The formation and role of MgO impurity phase in the glycine (Gly; C2H5NO2)-doped MgB2 were investigated by preparing the samples from multiple routes. On the contrary to the observation that the dielectric MgO aggregates at the grain boundary and deteriorates the superconductivity in the un-doped system, the MgO particles in the Gly-doped system dispersed uniformly on the MgB2 matrix and provided enhanced effect on the critical current density. Thermodynamic and kinetic (Ozawa-Flynn-Wall method) analyses were carried out by in situ X-ray diffraction (in situ XRD) and differential scanning calorimetry (DSC) techniques, which proved that the formation of MgO is owing to the reaction between Mg and the decomposition product of Gly (CO2) in advance. Due to this prior reaction, the solid-solid Mg-B reaction followed the first-order reaction mechanism in the Gly-doped MgB2, rather than the second-order Avrami-Erofeev mechanism, which is accorded by the un-doped sample. This reaction 2 Mg + CO2 -> 2 MgO + C is analogous to the dual reaction mechanism 2Mg + SiC -> Mg2Si + C in the SiC-doped MgB2, and similarly the MgO served as effective pinning centers and enhanced the critical current density in the Gly-doped MgB2, while the C atoms provided the scattering effect and improved the upper critical field. Combined with the effect of histidine on the superconductivity of MgB2, we summed up the requirements for an amino acid to be an effective dopant.