Liver quaternary structure

FIGURE 6.41 The quaternary structure of liver alcohol dehydrogenase. Within each subunit is a six-stranded parallel sheet. Between the two subunits is a two-stranded antiparallel sheet. The point in the center is a C9 symmetry axis. (Jane Richardson)... 

The breakdown of glycogen in skeletal muscles and the liver is regulated by variations in the ratio of the two forms of glycogen phosphorylase. The a and b forms differ in their secondary, tertiary, and quaternary structures the active site undergoes changes in structure and, consequently, changes in catalytic activity as the two forms are interconverted. 

Electrospray ionization has allowed the observation of a great number of non-covalent complexes protein-protein, protein-metal ion, protein-drug and protein-nucleic acid. About one-third of the proteins exist as multimeric forms. Mass spectrometry allows the study of their quaternary structure. This has been done for alcohol dehydrogenase (ADH) from horse liver and from yeast. The ESI spectra are displayed in Figure 8.22. The horse liver ADH is observed to be dimeric whereas that of yeast is tetrameric [131]. 

In rat liver ADH the presence of multiple molecular forms has been correlated to disulfide bridges involving the ligands to this zinc atom (Section II,B,3). These forms are active in ethanol oxidation (62). The lobe region which binds zinc is thus, in all probability, not essential for the catalytic action of alcohol oxidation. It has been suggested (133,134) that the extra zinc atom is essential for the structural stability of the enzyme. There is no evidence in the structure that this lobe region is necessary either for tertiary or quaternary structure stabilization. From the structural point of view, this region looks much more like a second catalytic center. The zinc atom is situated in one side of an obvious cleft into which the lone pair electrons of the sulfur atom of Cys-97 project. 

Phosphorylation of proteins probably occurs after translation of the protein (81). f-RNA specific for phosphoserine occurs in rat and rooster liver, but it has not been shown that the phosphorylated amino add can be incorporated directly into proteins (82). In this case, it appears that serine is phosphorylated after it combines with the specific f-RNA (83, 84). Sites of phosphorylation are probably determined by the amino acid sequence of die protein (see below), and extent of phosphorylation may be limited by formation of secondary, tertiary, and quaternary structure. 

The data cited thus far indicate that both alkaline phosphatase and liver alcohol dehydrogenase contain heterogeneous populations of metal atoms of the same species. In both instances, only two of the zinc atoms native to the enzyme appear to be involved in enzymatic activity. The remaining metal atoms do not have a catalytic role but appear to influence the quaternary structure of the protein, although the details of the manner in which this is accomplished are as yet uncertain. These observations have induced us to reexamine the possible effects of metals on structure in other proteins, including those having only single chains. 

Acid a-D-mannosidase (mol. wt. 1.84 X10, 8.1 X 10, and 4.54 x lO" by gel filtration, urea- el filtration, and sodium dodecyl sulphate-polyacrylamide gel electrophoresis, respectively pi 5.9 pH optimum 4.2) was purified more than 2000-fold — via conventional techniques, affinity chromatography on immobilized concanavalin A, and Pevikon electrophoresis — from porcine kidneys. The final product contained the multiple forms A and B and had a quaternary structure. The immunoglobulin fraction of rabbit antiserum against the pure a-D-mannosidase leads in immunodiffusion and precipitation experiments to precipitation not only with a-D-mannosidase from porcine tissues but also (though with lower affinity) from bovine liver and human placenta, urine, and skin fibroblasts. 

The Zn ion, among the series of transition metals, is a cofactor which is not involved in redox reactions under physiological conditions. As a Lewis acid similar in strength to Mg , Zn participates in similar reactions. Hence, substituting the Zn ion for the Mg ion in some enzymes is possible without loss of enzyme activity. Both metal ions can function as stabilizers of enzyme conformation and their direct participation in catalysis is readily revealed in the case of alcohol dehydrogenase. This enzyme isolated from horse liver consists of two identical polypeptide chains, each with one active site. Two of the four Zn ions in the enzyme readily dissociate. Although this dissociation has no effect on the quaternary structure, the enzyme activity is lost. As described under section 2.3.1.1, both of these Zn ions are involved in the formation of the active site. In catalysis they polarize the substrate s C—O linkage and, thus, facilitate the transfer of hydride ions from or to the cosubstrate. Unlike the dissociable ions, removal of the two residual Zn ions is possible only under drastic conditions, namely disruption of the enzyme s quaternary structure which is maintained by these two ions. 

FIGURE 6.2 Phosphatidylcholine (PC) synthesis by the CDP-ethanolamine pathway. Structures of ethanolamine and choline. The nucleotide moiety S-adenosyl methionine (SAM) is requiivct to transfer the methyl group of methionine to phosphatidylethanolamine (J E) to form PC. In the process, SAM is converted to S-adenosyl homocysteine (SAH). The nitrogen atom of PC has four covalent bonds and is called a quaternary amine. It bears a positive charge that is not influenced by changes in the pH of the suitoimding fluids. The PE methyltran.sferase pathway of I C synthesis occurs only in the liver. 

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