The features of energetics and electronic properties of carbon nanotubes, containing a pentagon-heptagon pair (5/7) topological defect in the hexagonal network of the zigzag configuration, are investigated using the extended Su-Schriffer-Heeger model based on the tight binding approximation in real space. Our calculations show that this pentagon-heptagon pair defect in the nanotube structures is not only responsible for a change in nanotube diameter,but also goves the electronic behaviour around Fermi level. Furthermore, we have calculated the densities of states of the (9,0)-(8,0) and (8,0)-(7,0) systems. For the (9,0)-(8,0) system, a narrow gap exists in the vicinity of the Fermi energy. In contrast, for the (8,0)-(7,0) system, a little peak of the density of states occurs at the Fermi energy. These can be attributed to the addition of a pair of pentagon-heptagon defects in the interface between two isolated carbon nanotubes.