Enzymes of the Glycolytic Pathway

Enzymes of the Glycolytic Pathway.—Several of the studies have led to low-resolution electron-density maps and should give more detailed results in the very near future. These studies have been reviewed by Campbell et al and are summarized in Table 2. Certain of the studies demand more detailed discussion. 

A preliminary study of the Patterson projections of hexokinase-B suggests that the molecules are identical and there is a molecular two-fold screw axis parallel to the crystallographic c axi involving translation of 3.6 A along this local axis. 

Crystallographic studies of both yeast and horse-musde phosphoi ycerate kinase (PGK) are progressing well. Watson et alJ have produced a good mercuric acetate derivative of the yeast enzyme and have recently calculated a 6 A electron-density map. Blake et have computed a 6 A electron-density map of the horse-muscle enzyme which shows that the long polypeptide chain of about 420 residues is folded into two distinct globular units, which are clearly different in chain conformation. There is no sug tion that the molecule is composed of two polypeptide chains. A single active site has been located on one of the subunits by calculating difference electron-density maps on the isomorphous binary complex PGK-Mg-ATP. 

4 A resolution crystallographic study. The crystals are isotnorphous with the apoenzyme and have one binding site for dye per monomer of the enzyme. The position of the dye is coincident with the adenosine portion of bound NAD+. 

The structure at 5 A resolution of horse liver alcohol ddiydrogenase is described by Branden et a/. A two-fold crystallographic axis relates two identical subunits of the dimeric enzyme. The subunit is about 45 x 55 x 110 A. The binding site of one zinc atom per subunit is clearly identified in a cleft close to the surface of the enzyme. The position of the second zinc atom is not evident. The active site cleft is clear but lies about 30 A from the position of the zinc atom found from the electron-density map, and so this zinc atom is probably not involved in biological activity. 

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The resting adult human brain consumes around 80 mg of glucose and 50 ml of 02 per minute, and once the glucose has been transported across the plasma membrane it is rapidly phosphorylated by hexokinase, the first enzyme of the glycolytic pathway. Hexokinase... 

As we look at some of the reactions of intermediary metabolism, we shall rationahze them in terms of the chemistry that is taking place. In general, we shall not consider here the involvement of the enzyme itself, the binding of substrates to the enzyme, or the role played by the enzyme s amino acid side-chains. In Chapter 13 we looked at specific examples where we know just how an enzyme is able to catalyse a reaction. Examples such as aldolase and those phosphate isomerase, enzymes of the glycolytic pathway, and citrate synthase from the Krebs cycle were considered in some detail. It may... 

Beginning with Eduard Buchner s discovery (c. 1900) that an extract of broken yeast cells could convert glucose to ethanol and C02, a major thrust of biochemical research was to deduce the steps by which this transformation occurred and to purify and characterize the enzymes that catalyzed each step. By the middle of the twentieth century, all ten enzymes of the glycolytic pathway had been purified and characterized. In the next 50 years much was learned about the regulation of these enzymes by intracellular and extracellular signals, through the kinds of allosteric and covalent mechanisms we have described in this chapter. The conventional wisdom was that 1860 1917 in a linear pathway such as... 

Roth EF Jr, Calvin MC, Max-Audit I et al (1988) The enzymes of the glycolytic pathway in erythrocytes infected with Plasmodium falciparum malaria parasites. Blood 72(6) 1922-1925... 

The results obtained by affinity chromatography gave good evidence but are not proof of a direct involvement of these proteins in the Lefiunomide mode of action. H owever, it was very interesting that four enzymes of the glycolytic pathway as well as MDH could be eluted specifically. 

The use of aldolases and transketolase has opened the way to many highly multifunctional organic compounds [1]. In organic synthesis, the most widely used dihydroxyacetonephosphate (DHAP) aldolase is the commercially available fruc-tose-1,6-bisphosphate aldolase from rabbit muscle (FruA). This enzyme is a key enzyme of the glycolytic pathway, reversibly catalyzing the cleavage of fructose-... 

Potassium ions are required by a variety of enzymes (Suelter, 1970), These enzymes are activated by K to a greater extent than by Na. Pyru ate kiirase, an enzyme of the glycolytic pathway, is the most well known K-requiring enzyme. It is activated by a variety of monovalent cations, as indicated in Table 10.7. Ammonium ions at 100 mM support catalytic activity however, the concentration of ammonium ions in the cell is under 1.0 mAI. Sodium at 100 mA/f weakly supports activity. The concentration of Na in the ccH is only about 10 mM, indicating that the importance of Na in supporting enzyme activity m uiuo is nil. Rubidium, which is chemically similar to K, supports activity however, Rb is not a physiological cation. 

Agents that inhibit one or more enzymes of the glycolytic pathway could act to deprive trypanosomes of energy and thus be useful for treating sleeping sickness. A difficulty is that glycolysis in the host cells also would be inhibited. 

The sustained generation of energy from fat requires large supply of citric acid cycle intermediates. Recall that oxaloac-etate is derived from glucose via the enzymes of the glycolytic pathway and pyruvate carboxylase. 

How do the names of the first three enzymes of the glycolytic pathway relate to the reactions they catalyze ... 

To synthesize glucose, each of these steps must be bypassed by different reactions, and different enzymes. All other steps in the synthesis utilize the same reactions and enzymes of the glycolytic pathway. 

Stehmann, C., Grant, B.R., Inhibition of enzymes of the glycolytic pathway and hexose monophosphate bypass by phosphonate. Pestic. Biochem. Physiol. (2000) 67, 13-24. 

Within the free space of the cytoplasm reside all those intracellular enzymes which are not located in organelles. For example, the enzymes of the glycolytic pathway, the fatty acid synthase complex, the protein synthesizing system and all other enzymes involved in the metabolism of the yeast cell. Additionally, metabolic intermediates and storage compounds such as trehalose and starch reside in the cytoplasm. 

The fructose 1,6-diphosphate is then split into two molecules of triosephosphate. This reaction is catalyzed by aldolase, a key enzyme of the glycolytic pathway. Homofermentative bacteria present a high fructose 1,6-diphosphate aldolase activity. The products of this reaction are glycer-aldehyde 3-P and dihydroxyacetone-P. 

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