Protein lability

Apart from the sheer complexity of the static stmctures of biomolecules, they are also rather labile. On the one hand this means that especial consideration must be given to the fact (for example in electron microscopy) that samples have to be dried, possibly stained, and then measured in high vacuum, which may introduce artifacts into the observed images [5]. On the other, apart from the vexing question of whether a protein in a crystal has the same stmcture as one freely diffusing in solution, the static stmcture resulting from an x-ray diffraction experiment gives few clues to the molecular motions on which operation of an enzyme depends [6]. 

Evaporation from a spray of charged droplets produced from a stream of liquid yields ions that can be analyzed in a mass spectrometer. Thermally labile and normally nonvolatile substances such as sugars, peptides, and proteins can be examined successfully. 

The FAB source operates near room temperature, and ions of the substance of interest are lifted out from the matrix by a momentum-transfer process that deposits little excess of vibrational and rotational energy in the resulting quasi-molecular ion. Thus, a further advantage of FAB/LSIMS over many other methods of ionization lies in its gentle or mild treatment of thermally labile substances such as peptides, proteins, nucleosides, sugars, and so on, which can be ionized without degrading their. structures. 

As a rule of thumb, one can say that the efficiency of separation of mixtures and the simplicity of operating and maintaining apparatus are much greater for GC than for LC. Hence, other things being equal, GC is most often the technique of first choice and can be used with a very wide variety of compound types. However, for nonvolatile or thermally labile substances like peptides, proteins, nucleotides, sugars, carbohydrates, and many organometallics, GC may be ruled out completely... 

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 simplest NHIP is rubredoxin, in which the single iron atom is coordinated (Fig. 25.9a) to 4 S atoms belonging to cysteine residues in the protein chain. It differs from the other Fe-S proteins in having no labile sulfur (i.e. inorganic sulfur which can be liberated as H2S by treatment with mineral acid sulfur atoms of this type are not part of the protein, but form bridges between Fe atoms.)... 

Preparation of Balanoglossus luciferin. The residue of the first pH 6 extraction above was re-extracted with 50 mM potassium phosphate buffer, pH 8. After centrifugation, the supernatant was used as the standard luciferin preparation. Luciferin was highly labile and easily inactivated at an extreme pH, by heat, and also by freezing and thawing. The instability resembled that of certain proteins. 

Owing to the weak hydrophobicity of the PEO stationary phases and reversibility of the protein adsorption, some advantages of these columns could be expected for the isolation of labile and high-molecular weight biopolymers. Miller et al. [61] found that labile mitochondrial matrix enzymes — ornitine trans-carbomoylase and carbomoyl phosphate synthetase (M = 165 kDa) could be efficiently isolated by means of hydrophobic interaction chromatography from the crude extract. 

Materials produced by crystalliferous bacilli which elicit a toxic response in susceptible insects may be separated into two types. The first type, the true toxins, include the crystalline protein inclusion body the parasporal body of Hannay (14)], a heat-stable, water-soluble exotoxin active against flies, a heat-stable, dialyzable water-soluble exotoxin, toxic to Lepidoptera on injection (23), and a heat-labile, water-soluble, filterable exotoxin, toxic toward larch sawfly larvae (Hymenoptera) which was reported by Smirnoff (31). 

Cholera toxin, heat labile coli toxins Gs proteins ADP-ribosylation Activation of adenylate cyclase (cholera, traveler -d iarrhea)... 

The diversity of these subcellular actin structures is remarkable and appears to be determined by the interactions of many actin-binding proteins (ABPs) as well as by changes in the concentrations of intracellular signaling molecules such as Ca and cAMP, by small GTP-binding proteins, and by signals arising from mechanical stress. Approximately 50% of the actin molecules in most animal cells are unpolymerized subunits in the cytosolic pool and exist in a state of dynamic equilibrium with labile F-actin filamentous structures (i.e., new structures are formed while existing structures are renewed) (Hall, 1994). 

The contractile apparatus may be thought of as the sum of those intracellular components which constitute the machinery of chemomechanical transduction. It is the set of proteins which convert the chemical energy of the terminal phosphate ester bond of ATP into mechanical work. The structure of the contractile apparatus is determined by the connections between the various protein molecules via specific binding sites or, in a minority of cases, via labile covalent linkages. The kinetics of the contractile machinery are determined by the regulation of changes in these connections. 

The metal and acid labile sulfur contents of these proteins was somewhat lower than expected by comparison with the MoFe proteins. AcP contained 2 0.3 vanadium atoms, 21 1 iron atoms, and 19 0.2 acid-labile sulfur atoms per a2)32 hexamer (172). The a2fi2(S) form of Avl was reported to contain 1.4 0.2 vanadium atoms, 21.4 iron atoms, and 24 acid-labile sulfur ions per molecule, and the afi2 form had lower levels of metal and sulfur (173). 

Fig. 4. Representation of the ligand sphere of the [2Fe-2S] cluster of the Rieske protein from spinach and the attribution of g-tensor to moleculEir axes as discussed in the text. Ser 130 has been observed to influence the redox potentiEd of the cluster via hydrogen interactions with the acid-labile bridging sulfur.

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