Supondo que você tenha linhas vazias entre cada bloco, conforme mostrado na sua pergunta, então:
$ awk -v RS='\n\n' '/...._...\.pdb; NMR;/' RS= infile
AADB1_KLEPN 2''-aminoglycoside nucleotidyltransferase; P0AE05 PDB; 4WQK; X-ray; 1.48 A; A=1-177.
PDB; 4WQL; X-ray; 1.73 A; A=1-177.
PDB; 5KQJ_GOL.pdb; NMR; -; A=1-177.
$ awk -v RS='\n\n' '/...._...\.pdb; X-ray;/' RS= infile
A4_RAT Amyloid-beta A4 protein; P08592 PDB; 1M7E; X-ray; 2.45 A; D/E/F=755-763.
PDB; 1NMJ; NMR; -; A=672-699.
PDB; 1OQN_I3P.pdb; X-ray; 2.30 A; C/D=755-763.
PDB; 2LI9; NMR; -; A/B=672-687.
AACP_AGRFC Aminoacyl carrier protein; A9CHM9 PDB; 2JQ4; NMR; -; A=1-83.
PDB; 4H2W_5GP.pdb; X-ray; 1.95 A; C/D=1-83.
PDB; 4H2X_G5A.pdb; X-ray; 2.15 A; C/D=1-83.
PDB; 4H2Y; X-ray; 2.10 A; C/D=1-83.
Para preservar as linhas vazias, remova RS=
e adicione a declaração {print $0"\n"}
:
$ awk -v RS='\n\n' '/...._...\.pdb; NMR;/{print $0"\n"}' infile
$ awk -v RS='\n\n' '/...._...\.pdb; X-ray;/{print $0"\n"}' infile