Membrane Proteins Are Associated With the Lipid Bilayer

Membrane phospholipids act as a solvent for membrane proteins, creating an environment in which the latter can function. Of the 20 amino acids contributing to the primary structure of proteins, the functional groups attached to the a carbon are strongly hydrophobic in six, weakly hydrophobic in a few, and hy-drophilic in the remainder. As described in Chapter 5, the a-helical structure of proteins minimizes the hy-drophilic character of the peptide bonds themselves. Thus, proteins can be amphipathic and form an integral part of the membrane by having hydrophilic regions protruding at the inside and outside faces of the membrane but connected by a hydrophobic region traversing the hydrophobic core of the bilayer. In fact, those portions of membrane proteins that traverse membranes do contain substantial numbers of hy-drophobic amino acids and almost invariably have either a high a-helical or P-pleated sheet content. For many membranes, a stretch of approximately 20 amino acids in an a helix will span the bilayer.

It is possible to calculate whether a particular sequence of amino acids present in a protein is consistent with a transmembrane location. This can be done by consulting a table that lists the hydrophobicities of each of the 20 common amino acids and the free energy val ues for their transfer from the interior of a membrane to water. Hydrophobic amino acids have positive values; polar amino acids have negative values. The total free energy values for transferring successive sequences of 20 amino acids in the protein are plotted, yielding a so-called hydropathy plot. Values of over 20 kcalmol-1 are consistent with—but do not prove—a transmembrane location.

Another aspect of the interaction of lipids and proteins is that some proteins are anchored to one leaflet or another of the bilayer by covalent linkages to certain lipids. Palmitate and myristate are fatty acids involved in such linkages to specific proteins. A number of other proteins (see Chapter 47) are linked to glycophos-phatidylinositol (GPI) structures.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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