Cell-surface receptor proteins specificity

Here are two of many known examples of specific cell-cell adhesion. The species-specific reaggregation of dissociated cells of marine sponges (Chapter 1) depends upon a 20-kDa proteoglycan of unique structure209-211 together with a cell surface receptor protein and calcium ions. The recognition of egg cell surfaces by sperm212-214 is species... 

Theories abound, which relate those two vocabularies, but no one of them has emerged as predominant. Many of the theories suppose the existence of specific receptor sites on the surface of the receptor neurons. One hypothesis posits a set of odors of specific objects (e.g., camphor, sperm, urine, fish) that correspond to pure compounds and represent fundamental submodalities (Beets, 1982). Another (based on molecular biology) proposes dozens—perhaps hundreds—of different types of cell surface receptor proteins, each of which is tuned to a specific odorant compound or class of compounds (Buck, 1996 Zhao et al., 1998). 

The sensitivity of cellular interactions to interfacial proteins probably is due to the presence of cell surface receptors for specific proteins and to the enhancement of receptor-protein interaction by the concentration of proteins at interfaces. To illustrate the role of specific proteins at interfaces,... 

Food vitamin B 2 appears to bind to a saUvary transport protein referred to as the R-protein, R-binder, or haptocorrin. In the stomach, R-protein and the intrinsic factor competitively bind the vitamin. Release from the R-protein occurs in the small intestine by the action of pancreatic proteases, leading to specific binding to the intrinsic factor. The resultant complex is transported to the ileum where it is bound to a cell surface receptor and enters the intestinal cell. The vitamin is then freed from the intrinsic factor and bound to transcobalamin II in the enterocyte. The resulting complex enters the portal circulation. 

Figure 2. Mechanism of PDH. The three different subunits of the PDH complex in the mitochondrial matrix (E, pyruvate decarboxylase E2, dihydrolipoamide acyltrans-ferase Ej, dihydrolipoamide dehydrogenase) catalyze the oxidative decarboxylation of pyruvate to acetyl-CoA and CO2. E, decarboxylates pyruvate and transfers the acetyl-group to lipoamide. Lipoamide is linked to the group of a lysine residue to E2 to form a flexible chain which rotates between the active sites of E, E2, and E3. E2 then transfers the acetyl-group from lipoamide to CoASH leaving the lipoamide in the reduced form. This in turn is oxidized by E3, which is an NAD-dependent (low potential) flavoprotein, completing the catalytic cycle. PDH activity is controlled in two ways by product inhibition by NADH and acetyl-CoA formed from pyruvate (or by P-oxidation), and by inactivation by phosphorylation of Ej by a specific ATP-de-pendent protein kinase associated with the complex, or activation by dephosphorylation by a specific phosphoprotein phosphatase. The phosphatase is activated by increases in the concentration of Ca in the matrix. The combination of insulin with its cell surface receptor activates PDH by activating the phosphatase by an unknown mechanism.

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