Proteins noncatalytic

In addition, Ee protein-1 has at least two noncatalytic roles related to nitrogen fixation, and presumably Ee protein-2 and Ee protein-3 do also. The first function is somehow involved in the early stages of EeMo-cofactor biosynthesis because mutant strains having a deletion in nifil produce neither Ee protein nor EeMo-cofactor, rather only a EeMo-cofactor-deficient apo-MoEe protein (104,105). The second role involves insertion of preformed EeMo-cofactor into these apo-MoEe proteins (106). 

In addition to enzymes, noncatalytic proteins may exhibit many of these properties hemoglobin is the classic example. The allosteric properties of hemoglobin are the subject of a Special Focus beginning on page 480. 

Noncatalytic phosphotyrosine binding (PTB) domains are 100-150 residue modules, which bind Asn-Pro-X-Tyr motifs. PTB-domain binding specificity is determined by residues at the amino-terminal side of the phosphotyrosine. In most cases, the tyrosine residue must be phosphorylated in order to mediate binding. PTB domain containing proteins are often found in signal transduction pathways. 

Under physiological conditions, NRPTKs are highly specific in directing tyrosine phosphorylation toward appropriate substrates. This specificity relies on the intrinsic predilection of the catalytic domain towards specific amino acid sequences within protein substrates. In addition, noncatalytic domains, e.g., SH2, SH3 and PH domains of NRPTKs, distribute these kinases to the subcellular region where appropriate substrates are in proximity or abundance, thus favoring phosphorylation of these proteins rather than other substrates. 

Autosomal dominant diseases are more likely to show late onset of symptoms. The genes involved often encode noncatalytic proteins, and may occasionally show incomplete penetrance or variable expression in a pedigree. 

Sadowski, I., Stone, J. C., and Pawson, T. (1986). A noncatalytic domain conserved among cytoplasmic protein-tyrosine kinases modifies the kinase function and transforming activity of Fujinami sarcoma virus P130gag- s, Mol Cell Biol 6, 4396-408. 

Enzymes represent a special case of protein function. Enzymes bind and chemically transform other molecules—they catalyze reactions. The molecules acted upon by enzymes are called reaction substrates rather than ligands, and the ligand-binding site is called the catalytic site or active site. In this chapter we emphasize the noncatalytic functions of proteins. In Chapter 6 we consider catalysis by enzymes, a central topic in biochemistry. You will see that the themes of this chapter—binding, specificity, and conformational change—are continued in the next chapter, with the added element of proteins acting as reactants in chemical transformations. 

Lipophilic materials require intracellular carrier proteins to be optimally mobilized, just as they required transport proteins in the blood (Figure 10.3). Several intracellular carrier proteins that mobilize specific endogenous chemical have been characterized, although less is known of which proteins typically mobilize xenobiotics. Some of the cytosolic glutathione S-transferase proteins have been shown to noncatalytically bind xenobiotics and to be coordinately induced along with xenobiotic biotransformation enzymes and efflux transporters, suggesting that these proteins may function to mobilize xenobiotics. 

While protein kinases are responsible for the phosphorylation of their substrates, protein phosphatases perform the opposite duty, removing phosphate groups from their substrates, thus countering the functional impact of the kinases. The two major types of protein phosphatases are the serine/threonine phosphatases and tyrosine phosphatases. Several natural compounds with potent serine/threonine phosphatase inhibitory activity have been identified, including the cyanobacterial metabolite microcystin [105,106]. This compound labels its targets via a Michael addition of a noncatalytic active site cysteine residue with an acceptor in the macrocyclic peptide backbone [107]. A fluorescent probe based on microcystin was synthesized by Shreder et al., and its use in Jurkat lysates identified two previously undescribed phosphatase targets of microcystin, PP-4 and PP-5 [108]. Whereas serine/threonine... 

Cross, T.A. Florida State University Protein stability - catalytic and noncatalytic solvents National Institute of General Medical Sciences... 

The bcf complexes form dimers in the membrane with molecular masses of approximately 480 kDa (mitochondria) and 130 kDa (bacteria), respectively. Each monomer has 10-13 membrane spanning helices, depending on the number of noncatalytic subunits. The membrane spanning helices of cytochrome b are in the center of the structure and form the dimer interface while the other membrane spanning helices are located around cytochrome b. Cytochrome c and the Rieske iron sulfur protein both have water soluble domains containing the redox centers, heme ci and the [2Fe-2S] cluster, respectively. These domains are at the outside of the iimer mitochondrial membrane, i.e., in the intermembrane space, and bound to the membrane via membrane spanning helices acting as membrane anchors. 

Figure 4-17 A simple representation of an equilibrium dialysis experiment where the volumes inside and outside the dialysis bag are unequal. The large circles inside the dialysis bag represent a nondiffusible protein, P, which may be an enzyme or a noncatalytic binding protein. The small dots represent a diffusible ligand, S, which may be a substrate, inhibitor, or activator, and so on. At equilibrium, the concentration of free ligand, [S]/, is the same on both sides of the membrane. The excess ligand inside the dialysis bag represents bound ligand, [S]t or [PS]. Note that if no protein were present inside the dialysis bagi the equilibrium [S] would be 1.2 (6/5 = 1.2 inside the bag. 12/10 =1.2 outside the bag, or 18/15= 1.2 overall). To minimize the amount of protein and ligand required, equilibrium dialysis is usually carried out with specially-made plastic chambers of equal volumes (e.g., 0.1 to 1.0 ml), separated by a semipermeable membrane (see Fig. 6-2).

These suggestions were tested experimentally by Wasteneys and Borsook (68), who found that a peptic hydrolysate of protein gave, in the presence of concentrated pepsin solution and emulsion droplets of benzene or benzaldehyde, appreciable synthesis of a complex, less soluble material. Without the oil drops, reaction was very much slower, as the data in Fig. 24 show. Emulsions of fats, however, were noncatalytic, and xylol, talc, kieselguhr and barium sulfate were but weakly effective in increasing the rate. If the original protein is only slightly degraded, the yield in the synthesis is independent of the presence of the catalyst, but. 

The detection of enzymes by activity staining is important not only for the direct detection of separated enzymes, but also for the detection of noncatalytic proteins after staining with enzyme-labeled antibodies to the protein of interest. The ability to detect a particular catalytic activity greatly reduces background staining, and allows the unequivocal identification of a particular analyte species in very complex sample mixtures. 

---