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 Activated by Mg(2+) ions. Inhibited by metal-chelating agents such as EDTA, EGTA, 1,10-phenanthroline, 2,2'-dipyridyl, and by PtCl4 and KPdCl4 as well as Ni(2+) and Co(2+). Also inhibited by iodoacetamide, N-tosyl-L-phenylalanine chloromethyl ketone, and 5,5'-dithiobis(2-nitrobenzoic acid), as well as glutamycin. Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; 5-aminolevulinate from L-glutamyl-tRNA(Glu): step 1/2. Homodimer. Interacts with glutamate-1-semialdehyde 2,1-aminomutase (GSA-AM), which forms a metabolic channeling between both enzymes to protect the reactive aldehyde species GSA. 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 (By similarity). 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.