Question

2. The PPP has three main phases, what are the outcomes of those phases?

3. What is Ribose-5-phosphate and why is it important?

4. Name all eight enzymes in the PPP and briefly describe the processes.

please answer all THANK YOU!

Answer 1

2.

The pentose phosphate pathway is the major source for the NADPH required for anabolic processes. There are three distinct phases each of which has a distinct outcome. Depending on the needs of the organism the metabolites of that outcome can be fed into many other pathways. Gluconeogenesis is directly connected to the pentose phosphate pathway. As the need for glucose-6-phosphate increases so does the activity of gluconeogenesis.

Introduction

The main molecule in the body that makes anabolic processes possible is NADPH. Because of the structure of this molecule it readily donates hydrogen ions to metabolites thus reducing them and making them available for energy harvest at a later time. The PPP is the main source of synthesis for NADPH. The pentose phosphate pathway is also responsible for the production of Ribose-5-phosphate which is an important part of nucleic acids. Finally the PPP can also be used to produce glyceraldehyde-3-phosphate which can then be fed into the TCA and ETC cycles allowing for the harvest of energy. Depending on the needs of the cell certain enzymes can be regulated and thus increasing or decreasing the production of desired metabolites. The enzymes reasonable for catalyzing the steps of the PPP are found most abundantly in the liver more specifically in the cytosol. The cytosol is where fatty acid synthesis takes place which is a NADPH dependent process.

Oxidation Phase

• The beginning molecule for the PPP is glucose-6-P which is the second intermediate metabolite in glycolysis. Glucose-6-P is oxidized in the presence of glucose-6-P dehydrogenase and NADP⁺. This step is irreversible and is highly regulated. NADPH and fatty acyl-CoA are strong negative inhibitors to this enzyme. The purpose of this is to decrease production of NADPH when concentrations are high or the synthesis of fatty acids is no longer necessary.
• The metabolic product of this step is gluconolactone which is hydrolytrically unstable. Gluconolactonase causes gluconolactone to undergo a ring opening hydrolysis. The product of this reaction is the more stable sugar acid, 6-phospho-D-gluconate.
• 6-phospho-D-gluconate is oxidized by NADP⁺ in the presence of 6-phosphogluconate dehydrogenase which yields ribulose-5-phosphate.
• The oxidation phase of the PPP is solely responsible for the production of the NADPH to be used in anabolic processes.

Isomerization Phase

• Ribulose-5-phosphate can then be isomerized by phosphopentose isomerase to produce ribose-5-phosphate. Ribose-5-phosphate is one of the main building blocks of nucleic acids and the PPP is the primary source of production of ribose-5-phosphate.
• If production of ribose-5-phosphate exceeds the needs of required ribose-5-phosphate in the organism, then phosphopentose epimerase catalyzes a chiralty rearrangement about the center carbon creating xylulose-5-phosphate.
• The products of these two reactions can then be rearranged to produce many different length carbon chains. These different length carbon chains have a variety of metabolic fates.

Rearrangement Phase

• There are two main classes of enzymes responsible for the rearrangement and synthesis of the different length carbon chain molecules. These are transketolase and transaldolase.
• Transketolase is responsible for the cleaving of a two carbon unit from xylulose-5-P and adding that two carbon unit to ribose-5-P thus resulting in glyceraldehyde-3-P and sedoheptulose-7-P.
• Transketolase is also responsible for the cleaving of a two carbon unit from xylulose-5-P and adding that two carbon unit to erythrose-4-P resulting in glyceraldehyde-3-P and fructose-6-P.
• Transaldolase is responsible for cleaving the three carbon unit from sedoheptulose-7-P and adding that three carbon unit to glyceraldehyde-3-P thus resulting in erythrose-4-P and fructose-6-P.
• The end results of the rearrangement phase is a variety of different length sugars which can be fed into many other metabolic processes. For example, fructose-6-P is a key intermediate of glycolysis as well as glyceraldehyde

The pentose phosphate pathway meets the need of all organisms for a source of nicotinamide adenine dinucleotide phosphate (NADPH) to use in reductive biosynthesis, such as fatty acid, cholesterol, neurotransmitter, and nucleotide biosynthesis, and synthesizes five-carbon sugars.

The pentose phosphate pathway takes place in the cytosol of the cell, the same location as glycolysis. The two most important products from this process are the ribose-5-phosphate sugar used to make DNA and RNA, and the NADPH molecules which help with building other molecules.

The HMP shunt is an alternative pathway to glycolysis and is used to produce ribose-5-phosphate and nicotinamide adenine dinucleotide phosphate (NADPH). This pathway occurs in the oxidative and non-oxidative phases, each comprising a series of reactions.

3  

Ribose-5-phosphate isomerase (Rpi) encoded by the RPIA gene is an enzyme that catalyzes the conversion between ribose-5-phosphate (R5P) and ribulose-5-phosphate (Ru5P). It is a member of a larger class of isomerases which catalyze the interconversion of chemical isomers (in this case structural isomers of pentose).

It has two important products: ribose 5-phosphate, which is needed for synthesis of nucleotides and nucleic acids (DNA and RNA), and NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate), which provides the reducing equivalents for synthetic reactions such as fatty acid biosynthesis.

Pentose phosphate pathway (PPP) plays a critical role in regulating cancer cell growth by supplying cells with not only ribose-5-phosphate but also NADPH for detoxification of intracellular ROS, reductive biosynthesis, and ribose biogenesis.They are composed of a nitrogenous base, a pentose sugar, and at least one phosphate group. Nucleotides contain either a purine or a pyrimidine nitrogenous base. ... During nucleotide biosynthesis, R5P undergoes activation by ribose-phosphate diphosphokinase (PRPS1) to form phosphoribosyl pyrophosphate.

Ribulose-5-phosphate and NADPH are the most significant products of this pathway. As mentioned earlier, NADPH is important as a general reducing agent. The mechanism for this involves glutathione and glutathione reductase.

3.

The enzymes that execute the PPP

• Glucose-6-Phosphate Dehydrogenase. ...
• 6-Phosphogluconolactonase. ...
• 6-Phosphogluconate Dehydrogenase. ...
• Ribulose-5-phosphate isomerase and Ribulose-5-phosphate epimerase. ...
• Transketolase and Transaldolase.
• Lactonase
• xyulose 5-phosphate
• fructose 6-phosphate

PENTOSE PHOSPHATE PATHWAY

Objectives

1. To understand the function of pentose phosphate pathway in production of NADPH and ribose precursors for synthesis of nucleic acid.

2. To examine the importance of NADPH in protection of cell against highly reactive oxygen species.

3. To relate defects in pentose phosphate pathway to disease condition.

Introduction

1. Glucose is catabolised by way of glycolytic pathway in to two molecules of pyruvate , then pyruvate is oxidized by citric acid cycle to produce ATP In animal tissue
2. There is an another metabolic pathway, called as the Pentose Phosphate Pathway (PPP), which is also termed as Hexose Monophosphate Pathway (HMP shunt) -or Phosphogluconate pathway
3. The oxidation of glucose without direct consumptions occur by pentose phosphate pathway
4. Thus, this pathway generate Ribose sugar, ATP, two NADPH.

The PPP can be divided into following phases

 The oxidative (non- reversible) phase

 The non oxidative phase

Phase of Pentose Phosphate Pathway (PPP)

 In the first step, glucose -6-phosphate (G6P) is converted into ribulose-5-phosphate and CO2.During this oxidation reaction, NADP+ is reduced in to NADPH

 In subsequent step of pathway, ribulose 5-phosphate (R5P) converted into other pentose 5-phosphate (P5P) containing ribose 5-phosphates used to produced nucleic acids

 During succeeding step, a sequence of reactions occur which convert three of pentose -5 phosphate into one molecules of triose and two molecules of hexose

 During last step, some of these sugars are converted back into glucose -6-phosphate so the cycle can be repeat again

OXIDATIVE PHASE

1. Enzyme: Glucose -6-phosphate dehydrogenase

 First enzymatic step in oxidative phase reduce NADP+ to NADPH Mechanism: Oxidation reaction of C1 position  Lactone is form by hydride transfer to the NADP+ , which is an intra molecular ester

 Reaction start with glucose 6- phosphate dehydrogenase, which oxidize the aldehyde C1 of glucose 6-phosphate to a carboxylate group

 Glucose aldehyde normally exists a hemicetal pyran ring, in which an aldehyde has combined with O-5

 The caboxylate anion derive from glucose is gluconate,and cyclic ester of this type are called lactones, hence the product name is 6-phospho glucono lactone Energetic: Very favourable reaction in the forward direction for NADPH production.

. Enzyme: Lactonase

 It is a specific enzyme that target 6- phosphoglucono-δ-lactone for hydrolysis Mechanism

 The lactoneis openedby hydrolysis, the additional water molecules to break a bond usually a kind of amide or ester

 In this case, since lactone is intra molecular, then 6- phospho-glucono-δ-lactone is opened up to the acid form, gluconate Energetic: Thistype of hydrolysis reactionsare favourable.

Enzyme: 6-Phospho gluconate dehydrogenase

 It carryout the conversion of 6 carbon skeleton to a pentose with oxidative decarboxylation Mechanism: The C1 carboxylate is removed. The C3 position is oxidized to a ketone, creating 5 carbon ketose, ribulose 4. Enzyme: Phosphopentoseisomarase Mechanism: Typical ketose to aldose conversion

 This is a acid base catalysed reaction

 Ribulose 5-phosphate has noimmediateuse in the cell, so is rearranged in to other form

 Isomerization of ribose -5 phosphate,which can be used for RNA ,DNA and nucleotide

NON OXIDATIVE PHASE

Conversion of pentose phosphate to glucose -6 phosphate

 In cell that require high level of NADPH for biosynthetic reaction, the ribulose -5P produced in the oxidative phase need to be a turn back in to a glucose -6 phosphate to maintain flux through the glucose -6P dehydrogenase reaction

 The carbon shuffle reaction of non-oxidative phase which ultimately are used to regenerate glucose -6P using transketolase and trans aldolase enzyme.

. Enzyme: Ribulose -5 phosphate epimerase

Mechanism: The turn of stereo centre of the substrate in the epimerization reaction. When only one stereo centre is flipped an epimer of compound occur.

Enzyme: Trans ketolase and trans aldolase

 This step isessential to change pentose back to glucose ,but beside to epimarase, two enzymes namely

(i) Trans ketolase and

(ii) Trans aldolase are needed

REGULATION OF PPP

 The initial step of pentose phosphate pathaway is irreversible commited step

 This step is catalyzed by the enzyme glucose -6 phosphate dehydrogenase and regulated allostrerically

 Product of this reaction is NADPH which is a strong inhibitor of enzyme glucose 6- phosphate dehydrogense, so, when the concentration of NADPH is high, the enzyme glucose 6- phosphate dehydrogense activity is low. As, NADPH is used in different pathway, inhibition is relived,and the enzyme is incresed to produce more NADPH. This reaction is also allosterically regulated

 The transcription of the gene for this enzyme is regulated by hormon

Importance of PPP

 PPP is the main generator of NADPH

 NADPH is needed for reductive biosynthesis of fatty acid, cholesterol, steroid hormon & spingoshine. Thus it is active in lactating mammary gland, liver, gonads adipose tissue & adrenal cortex

 NADPH is used in hydroxylation reaction during metabolisms of phenylalanine and tryptophan

 NADPH is used for the synthesis of Nitric Oxide (NO): Arginine + O2 + NADPH +H+ → NADP + NO + Citruline

 NO is a laughing gas, used as ansthetic, causes relaxation of vascular smooth muscles

 In macrophages, NO is effective against viral, fungal, protozoal infections.

 NO potent inhibitors of platelet aggeregation

 NO act as a neurotransmitter in brain

 PPP provides a way for oxidation of glucose by other then TCA cycle with no production energy

 PPP provides Ribose -5 P to the cells needed for nucleoside, nucleotides, nucleic acid & coenzyme biosynthesis