Protein–lipid interaction
Protein–lipid interaction is the influence of membrane proteins on the lipid physical state or vice versa. The questions which are relevant to understanding of the structure and function of the membrane are: 1) Do intrinsic membrane proteins bind tightly to lipids (see annular lipid shell), and what is the nature of the layer of lipids adjacent to the protein? 2) Do membrane proteins have long-range effects on the order or dynamics of membrane lipids? 3) How do the lipids influence the structure and/or function of membrane proteins? 4) How do peripheral membrane proteins which bind to the layer surface interact with lipids and influence their behavior? A large research effort involves approaches to know whether proteins have binding sites which are specific for particular lipids and whether the protein–lipid complexes can be considered to be long-lived, on the order of the time required for the turnover a typical enzyme, that is 10−3 sec. This is now known through the use of 2H-NMR, ESR, and fluorescent methods. There are two approaches used to measure the relative affinity of lipids binding to specific membrane proteins. These involve the use of lipid analogues in reconstituted phospholipid vesicles containing the protein of interest:
- Spin-labeled phospholipids are motionally restricted when they are adjacent to membrane proteins. The result is a component in the ESR spectrum which is broadened. The experimental spectrum can be analyzed as the sum of the two components, a rapidly tumbling species in the "bulk" lipid phase with a sharp spectrum, and a motionally restricted component adjacent to the protein. Membrane protein denaturation causes further broadening of ESR spin label spectrum and throws more light on membrane lipid-proteins interactions
- Spin-labeled and brominated lipid derivatives are able to quench the intrinsic tryptophan fluorescence from membrane proteins. The efficiency of quenching depends on the distance between the lipid derivative and the fluorescent tryptophans.
