http://www.ncbi.nlm.nih.gov/pubmed/18335520

Excess sugar binds with proteins creating Advanced Glycation End products (AGE's) that interfere with protein functions. This has been studied well in diabetes where it causes blindness, neuropathy, etc. but not so much in neurology.

It apparently damages neurons as well:

"Our results show that AGEs promote in vitro cross-linking of alpha-synuclein, that intracellular accumulation of AGEs precedes alpha-synuclein-positive inclusion body formation, and that extracellular AGEs accelerate the process of intracellular alpha-synuclein-positive inclusion body formation."

This may be a mechanism by which fructose damages neurons in PD.