Liver subunit association

Inspection of the size of the various nucleotide-binding proteins, whether coenzyme B12- or iron subunit-associated, reveals a preference for the 75,000 to 100,000 dalton range. The 45,000 dalton L1 subunits of rat liver reductase with one half this value still fit the picture although in no other case dissociation into two polypeptide chains has been found. One, two, or four of these large subunits can apparently form a holoenzyme like in the Lactobacillus (1), Corynebacterium, E. coli, Scenedesmus, mammalian enzymes... 

The release of glucose from the glycogen stores in the liver is mediated by glucose 6-phosphatase, which is apparently embedded within the membranes of the endoplasmic reticulum. A labile enzyme, it consists of a 357-residue catalytic subrmit,251/252 which may be associated with other subunits that participate in transport.252 253 A deficiency of this enzyme causes the very severe type la glycogen storage disease (see Box 20-D).251 253 Only hepatocytes have significant glucose 6-phosphatase activity. 

Several GPCRs, their associated heterotrimeric G protein subunits and a family of Rho GTFIases have been identified and characterized in trematodes (Fig. 10.2C). Mansour (1984) was the first to study, at the biochemical level, tegumental receptors of 5. mansoni and the liver... 

Bioenergetic dysfunction of mitochondria has been reported as a hallmark of many types of cancers (i.e., downregulation of ATP synthase /3-subunit expression in liver, kidney, colon, squamous esophageal, and lung carcinomas, as well as in breast and gastric adenocarcinomas). ATP synthase d-subunit was found to be associated with chemoresistance to 5-fluorouracil (5-FU) in CRC using 2D-PAGE. In a functional assay, suppressed ATP synthase d-subunit expression by siRNA transfection increased cell viability in the presence of 5-FU [115]. 

The enzyme pyruvate carboxylase (Chap. 12) from chicken liver is an oligomer composed of four identical subunits. The enzyme loses its catalytic activity when cooled below 277 K. Assuming that this loss of activity reflects dissociation of the tetrameric enzyme into its subunits, what can you deduce about the relative importance of enthalpic and entropic effects in the association of the subunits in the tetrameric enzyme ... 

In the mitochondrial matrix, carnitine palmitoyltransferase II (CPT II) catalyzes the reversible transfer of acyl residues with 10-18 carbon atoms between carnitine and CoA to form acyl-CoA thioesters that are the substrates of P-oxidation [4]. CPT II purified from mitochondria of bovine heart and rat liver has a subunit molecular mass of approximately 70 kDa. The crystal stmcture of CPT II revealed the presence of two antiparallel helices that are absent from soluble carnitine acyltransferases and are believed to facilitate the association with the inner mitochondrial membrane (M. Henning, 2006). 

The glutathione transferases are soluble dimeric proteins that are composed of identical subunits, although some forms are heterodimers. These enzymes are present in most tissues and are primarily cytosolic (approximately 95%), although a small percentage are found in the endoplasmic reticulum (approximately 5%). The cytosolic glutathione S-transferases (GSTs) are divided into six families or classes alpha, kappa, mu, pi, sigma, and theta. The alpha, mu, and pi GST classes are primarily responsible for most of the catalytic activity associated with liver detoxication of xenobiotics. 

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