Sorry - made a mistake. Try this:
def mutate_all_mets_to_mse():
mol_id=active_residue()[0]
for ch_id in chain_ids(mol_id):
for resn in range(0,chain_n_residues(ch_id,mol_id)):
if (resname_from_serial_number(mol_id,ch_id,resn)=="MET"):
seqnum=seqnum_from_serial_number(mol_id,ch_id,resn)
mutate(mol_id,ch_id,seqnum,'',"MSE")
mutate_all_mets_to_mse()
