Theanine, a unique amino acid found in tea plants, has many important physiological functions. Theanine can be enzymatically synthesized via the ${gamma}$-glutamyltranspeptidation reaction. In this study, we described a new method of theanine synthesis using the L-glutamine-Zn(II) ($Zn(Gln)_2$) complex instead of glutamine as the donor, which successfully reduced the side autotranspeptidation reaction and led to higher yield of theanine. We prepared the $Zn(Gln)_2$ complexes and showed that they are stable in liquid bulk under 9.0 pH. After using the $Zn(Gln)_2$ in the ${gamma}$- glutamyltranspeptidation reaction, we utilized HPLC and Mass spectrometry analysis to demonstrated that $Zn(Gln)_2$ was an more effective ${gamma}$-glutamyl donor than glutamine. The autotranspeptidation reaction was restrained effectively. As a result, the theanine yield and the conversion rate for glutamine were vastly improved. In a reaction mixture containing 48 mM of $Zn(Gln)_2$, 1.6 M ethylamine, and 0.5 U/mL GGT, the final concentration of theanine obtained was 61.3 mM after incubation at $37^{circ}C$ for three hours. The conversion rate for glutamine was 63.8%, which showed a 16.9% increase as compared to when using glutamine alone as the donor substrate.