if subunit.species_type.id in gvar.protein_aa_usage:                                        
                                        for aa, aa_id in amino_acid_id_conversion.items():
                                            if gvar.protein_aa_usage[subunit.species_type.id][aa]:
                                                aa_content[aa_id] = gvar.protein_aa_usage[subunit.species_type.id][aa]