Plasma proteins noncovalent binding

As hormone-sensitive lipase hydrolyzes triacylglyc-erol in adipocytes, the fatty acids thus released (free fatty acids, FFA) pass from the adipocyte into the blood, where they bind to the blood protein serum albumin. This protein (Mv 66,000), which makes up about half of the total serum protein, noncovalently binds as many as 10 fatty acids per protein monomer. Bound to this soluble protein, the otherwise insoluble fatty acids are carried to tissues such as skeletal muscle, heart, and renal cortex. In these target tissues, fatty acids dissociate from albumin and are moved by plasma membrane transporters into cells to serve as fuel. 

Data for the metabolites in plasma are generally for the unbound forms, but there is ample evidence that PPT bind noncovalently to proteins. Most studies on PPT-protein interaction have focused on protein utilization or astringency but a few studies have addressed binding to plasma proteins and lipoproteins. " " Strongest binding has been associated with 1,2-dihydroxyphenols and proline-rich proteins such as those character-istic of human saliva and structure-activity relationships have been reported. 

Mechanism of activation of the epidermal growth factor (EGF) receptor, a representative receptor tyrosine kinase. The receptor polypeptide has extracellular and cytoplasmic domains, depicted above and below the plasma membrane. Upon binding of EGF (circle), the receptor converts from its inactive monomeric state (/eft) to an active dimeric state (right), in which two receptor polypeptides bind noncovalently. The cytoplasmic domains become phosphorylated (P) on specific tyrosine residues (Y) and their enzymatic activities are activated, catalyzing phosphorylation of substrate proteins (S). 

The noncovalent binding to plasma proteins may involve four types of interaction (Fig. 3.17) ... 

Lipids, by virtue of their immiscibility with aqueous solutions, depend on protein carriers for transport in the bloodstream and extracellular fluids. Fat-soluble vitamins and free fatty acids are transported as noncovalent complexes. Vitamin A is carried by retinol-binding protein and free fatty acids on plasma albumin. However, the bulk of the body s lipid transport occurs in elaborate molecular complexes called lipoproteins. 

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