Glutaminase-interacting protein (GIP) is a 124 residue human brain protein involved in the synthesis of neurotransmitters glutamate and glutamine. Both glutamate and glutamine serve as main precursors of the inhibitory neurotransmitter GABA (g-amino butyric acid) and are involved in a variety of general metabolic processes, such as the synthesis of nucleotides and
proteins and in energy metabolism. Deterioration of any component of the glutamate-glutamine cycle is known to lead to degenerative diseases such as epilepsy and Parkinson's disease. Lead is a well-known environmental pollutant found in many everyday materials. Although exposure from lead is often received at low levels, cumulative amounts of lead are known to cause cerebral vascular damage. There have been studies on the effect of lead on the glutamate-glutamine
cycle resulting in reduced activity of glutamine, glutamate, and GABA. This project ventures further to investigate the effect of lead on the structure and stability of GIP.
The effect of increasing lead acetate titrations on the conformation of GIP was investigated using various biophysical techniques such as fluorescence and circular dichroism. Both techniques utilize light to study structural changes in protein. In fluorescence, emission from aromatic amino acids such as tryptophan is measured as a result of excitation by light, giving
indication of its environment within the protein. Circular dichroism examines the secondary and tertiary structure of the protein by use of the absorption of left-handed polarized light versus right-handed polarized light which arises due to structural asymmetry. Our results show a gradual denaturing of the secondary structure of GIPthroughout lead acetate titrations from a predominantly beta sheet structure to a random coil. Results of tryptophan fluorescence indicate a blue shift
and quenching of GIP. This demonstrates the tryptophan aromatic amino acid is becoming more exposed to lead acetate. Lead acetate then hinders the energy emission of tryptophan possibly by collisional quenching. Therefore, it can be concluded that lead causes a loss of stability and structure within GIP and in fact can be detrimental to the central nervous system at high concentrations. This work is supported with funding from the Simons Foundation.