Protein folding water-soluble proteins

Like Complex III of mitochondria, cytochrome b6f conveys electrons from a reduced quinone—a mobile, lipid-soluble carrier of two electrons (Q in mitochondria, PQb in chloroplasts)—to a water-soluble protein that carries one electron (cytochrome c in mitochondria, plastocyanin in chloroplasts). As in mitochondria, the function of this complex involves a Q cycle (Fig. 19-12) in which electrons pass, one at a time, from PQBH2 to cytochrome bs. This cycle results in the pumping of protons across the membrane in chloroplasts, the direction of proton movement is from the stromal compartment to the thylakoid lumen, up to four protons moving for each pair of electrons. The result is production of a proton gradient across the thylakoid membrane as electrons pass from PSII to PSI. Because the volume of the flattened thylakoid lumen is small, the influx of a small number of protons has a relatively large effect on lumenal pH. The measured difference in pH between the stroma (pH 8) and the thylakoid lumen (pH 5) represents a 1,000-fold difference in proton concentration—a powerful driving force for ATP synthesis. 

In contrast to their limited importance in nature, the /1-barrel proteins are most prominent in the list of established membrane protein structures. Moreover, they show a high degree of internal chain-fold symmetry and therefore convey the impression of beautiful proteins (Fig. 1). One should not forget that the first protein structure, myoglobin, caused some disappointment among those who solved it as it showed no symmetry whatsoever even the a-helices were not whole-numbered but about 3.6 residues per turn. Accordingly, the symmetric transmembrane /1-barrels stand out from the bulk of asymmetric chain folds of water-soluble proteins. 

Tertiary Structure Water-Soluble Proteins Fold Into Compact Structures with Nonpolar Cores... 

The -carboxylic acid group of aspartic acid has a pK of 3.86 and is ionized at pH 7.0 (the anionic form is called aspartate). The anionic carboxylate groups tend to occur on the surface of water-soluble proteins, where they interact with water. Such surface interactions stabilize protein folding. 

The answer is d. (Murray, pp 48-62. Scrivei pp 3-45. Sack, pp 1-3. Wilson, pp 101—120.) The structure of myoglobin is illustrative of most water-soluble proteins. Globular proteins tend to fold into compact configurations with nonpolar cores. The interior of myoglobin is composed almost exclusively of nonpolar, hydrophobic amino acids like valine, leucine, phenylalanine, and methionine. In contrast, polar hydrophilic residues such as arginine, aspartic acid, glutamic acid, and lysine are found mostly on the surface of the water-soluble protein. 

How would a protein that resides in the interior of a membrane fold, compared with the water-soluble protein just discussed ... 

TERTIARY STRUCTURE WATER-SOLUBLE PROTEINS FOLD INTO COMPACT STRUCTURES WITH NONPOLAR CORES... 

The tertiary structure of proteins describes the pattern of folding of secondary structures into a compact, more sophisticated molecule that can carry out biological functions. The tertiary structures of water-soluble proteins have the following common morphological features (1) an interior formed of amino acids with hydrophobic side chains, and (2) a surface formed largely of hydrophilic amino acids that interact with the aqueous environment, directed by the hydrophobic interactions between the interior residues. 

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