Catalyzes the NADPH-dependent reduction of glutamyl-tRNA(Glu) to glutamate 1-semialdehyde (GSA). In the absence of NADPH, exhibits substrate esterase activity, leading to the release of glutamate from tRNA. (S)-4-amino-5-oxopentanoate + NADP(+) + tRNA(Glu) = H(+) + L-glutamyl-tRNA(Glu) + NADPH Inhibited by heavy metal compounds, Zn(2+), and heme. Also competitively inhibited by glutamycin. Optimum pH is 8.1. Optimum temperature is 90 degrees Celsius. Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; 5-aminolevulinate from L-glutamyl-tRNA(Glu): step 1/2. Homotetramer. Belongs to the glutamyl-tRNA reductase family.