<Table> <Tr> <Th_colspan="2"> (show) Available protein structures: </Th> </Tr> <Tr> <Th> Pfam </Th> <Td> structures </Td> </Tr> <Tr> <Th> PDB </Th> <Td> RCSB PDB; PDBe; PDBj </Td> </Tr> <Tr> <Th> PDBsum </Th> <Td> structure summary </Td> </Tr> </Table> <Tr> <Th_colspan="2"> (show) Available protein structures: </Th> </Tr> <Tr> <Th> Pfam </Th> <Td> structures </Td> </Tr> <Tr> <Th> PDB </Th> <Td> RCSB PDB; PDBe; PDBj </Td> </Tr>

Why is it difficult to purify taq from its original source t. aquaticus