Catalyzes the NADPH-dependent reduction of glutamyl-tRNA(Glu) to glutamate 1-semialdehyde (GSA). (S)-4-amino-5-oxopentanoate + NADP(+) + tRNA(Glu) = H(+) + L-glutamyl-tRNA(Glu) + NADPH Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; 5-aminolevulinate from L-glutamyl-tRNA(Glu): step 1/2. Porphyrin-containing compound metabolism; chlorophyll biosynthesis. Homodimer. Possesses an unusual extended V-shaped dimeric structure with each monomer consisting of three distinct domains arranged along a curved 'spinal' alpha-helix. The N-terminal catalytic domain specifically recognizes the glutamate moiety of the substrate. The second domain is the NADPH-binding domain, and the third C-terminal domain is responsible for dimerization. During catalysis, the active site Cys acts as a nucleophile attacking the alpha-carbonyl group of tRNA-bound glutamate with the formation of a thioester intermediate between enzyme and glutamate, and the concomitant release of tRNA(Glu). The thioester intermediate is finally reduced by direct hydride transfer from NADPH, to form the product GSA. Belongs to the glutamyl-tRNA reductase family.