Glycoproteins, heat-stable

Kanno, C. and Yamauchi, K. 1978. Antigenic identity between the soluble glycoprotein of milk fat globule membrane ami a heat-stable protein fraction of whey. Agr. Biol. Chem. (Japan) 42, 1697-1705. 

Cholesteryl ester transfer protein (CETP) promotes exchange and transfer of neutral lipids such as cholesteryl ester (CE) and TG between plasma lipoproteins [63-65], The function of CETP is illustrated in Fig. 3. CETP is a very hydrophobic and heat-stable glycoprotein with an apparent molecular weight of 74 kDa as determined by SDS-PAGE analysis [66,67], The cDNA from human liver was cloned and sequenced [68], It encodes for a 476-amino acid protein (53 kDa), suggesting that the apparent higher molecular weight is due to the addition of carbohydrate residues by posttranslational modification. 

Some receptors appear to be linked to the stimulation of guanylyl-cyclase. These membrane receptors mediate the action of the atrial natriuretic peptide (ANP), the heat-stable enterotoxin of E. coli and the receptors involved in fertilization in some species. These membrane receptors are glycoproteins of about 130-160 kDa spanning the membrane only once. The binding of the peptides occurs at the extracellular domain, while the intracellular regions contain both cyclase catalytic domains, converting GTP to cGMP, and a protein kinase-like domain. 

Saposins, a family of four small heat-stable glycoproteins derived from the large precursor protein prosaposin (70 kDa). The saposins A-D are involved as cofactors in sphingolipid catabolism [Y. Kishimoto et al, J. Lipid Res. 1992, 33, 1255 A. M. Vaccaro et al., Neurochem. Res. 1999, 24, 307]. 

The system consists of LP (EC 1.11.1.7) and the substrates thiocyanate and H2O2. The enzyme, a glycoprotein (carbohydrate content 10%), consists of 612 amino acid residues (Mr 78,000, IP 9.6) and Fe-protoporphyrin IX, which as the prosthetic group carries out the catalysis, as described in 2.3.2.2. Thiocyanate takes part in this reaction process as the electron donor AH. Lactoperoxidase is one of the heat stable enzymes of milk, especially when the structure is fixed by Ca ions. It is still active after 30 min at 63 °C (neutral pH) and after 15 s at 72 °C, but it is inactive after only 2.5 s at 80 °C. Acidification (pH 5.3) accelerates the inactivation by liberating the Ca ions. After... 

Many foodstuffs contain a metabolic intermediate of biotin, biocytin s-N-biotinyUysine), which is cleaved in the intestinal tract by the enzyme biotinidase. Only free biotin can be resorbed in the proximal small intestine, a process, which can be blocked by avidin, a glycoprotein with a molar mass of ca. 70,000. Avidin occurs in greater amounts in egg-white, and forms with biotin an extraordinarily stable molecular complex (dissociation constant at 25 °C K = 10 M), which can be cleaved neither by acids nor by peptidases. Only irradiation or longer exposure to heat leads to denaturation of avidin and thereby the release of biotin. This is another reason why a breakfast egg ought to be cooked for at least AVi minutes. In this way avidin is denatured and loses its harmful effect. Similarly stable complexes are formed by biotin with neutravidin (de-glycosyl-ated avidin), streptavidin and stravidin from certain Streptomyces and Saccharo-myces species respectively. 

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