Question: what are the different classes of membrane proteins
[{'answer': 'Integral polytopic proteins', 'provenance': [{'wikipedia_id': '168369', 'title': 'Membrane protein', 'start_paragraph_id': 12, 'start_character': 12, 'end_paragraph_id': 12, 'end_character': 39, 'bleu_score': 1.0, 'section': 'Section::::Function.:Integral membrane proteins.\n'}]}, {'answer': 'Integral monotopic proteins', 'provenance': [{'wikipedia_id': '168369', 'title': 'Membrane protein', 'start_paragraph_id': 16, 'start_character': 12, 'end_paragraph_id': 16, 'end_character': 39, 'bleu_score': 1.0, 'section': 'Section::::Function.:Integral membrane proteins.\n'}]}]