively) are higher than the stall forces of enzymes having helicase activities. From a mechanical perspective alone, these data support the hypothesis that G quadruplexes may cause replication slippage by blocking replication process. Using the unique combination of the rupture force and the contour length measured by laser tweezers, the simultaneous determination of probable parallel and antiparallel G quadruplex structures in a solution mixture has been achieved. Jarzynski′s equality analysis has revealed that the antiparallel G quadruplex is thermodynamically more stable than the parallel conformer (ΔG unfold: 23 vs 14 kcal/mol, respectively). On the other hand, kinetic measurements have indicated that both parallel and antiparallel structures fold rather rapidly (kfold: 0.4 vs 0.3 s−1, respectively), suggesting that they may be kinetically accessible for gene control. This work provides an unprecedented mechanical perspective on G quadruplex stability, presenting a unique opportunity to predict the functional consequence when motor enzymes encounter such structures.
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