def findAssociatedEnzyme(rx, blocks):
    """
    Return enzyme species, associated with reaction.

    Returns
    -------
    String with enzymeID, empty string if reaction is spontaneous.
    """
    RN = blocks.metabolism.reactions[rx]
    if RN in blocks.enzymes.reaction_catalyzed:
        return(blocks.enzymes.ids[blocks.enzymes.reaction_catalyzed.index(RN)])
    else:
        return(str(''))