Protein as substrates

FIGURE 25.20 Triacylglycerols are formed primarily by the action of acyltransferases on mono- and diacylglycerol. Acyltransferase in E. coli is an integral membrane protein (83 kD) and can utilize either fatty acyl-CoAs or acylated acyl carrier proteins as substrates. It shows a particular preference for palmitoyl groups. Eukaryotic acyltransferases nse only fatty acyl-CoA molecnles as substrates. 

Gabriel et al. (2007) Novel mitochondrial intermembrane space proteins as substrates of the MIA import pathway. J Mol Biol 365 612-620 Gakh O, Cavadini P, Isaya G (2002) Mitochondrial processing peptidases. Biochim Biophys Acta 1592 63-77... 

Del Duca, S., Tidu, V., Bassi, R., Esposito, C., and Serafini-Fracassini, D., Identification of chlorophyll-a/b proteins as substrates of transglutaminase activity in isolated chloroplasts of Helianthus tuberosus L., Planta, 193, 283-289, 1994. 

The exact function of the annexin fold family is at present unclear. All of these proteins appear to show calcium-dependent binding to phosphatidylethanolamine or phosphatidylinositol liposomes. In addition, they can promote fusion of liposomes, and because of this property, it has been suggested that these proteins might mediate calcium dependent exocytosis. P36 and p35 have also been shown to bind to F-actin and spectrin [65,66]. Recently, Khanna et al. [70] have reported a procedure for the simultaneous purification of p35, p36 oligomer and p36 monomer from bovine lung, and identified all three proteins as substrates of protein kinase C. Furthermore, the work of Huang et al. [86] and Khanna et al. [69] has suggested that all three proteins are inhibitors of phospholipase A2. Further experiments will be required to clarify the function of these proteins. 

Apart from the application of dyed proteins as substrates of proteolytic activity assay so far mentioned (see also 7.2), proteins are labeled with chromophores for studying membrane surfaces, for detecting proteins after several different [ ation procedures (i.e. chromatographic and electrophoretic techniques), for the determination of proteins in the mixture with many other substances, etc. 

As amply discussed in Chapter 2 [Secs. Ill, IV], proteins from different sources can be modified to produce a wide variety of PBS. Some of the techniques used in protein modification have been indicated in Chapter 5 [Secs. II-VII], with reference to PBS produced via a degradation process such as proteolysis and deamidation, in addition to the cases of the surfactants produced by covalent attachment of nonprotein moieties. Enzymatic modification of proteins with specific reference to food application has been discussed in Chapter 1, Section II.B. Briefly, it was hypothesized that by reacting a hydrophilic protein as the substrate with highly hydrophobic amino acid ester as the nucleophile, a product with amphiphilic properties would result from the localized regions of hydrophobicity [76]. To obtain adequately hydrophilic proteins as substrates, succinylation could be used to modify the proteins prior... 

Enzymes that hydrolyze proteins and other compounds composed of amino acids were among the first biological catalysts to be discovered, and they have continued to be prominent in studies of enzyme structure, kinetics, activation, and mechanism of action. The crystallization of the enzyme urease by Sumner was followed by the crystallization of various proteolytic enzymes in the laboratory of Northrop. These studies established that catalytic activity is associated with what appear to be pure proteins all well-defined enzymes isolated subsequently have also proved to be proteins, although many contain additional components. The study of enzymatic reactions involving proteins as substrate, therefore, gives insight into the chemical nature of enzymes as well as the mechanisms by which they act. 

Posch, K. C., Boerman, M H E M, Burns, R D, and Napoli, J. L (1991) Holocellular retinol-bindmg protein as substrate for microsomal retinal synthesis. Biochemistry 30, 6224-6230. 

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