Date of Graduation

5-2016

Document Type

Thesis

Degree Name

Bachelor of Science

Degree Level

Undergraduate

Department

Chemistry & Biochemistry

Advisor

Koeppe, Roger

Reader

Millett, Frank

Second Reader

Shew, Woodrow

Third Reader

Kumar, Suresh

Abstract

An essential component of animal cells, cholesterol exerts significant influence on the physical properties of the cell membrane and in turn, its constituents. One such category of constituents, the membrane proteins, are responsible for diverse and essential biological functions and often contain polar amino acids. Although sparse within the hydrophobic interior of lipid-bilayer membranes, polar amino acid residues are highly conserved and may play pivotal roles in determining specific structural and functional properties of key proteins. To gain greater understanding of the lipid membrane environment, and more broadly, cellular function, a model peptide framework termed “GWALP23” (acetyl-GGALWLALALAL12AL14ALALWLAGA-amide) can be useful. Designed and created at the University of Arkansas, the peptide is composed of amino acid residues of glycine (G), tryptophan (W), alanine (A), and leucine (L). The central helical core consisting of the W(LA)6LW sequence is critical for defining the transmembrane configuration of this host peptide framework. Furthermore, the limited dynamic averaging of NMR observables such as the deuterium quadrupolar splittings of labeled alanine residues makes GWALP23 favorable for examining the influence of single “guest” residue replacements within the core sequence. Previously, GWALP23 family peptides were characterized with single leucine to arginine (R) mutations at positions 12 and 14 in single-lipid membranes. GWALP23-R14 adopts a defined tilted orientation in DOPC bilayers, whereas GWALP23-R12 displays multi-state behavior. The goal of this research is to further characterize these peptides in cholesterol-containing lipid bilayers. Specific deuterium-labeled alanine residues were incorporated into the R12 and R14 sequences to identify transmembrane peptide orientations by means of solid-state deuterium NMR spectroscopy. The peptides were incorporated into phospholipid bilayers with varying cholesterol content (0%, 10%, or 20%). Our findings suggest that 10% or 20% cholesterol content has minimal impact on the orientation of the GWALP23-R14 peptide. (Although the NMR signals are broader and weaker in the presence of 20% cholesterol, the deuterium quadrupolar splittings for 2H-Ala residues in GWALP23-R14 change little.) Conversely, cholesterol appears to reduce the multi-state behavior of GWALP23-R12, favoring a single well-defined state for the helix. With 10% or 20% cholesterol content, the spectra exhibit defined quadrupolar splittings, suggesting that GWALP23-R12 adopts a predominant orientation at the membrane surface in the presence of cholesterol. These results convey a conditional sensitivity of a complex multi-state peptide helix to the presence of cholesterol.

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