Pentose+Pathway

Pentose Phosphate Pathway by: Curtis Converse and Kyle Smith =Introduction:=

toc The pentose phosphate pathway (also known as the hexose monophosphate shunt or the phosphogluconate pathway)is a metabolic pathway that serves several purposes in plants, animals, and some microbes. It allows for the metabolism of 5-Carbon sugars (such as ribose), provides reduced NADPH to be used as energy carriers for other cellular reactions, and produces the ribose-5-phosphate that is required for ribonucelotide synthesis. Plants and microbes also use this pathway to generate erythrose-4-phosephate for the synthesis of aromatic acids. The pentose phosphate pathway is both anabolic and catabolic, producing necessary compounds for later use in various processes but also producing high-energy NADPH. Based on the energetic requirements of the cell, glucose 6-phosphate can either enter glycolysis (if cellular ATP is low) or the pentose phosphate pathway (if cellular ATP is high). The enzymes for the first step of both reactions (glycolysis and the pentose phosphate pathway) are highly regulated to ensure that the energy and substrate requirements of the cell are met. The pentose phosphate pathway takes place mainly in liver and adipose cells but not muscle cells, where glucose 6-phosphate is used for ATP production, not anabolism. The pathway proceeds two stages. Stage 1, also known as the "oxidative stage" consists of three steps in which the substrate is oxidized and NADP+ is reduced. The second stage, or "nonoxidative stage," consists of four steps in which C-C bonds are broken and rearranged. The overall reaction is:

3 Glucose 6-phosphate + 6 NADP+ +3 H2O ––> 2 Fructose-6-phosphate + Glyceraldehyde-3-phosphate + 3 CO2 + 6 NADPH + 6 H+

= **Pathway:** =

[[image:http://www.uic.edu/classes/phar/phar332/Clinical_Cases/vitamin%20cases/thiamin/ppp.gif caption="Reaction Scheme of the Pentose Phosphate Pathway. (5)"]] Step 1:
β Glucose 6-phosphate is oxidized at the anomeric carbon to the corresponding lactone, 6-phosphogluconolactone, by glucose 6-phosphate dehydrogenase (E.C. 1.1.1.49). (1) The removed α hydrogen is transferred to NADP+ and the hydroxyl hydrogen to a His residue in the enzyme. glucose 6-phosphate dehydrogenase is inhibited by NADPH and fatty acid esters of coenzyme A. The NADP+/NADPH ratio is significantly lower in the liver than in other tissues, indicating the high level of control of this pathway. (4)

Step 2:
6-phosphogluconolactone from step 1 is hydrolyzed to the open chain 6-phosphogluconate by nucelophilic addition of water to the acyl carbon. The reaction is catalyized by gluconolactonase (E.C. 3.1.1.17). (2) This reaction occurs spontaneously in aqueous solution but is accelerated by the enzyme. (4)

Step 3:
6-phosphogluconate from step 2 is decarboxylated in a three-step mechanism to ribose 5-phosphate. First, the 6-phosphogluconate is oxidized at the C3 hyroxyl group to make an β-keto acid and reduced NADPH. Secondly, The C1 carbon is removed in a decarboxylation, releasing CO2. The resulting endiol is tautomerized to rubulose 5-phosphate in the third step of the mechanism, ending the oxidative phase of the pathway. All three of the reactions in the mechanism are catalyzed by phosphogluconate dehydrogenase (E.C. 1.1.1.43). (3)

Step 4:
From step 3, there are 3 molecules of Ribulose-5-phosphate produced; Ribulose-5-phosphate is isomerized and epimerized to 2 molecules of ribose-5-phosphate and one molecule of Xylulose-5-phosphate, respectively. Both of these reactions undergo an enediol intermediate to proceed to their products and both reactions are reversible. In different numbering schemes, step 4 is divided into two separate steps, 4 and 5, but in this numbering scheme, the epimerization and isomerization are both considered to be part of step. There is no order as to when Ribulose-5-phosphate is isomerized or epimerized but we will divide the step 4 in step 4a and step 4b to differentiate between the isomerization and epimerization. (4,7)

Step 4a:
The carbonyl group of Ribulose-5-phosphate is reduced by a proton from Asp-38 after a glumate removes an alpha hydrogen on the C1 carbon; a 1,2-Enediol is produced. From here, the hydroxyl group on the C1 carbon of the enediol is oxidized and the 2 electrons from the diol are transferred to a proton on a glutamate molecule. This isomerization changes switches the placement of the C1 hydroxyl group and C2 carbonyl to a C1 carbonyl group and C2 hydroxyl group. (7)

Step 4b:
An epimerase, phosphopentose epimerase (E.C. 5.1.3.22) (6), is responsible for catalyzing a keto-enol tautomerization that leads to one of ribose-5-phosphate’s epimers, Xylulose-5-phosphate. Instead of the 1,2-Enediol intermediate in Step 4a, a 2,3-Enediolate is used because the alpha hydrogen on a different carbon is used, C3, (instead of the alpha carbon on C1 used in Step 4a). A polar bond is formed between zinc and the oxygen at the C2 carbon, which allows the C2 and C3 carbons to be oxidized; this double bond between the C2 and C3 carbons is produced after the alpha hydrogen is accepted by Asp-38 mentioned above. After the 2,3-Enediolate is formed, electrons flow back to form a carbonyl on the C2 carbon from the polar bond with zinc. The double-bond is broken when a proton is transferred from Asp-178 and Xylulose is formed. (7)

Step 5:
A two carbon unit is transferred from Xylulose-5-phosphate by a thiamin diphosphate-dependent transketolase (E.C. 2.2.1.1) (8) to a Ribose-5-phosphate, forming Sedoheptulose-7-phosphate. Thiamin diphosphate (TPP) back-attacks the C2 carbonyl carbon of Xylulose and the carbonyl oxygen is reduced by a proton from an acid. From here, the bonded TPP leaves with the C1 and C2 carbon groups of the formerly Xylulose-5-phosphate, forming glyceraldehyde-3-phosphate. For the next part of this step, the carbonyl C1 carbon of Ribose-5-phosphate is reduced by an acid after a nucleophilic addition from the enamine from the previous part of this step. The enamine is cleaved and the C2 carbon of the new molecule is oxidized by histidine removing its proton, forming Sedoheptulose-7-phosphate. (7)

Step 6:
In this step, Sedoheptulose-7-phosphate loses a 3-carbon unit in a transaldolase (E.C. 2.2.1.2) (9) reaction to glyceraldehyde-3-phosphate to form Fructose-6-phosphate, leaving Erythrose-4-phosphate. A lone pair of electrons from the nitrogen of a lysine residue on the transaldolase initiate a nucleophilic addition to the C2 carbon of Sedoheptulose-7-phosphate, replacing the carbonyl oxygen, which leaves as water. C1-3 (double bonded to Lys at C2) break from the rest of the molecule when a base accepts a proton from the C4 hydroxyl group, which donates its electrons to the C-O bond, leaving Erythrose-4-phosphate. More electrons flow in from the amino group of the enzyme and the C2-3 double bond is broken by lending a pair of electrons to the C1 carbon of Glyceraldehyde-3-phosphate; the carbonyl group of Glyceraldehyde-3-phosphate was protonated by an acid, leaving an iminium ion. Hydrolysis of a water molecule protonates the enzyme and the C2 carbon of iminium ion is oxidized to a carbonyl afterwards, forming Fructose-6-phosphate. (4, 7)

Step 7:
Like in step 5, Xylulose-5-phosphate gives 2 carbon groups to another molecule by a transketolase (E.C. 2.2.1.1) (8) with a similar mechanism to the other transketolase used in this pathway; in this case, the molecule is Erythrose-4-phosphate. The Xylulose molecule in this step is one of the Xylulose molecules produced in step 4. Like in step 5, a Glyceraldehyde-3-phosphate is produced; a Fructose-6-phosphate is also formed. (7)

=Regulation of the Pentose Phosphate Pathway:=

The Pentose Phosphate Pathway is regulated by the number of its molecules present in the cytosol that can go into other pathways and by regulated enzymes in its pathway. The cell requires the pentose sugars for biosynthesis of nucleotides as well as NADPH as an energy source. However, if there is an abundance of NADPH and a relatively lower ratio of ATP/AMP, Glucose-6-phosphate would proceed toward glycolysis to make more ATP. If the ATP/AMP were high, Glucose-6-phosphate dehydrogenase would be inhibited, which would shift the Glucose-6-phosphate toward the pentose phosphate pathway. The controls of other pathways are relevant for considering the activation of the Pentose Phosphate Pathway because if other pathways are inhibited, then, as shown above, there must be favorable factors like a lack of ribose-5-phosphate or NADPH or an abundance of ATP or glucose to stimulate the pentose phosphate pathway, of which products can continue on into glycolysis. (4, 7) There are molecules in the Pentose Phosphate Pathway that feed into other cycles like Glyceraldehyde-3-phosphate or fructose-6-phosphate into glycolysis or gluconeogenesis. Based on the cell’s need for intermediates or energy or inhibitors and stimulators, various cycles will be initiated and discontinued. One of the products of the last half of the pentose phosphate pathway, Xylulose-5-phosphate, also plays a role in regulation of glucose in the blood, stimulating enzymes that enable rapid glycolysis and inhibit gluconeogenesis. (4)

=References:=

1) "EC 1.1.1.49." //IUBMB Enzyme Nomenclature//. 09 Dec. 2005. Web. 11 Dec. 2010. .

2) "EC 3.2.2.27." //IUBMB Enzyme Nomenclature//. 09 Dec. 2005. Web. 11 Dec. 2010. 

3) "EC 1.1.1.43." //IUBMB Enzyme Nomenclature//. 09 Dec. 2005. Web. 11 Dec. 2010. 

4) Garrett, R., and Charles M. Grisham. //Biochemistry//. Fort Worth: Harcourt College, 2010. Print. 684-693.

5) // PPP // . Digital image. //Thiamin//. University of Illinois at Chicago, 31 Dec. 2002. Web. 12 Dec. 2010. .

6) "EC 5.1.3.22." //IUBMB Enzyme Nomenclature//. 09 Dec. 2005. Web. 11 Dec. 2010. 

7) McMurry, John, and Tadhg P. Begley. //The Organic Chemistry of Biological Pathways//. Englewood: Roberts and, 2005. Print.203-211.

8) "EC 2.2.1.1." //IUBMB Enzyme Nomenclature//. 09 Dec. 2005. Web. 11 Dec. 2010. 

9) "EC 2.2.1.2." //IUBMB Enzyme Nomenclature//. 09 Dec. 2005. Web. 11 Dec. 2010. http://www.chem.qmul.ac.uk/iubmb/enzyme/EC2/2/1/2.html>