Protein interactions, ionic

Figure 1. Simplified schematic of receptor-mediated signal transduction in neutrophils. Binding of ligand to the receptor activates a guanine-nucleotide-binding protein (G protein), which then stimulates phospholipase C. Phosphatidylinositol 4,5-bis-phosphate is cleaved to produce diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). DAG stimulates protein kinase C. IP3 causes the release of Ca from intracellular stores, which results in an increase in the cytosolic Ca concentration. This increase in Ca may stimulate protein kinase C, calmodulin-dependent protein kinases, and phospholipase A2. Protein phosphorylation events are thought to be important in stimulating degranulation and oxidant production. In addition, ionic fluxes occur across the plasma membrane. It is possible that phospholipase A2 and ionic channels may be governed by G protein interactions. ...

Ge (2000) carefully designed a universal protein array (UFA) system based on the use of various transcription factors, their activators, and cofactors as probes. A total of 48 different, highly purified factors were used to create the UFA on nitrocellulose filters. Frotein-protein interactions of various binding affinities could be assessed using different ionic strength buffers (e.g., 100 mM KCl vs. 1000 mM KCl). The relative binding of radiolabeled ( T) GST-K-p52 proteins to various transcription factors was studied. 

Proteins interact with the membrane (support) by hydrophobic and charge-transfer forces and hydrogen bridges. The extent of these interactions depends on the accessibility of respective area of a protein. The accessibility is influenced, among other things, by the composition of the surrounding buffer, e.g., pH, ionic strength and/or chaotropic additives. 

In general, species containing transition metals and metalloids such as As, Sb, Se and Sn are thermodynamically more stable than those of the alkali and alkaline earth metals. Transition metals and metalloids form an integral part and are linked to the organic constituents by covalent bonds. In contrast alkali and alkaline earth metals are attached loosely by predominantly ionic bonds. Readers interested in the fundamentals of metal-protein interactions are referred to books... 

The major casein monomer subunits have random coil conformation that facilitates strong protein-protein interaction via hydrophobic and ionic bonding. The unique amphiphilic structure, which arises from separately clustered hydrophobic and negatively charged (acidic and ester phosphate) amino acid residues along the polypeptide chain, makes them susceptible to pH and Ca ion concentration effects. This amphiphilic nature is probably responsible for the excellent surfactant properties of commercial caseinate in a variety of food applications. 

The thermostabilization of biomolecules is a result of the mutual contribution from fundamental interactions [e.g., hydrophobic forces (4, 5) or ionic interactions (3, 6, 7)] that stabilize individual molecules and prevent their aggregation (6), structure modifications [such as DNA superhelicity (8, 9) and posttrans-lational modification of proteins], interactions with an environment (10), intermolecular interactions (11), and oligomerization (12). The possible dependence of fundamental interactions, for example, hydrophobic forces, on temperature may also affect stability. However, it remains a subject of controversy as to how and to what extent the dependence of the interaction strength on temperature should be taken into account (13-16). This article reviews the very basic level of protein and DNA thermostability. 

---