Amino acids hydrophobic association

Soybean Protein Isolates. Soybean protein isolates, having a protein content of >90 wt%, are the only vegetable proteins that are widely used in imitation dairy products (1). Most isolates are derived from isoelectric precipitation, so that the soybean protein isolates have properties that are similar to those of casein. They are insoluble at thek isoelectric point, have a relatively high proportion of hydrophobic amino acid residues, and are calcium-sensitive. They differ from casein in that they are heat-denaturable and thus heat-labile. The proteins have relatively good nutritional properties and have been increasingly used as a principal source of protein. A main deterrent to use has been the beany flavor associated with the product. Use is expected to increase in part because of lower cost as compared to caseinates. There has been much research to develop improved soybean protein isolates. 

The major stmctural feature of the HAz chain (blue in Figure 5.20) is a hairpin loop of two a helices packed together. The second a helix is 50 amino acids long and reaches back 76 A toward the membrane. At the bottom of the stem there is a i sheet of five antiparallel strands. The central i strand is from HAi, and this is flanked on both sides by hairpin loops from HAz. About 20 residues at the amino terminal end of HAz are associated with the activity by which the vims penetrates the host cell membrane to initiate infection. This region, which is quite hydrophobic, is called the fusion peptide. 

Fig. 3-10 The 20 protein amino acids divided by R group character as (a) hydrophobic, (b) hydrophilic, and (c) mixed. (Reprinted with permission from W. K. Purves and G. H. Orians, Life The Science of Biology," pp. 63-81, Copyright 1987 by Sinauer Associates, Inc., Sunderland, MA.)...

Loss of the native conformation of a protein generally exposes hydrophobic amino acid residues that are normally buried on the inside of the self-associated structure and are shielded from the aqueous environment. This leads to association between the exposed hydrophobic residues of neighboring proteins (aggregation) or between these exposed residues and hydrophobic surfaces that the protein may encounter either in the manufacturing process or in the primary package. 

Fukuoka was found to be homozygous for the 1615 G to A (539 Asp to Asn) mutation. This mutation occurred at relatively conserved amino acid residues and caused an alteration in hydrophobicity. Recently, we examined the structure-function relationship of these variants using the recombinant protein (F14). Although all of the four variants were found to be heat labile, the residual GPI activity seems to reflect clinical severity, such as the degree of anemia and episodes of hemolytic crisis. GPI Matsumoto, associated with severe anemia and hemolytic crisis, was extremely unstable, and GPI Iwate, which is associated with compensated hemolytic anemia, showed moderate heat instability. Affinity for substrate, fructose-6-phosphate, was slightly decreased in GPI Narita and GPI Fukuoka, which were associated with moderate anemia and hemolytic crisis. 

The membrane establishes in and out. The membrane is asymmetric because the inner and outer leaflets can have a different lipid composition and contain different proteins (Fig. 3-3). Proteins can be associated with either side of the membrane, or they can pass through the membrane using membrane-spanning segments. The functional part of the protein can be on the cytosolic side, the external side, or even in the membrane itself. A common structure for spanning a membrane is an a-helix (but there are examples of sheets spanning a membrane). It takes about 20 amino acid residues arranged in a helix to span to a 30 A hydrophobic interior of the bilayer. 

Proteins represent another major group of membrane components. They play structural roles and/or are involved in many cellular processes, which are strictly coupled to membranes. Proteins can be either entirely embedded within the bilayer, or they might be firmly anchored (e.g. by a hydrophobic transmembrane segment composed of hydrophobic amino acid side-chains or as lipoprotein), or they can be just associated with the surface. 

A common feature of enzymes is the presence of a cleft or depression in the structure which is lined with mainly hydrophobic amino acid residues and into which the substrate fits. Certain amino acid residues which are concerned with either the orientation of the substrate, and hence the specificity of the enzyme, or are involved in the catalysis of the reaction, are located in this cleft. Those amino acid residues that are associated with the latter role form the active site of the enzyme and are often located towards the base of this cleft. In most cases they are ionic or reactive and include histidine, lysine, cysteine... 

Interactions between proteins and salts in the binding buffer are also a major determinant of selectivity. Salts that are strong retention promoters in HIC are excluded from protein surfaces by repulsion from their hydrophobic amide backbones and hydrophobic amino acid residues.8,9 This causes the mobile phase to exert an exclusionary pressure that favors the association of proteins with the column, regardless of stationary-phase hydrophobicity.1(W2 Because this mechanism involves the entire protein surface, the degree of exclusion is proportional to average protein hydrophobicity, regardless of the distribution of hydrophobic sites. 

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