Parses the protein tag to get the names and EC numbers.

def _parse_protein_tag(self):
        '''Parses the protein tag to get the names and EC numbers.'''

        protein_nodes = self._dom.getElementsByTagName('protein')
        assert(len(protein_nodes) == 1)
        self.protein_node = protein_nodes[0]

        self._get_recommended_name()
        self._get_submitted_names()
        self._get_alternative_names()