Protein general features

General features that apply to the import of proteins into organelles, including mitochondria and some of the other organelles to be discussed below, are summarized in Table 46-1. 

TaWe 46-1. Some general features of protein import to organelles. ... 

A general feature of optimum sample preparation is that maximum recovery of the analyte is observed. Consider a graph of recovery vs. variation in one experimental condition. Figure 5 shows such a graph, with temperature as the experimental variable. The curve exhibits a maximum and a decline on either side of the maximum. The assay will be most reproducible at the point of zero slope, i.e., at the maximum recovery, because small variations in conditions will not affect the result. In hydrolysis of a protein to its constituent amino acids, for example, it will be found that at very high temperatures or long hydrolysis times, degradation of the product amino acids occurs, while at low temperatures or short hydrolysis times, the protein... 

In view of the large variations in phi/psi preferences for the different amino acids observed in folded protein (see Fig. 5), it follows that the task of encoding the general features of long-range structure does not... 

Before we conclude the functions of the elements and the proteome, there is a second general feature of eukaryote cells, much of which evolved from that of the prokaryotes - the types of metal-binding protein. The general supposition is that the number of folds are limited and certainly the number of metal-binding sites for any one metal ion is closely limited (see Section 4.15). We find that there are some general rules for protein-binding centres of metal ions and their geometry, mentioned only in brief in Chapters 5 and 6. 

The linking of a metal to an antibody could, in principle, be accomplished by forming the metal chelate either prior to or after attachment to protein. Success to date has been achieved only by formation of the protein-ligand conjugate before metal chelation. The complexation reaction has several general features. First, reactions between the metallic radionuclides and antibodies are almost always performed with sub-stoichiometric quantities of chelate and metal ion. It is therefore of the utmost importance that no carrier added metals obtained from commercial sources be exceedingly pure or else be purified prior to use. Reactions of "carrier added" metal solutions are not likely to be of use because of the ease with which available chelate sites become saturated. Because the formation of chelate complexes is usually a bimolecular reaction, the complexation will proceed optimally when more chelation sites are available. Similarly, the more isotope in solution, the faster the reaction. Employment of a carrier chelate to insure solubilization of the radiometal is of value to maximize available isotope and the acetate ion has proven useful. 

Eleven IgA myeloma proteins that precipitate with Pneumococcus C polysaccharide (PnC) have been described.65-69 This antigen has a structure whose general features are known,70 71 and it is a somatic, species-specific polysaccharide. 

Whereas a major function of biological membranes is to maintain the status quo by preventing loss of vital materials and entry of harmful substances, membranes must also engage in selective transport processes. Living cells depend on an influx of phosphate and other ions, and of nutrients such as carbohydrates and amino acids. They extrude certain ions, such as Na+, and rid themselves of metabolic end products. How do these ionic or polar species traverse the phospholipid bilayer of the plasma membrane How do pyruvate, malate, the tricarboxylic acid citrate and even ATP move between the cytosol and the mitochondrial matrix (see figs. 13.15 and 14.1) The answer is that biological membranes contain proteins that act as specific transporters, or permeases. These proteins behave much like conventional enzymes They bind substrates and they release products. Their primary function, however, is not to catalyze chemical reactions but to move materials from one side of a membrane to the other. In this section we discuss the general features of membrane transport and examine the structures and activities of several transport proteins. 

Calorimetric studies have established several general features of protein denaturation.12 13 14 For small globular proteins, the denaturation process is well represented by a two-state process,... 

Figure 5 also illustrates other general features of globular protein stability. The positive overall ACp results in the existence of two temperatures at which AG° is zero. The low temperature point defines the so-called cold denaturation of the protein and the high temperature point defines the heat denaturation. Additionally, the curvature in AG° implies the existence of a temperature of maximal stability. This temperature occurs at the point where AS0 is equal to zero. The... 

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