code: def find_protein_complexes(model):
      
      complexes = []
      for rxn in model.reactions:
          if not rxn.gene_reaction_rule:
              continue
          size = find_top_level_complex(rxn.gene_reaction_rule)
          if size >= 2:
              complexes.append(rxn)
      return complexes
docs: Find reactions that are catalyzed by at least a heterodimer.
  
      Parameters
      ----------
      model : cobra.Model
          The metabolic model under investigation.
  
      Returns
      -------
      list
          Reactions whose gene-protein-reaction association contains at least one
          logical AND combining different gene products (heterodimer).