This study focused on the trans-cis photoisomerization process of p-coumaric acid (pCA) in photoactive yellow protein (PYP), which is an initial step of the photocycle. Quantum molecular dynamics simulations for the whole PYP system were performed using the divide-and-conquer density functional tight-binding method. The hydrogen bond between Glu46 and pCA was clarified to be shorter than that of the initial structure, which hinders the hula-twist isomerization process for structures after passing through the conical intersection.