def add_point_feature(self, resnum, feat_type=None, feat_id=None, qualifiers=None):
        """Add a feature to the features list describing a single residue.

        Args:
            resnum (int): Protein sequence residue number
            feat_type (str, optional): Optional description of the feature type (ie. 'catalytic residue')
            feat_id (str, optional): Optional ID of the feature type (ie. 'TM1')

        """
        if self.feature_file:
            raise ValueError('Feature file associated with sequence, please remove file association to append '
                             'additional features.')

        if not feat_type:
            feat_type = 'Manually added protein sequence single residue feature'
        newfeat = SeqFeature(location=FeatureLocation(ExactPosition(resnum-1), ExactPosition(resnum)),
                             type=feat_type,
                             id=feat_id,
                             qualifiers=qualifiers)

        self.features.append(newfeat)