Pointing out that a functional 150 unit chain of amino acids is improbable and therefore could not have been produced by random mutation is exactly like pointing out that an eye is improbable and therefore could not be produced by random mutation, and both statements are wrong for the same reason. In order for Natural selection to work, there simply has to have been a series of functional and beneficial (or at least not detrimental) intermediates between utter simplicity and any complex organ/tissue/molecule that we observe.

You can get an modern eye from a non-eye if there exist many millions of intermediates between the two, each of which is functionally superior to the last and is therefore favored by natural selection. It is the same with proteins. You can get to a very complex protein from an extremely simple one so long as there exist intermediates along the way, each of which confers some type (any type) of advantage on the organism. The function of any intermediate protein need not be in any way related to the function of the modern 150-unit protein in question.

All this is is a doctored up irreducable complexity argument. "if this protein were off by one amino acid, it wouldn't do what it does, therefore it couldn't have evolved!"