Fructose-l,6-bisphosphate

FIGURE 19.13 (a) A mechanism for the fructose-l,6-bisphosphate aldolase reaction. The Schlff base formed between the substrate carbonyl and an active-site lysine acts as an electron sink, Increasing the acidity of the /3-hydroxyl group and facilitating cleavage as shown. (B) In class II aldolases, an active-site Zn stabilizes the enolate Intermediate, leading to polarization of the substrate carbonyl group. 

Cleavage of fructose-l,6-bisphosphate, an aldolase-catalyzed reaction. The aldolase reaction entails a reversal of the familiar aldol condensation. The first step involves abstraction of the hydrogen of the C-4 hydroxyl group, followed by elimination of an enolate anion. 

Glucose (Glc) is taken up and phosphorylated into glucose-6-phosphate (Glc6P), with consumption of ATP. Isomerization and phosphorylation afford fructose-l,6-bisphosphate (Frul,6P2), which is cleaved into two triose molecules D-glyceraldehyde-3-phosphate (GA3P) and dihydroxyacetone monophosphate (DHAP). These are equilibrated by triose phosphate isomerase as only GA3P is metabolized further, except approximately 5 mol% of DHAP that leaks out of the pathway via reduction to glycerol, which is excreted as a side-product. 

Pructoae 6-phosphate is then converted to fructose l.6-bisphosphate by phosphofmctokmose-catolyzed reaction witli ATP (the prefix bis" means two). The result is a moleciLlv ready to be split into the two three-carbon intermediates that will ultimately become two molecules of pyruvate. 

Connecting peptide of insulin Cytosine triphosphate Extracellular fluid Essential fatty acid Endoplasmic reticulum Fructose-l,6-bisphosphate Flavin adenine dinucleotide Free fatty acid Formiminoglutamic acid Glucose transporter gene or protein... 

Phosphatases specific for such substrates as glucose-6-phosphate, fructose-l,6-bisphosphate, and phospho-glycolate help to drive metabolic cycles (Chapter 17). The 335-residue fructose-1,6-bisphosphatase associates to form a tetramer with D2 symmetry. ° The allosteric enzyme exists in two conformational states (see Chapter 11). Activity is dependent upon Mg + or other suitable divalent cation, e.g., Mn or Zn, and is further enhanced by K+ or NH3. While the dimetal sites depicted in Figs. 12-23 and 12-24 are quite rigid and undergo little change upon formation of complexes with substrates or products, the active site of fructose-1,6-bisphosphatase is more flexible. There are three metal-binding sites but they contain no histidine side chains and have been seen clearly only in a product complex. Perhaps because of the need for... 

Closely related to aldolases is transaldolase, an important enzyme in the pentose phosphate pathways of sugar metabolism and in photosynthesis. The mechanism of the transaldolase reaction (Eq. 17-15) is similar to that used by fructose-l,6-bisphosphate aldolase with a lysine side chain forming a Schiff base and catalytic aspartate and glutamate side chains. ... 

Aldol condenBation. Dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, the two three-carbon units produced in step 7, join in step 8 to give fructose l,6-bisphosphate. The reaction looks like an aldol condensation (Section 23.2) between the enolate ion from dihydroxyacetone phosphate and the carbonyl group of glyceraldehyde 3-phosphate. 

D-Fructose-l,6-bisphosphate d- glycer aldehyde-3 -phosphate-lyase O .. Aldolase ALD... 

Skeletal muscle and yeast phosphofructokinases will catalyse the phosphorylation of 5-keto-D-fructose-l,6-bisphosphate (9). The latter has been isolated chromatographically and identified by its phosphorus content and the rather doubtful method of acid lability of the phosphate groups. The bis-phosphate is a competitive inhibitor of the reaction between aldolases and fructose-1,6-bisphosphate probably because of Schiff base formation with the enzyme. ... 

The speed and specificity of this reaction are ensured by the enzyme aldolase. The product is the sugar fructose-l,6-bisphosphate, which is converted by another enzyme into glucose-l,6-bisphosphate. Removal of the two phosphoryl groups results in a new molecule of glucose for use by the body as an energy source. 

A second energy "investment" is catalyzed by the enzyme phosphofructokimse. The phosphoanhydride bond in ATP is hydrolyzed, and a phosphoester linkage between the phosphoryl group and the C-1 hydroxyl group of fructose-6-phosphate is formed. The product is fructose-l,6-bisphosphate. 

Fructose-l,6-bisphosphate is split into two three-carbon intermediates in a reaction catalyzed by the enzyme aldolase. The products are glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate. 

In aldol condensation, aldehydes and ketones react to form a larger molecule (Section 14.4). This reaction is a reverse aldol condensation. The large ketone sugar fructose-l,6-bisphosphate is broken down into dihydroxyacetone phosphate (a ketone) and gIyceraldehyde-3-phosphate (an aldehyde). 

The last enzyme in glycolysis, pyruvate kinase, is also subject to allosteric regulation. In this case, fructose-l,6-bisphosphate is the allosteric activator. It is interesting that fructose-l,6-bisphosphate is the product of the reaction catalyzed by phosphofructokinase. Thus, activation of phosphofructokinase results in the activation of pyruvate kinase. This is an example of feedforward activation because the product of an earlier reaction causes activation of an enzyme later in the pathway... 

D-fructose-6-phosphate + ATP <=> D-fructose-l,6-bisphosphate Enzyme Phosphofructokinase... 

D-fructose-l,6-bisphosphate <=> Dihydroxyacetone phosphate + D- Enzyme Fructose-1,6-... 

Glyceralciehyde-3-plnwphate + dihydrQ.cyacetoiiephr>sphate fructose-l,d-bisphosphate Fructose-l,6-bisphosphate + H O ----- tructose-6-phosphate + Pj... 

Water-in-oil (W/O) gel emulsions has been applied for the first time in a-chymotrypsin-catalyzed peptide synthesis (Clapds et al. 2001) and in aldolic condensation of DHAP with acceptor aldehydes such as phenylacetaldehyde and ben-zyloxyacetaldehyde, catalyzed by D-fructose-l,6-bisphosphate aldolase from rabbit muscle (RAMA). Gel emulsions of the ternary systems such as water/Ci4E4/oil, where C14E4 is a technical grade poly(oxyethylene) tetradecyl ether surfactant, with an average of four moles of ethylene oxide per surfactant molecule and oil can be... 

These systems were tested in the enzymatic aldolization of a variety of A/-Cbz-aminoaldehydes catalyzed by D-fructose-l,6-bisphosphate aldolase from rabbit muscle (RAMA) and L-rhamnulose-1-phosphate aldolase and L-fuculose-1-phosphate aldolase from E. coli (Espelt et al. 2003 a,b, 2005). The largest differences between conventional DMF/water cosolvent systems and gel emulsions were observed with RAMA catalyst (Fig. 6.5.11). 

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