KHADIJAH HANIM ABDUL RAHMANDEPARTMENT OF CHEMICAL

ENGINEERING TECHNOLOGY, UNIMAPSEM1, 2013/2014

CARBOHYDRATE METABOLISM: GLYCOLYSIS

Learning Outcomes

Able to ILLUSTRATE processes involved in cellular respiration comprise of Glycolysis, Citric acid Cycle Electron Transport Chain and photosynthesis.

Introduction

Glycolysis- metabolic pathway that converted a glucose into 2 pyruvate molecules.

Major pathways in Carbohydrate metabolism

• excess glucose- converted into glycogen by glycogenesis• when glucose is needed- glycogen degraded to glucose by glycogenolysis• in some cells, glucose is converted to ribose-5-phosphate and NADPH by pentose phosphate pathway• glucosepyruvate, energy-generating pathway, glycolysis. •In the absence of O2, pyruvate lactate

GLYCOLYSIS

Enzymes and mechanisms that are involved in the pathway are highly homologous in prokaryotes and eukaryotes.

In glycolysis, each glucose (C6H12O6) molecule is split and converted to 2 three-carbon units known as pyruvate (CH3COCOO- + H+).

The small amount of energy captured during glycolytic reactions is stored temporarily in 2 molecules each of ATP and NADH.

In anaerobic organisms (without O2 to generate energy), pyruvate converted to waste products ie. Ethanol, lactic acid, acetic acid and etc.

Using O2 as terminal acceptor, aerobic organisms completely oxidized pyruvate to form CO2 and H2O- process known as aerobic respiration.

Glycolysis, which consists of TEN reactions, occurs in 2 stages:

1) Glucose is phosphorylated twice and cleaved to form 2 molecules of glyceraldehyde-3-phosphate (G-3-P). The 2 ATP molecules consumed during this stage are like an investment- this stage create the actual substrate for oxidation.

2) G-3-P is converted to pyruvate. 4 ATP molecules and 2 NADH are produced. Since 2 ATP molecules are consumed in stage 1, the net ATP molecules produced is 2 molecules.

The glycolytic pathway equation:D-Glucose + 2ADP + 2Pi + 2 NAD+ 2

pyruvate + 2 ATP + 2 NADH + 2H+ + 2H2O

The reactions of the glycolytic pathway

1) Synthesis of glucose-6-phosphate- After entering a cell, glucose and other

sugar molecules are phosphorylated. - Phosphorylation-prevents transport of

glucose out of the cell and increases the reactivity of O2 in the resulting phosphate ester

2) Conversion of glucose-6-phosphate to fructose-6-phosphate

- The aldose glucose-6-phosphate is converted to the ketose fructose-6-phosphate by phosphoglucose isomerase (PGI) in a readily reversible reaction

- This transformation makes C-1 of the fructose available for phosphorylation

3) Phosphorylation of fructose-6-phosphate- Phosphofructokinase-1 (PFK-1) catalyzes the

phosphorylation of fructose-6-phosphate to form fructose-1,6-biphosphate:

- The 2nd molecule of ATP serves as:- Phosphorylating agent- To prevent any later product from diffusing

out of the cell- fructose-1,6-biphosphate splits into 2 trioses.

4) Cleavage of fructose-1,6-biphosphate

- Stage 1 of glycolysis ends by cleaving fructose-1,6-biphosphate into 2 three-carbon molecules: G-3-P and dihydroxyacetone phosphate (DHAP).

- In aldol cleavage: products are aldehyde and a ketone.

5) The interconversion of G-3-P and dihydroxyacetone phosphate.

- Only G-3-P serves as substrate for the subsequent reaction of glycolysis.

- To prevent the loss of the other 3-carbon unit from the glycolytic pathway, triose phosphate isomerase catalyzes the interconversion of DHAP and G-3-P:

6) Oxidation of glyceraldehyde-3-phosphate- G-3-P undergoes oxidation and

phosphorylation.- The product glycerate-1,3-biphosphate

contains a high-energy bond- used in the next reaction to generate ATP:

7) Phosphoryl group transfer- In this reaction, ATP is synthesized as

phosphoglycerate kinase catalyzes the transfer of the high-energy phosphoryl group of glycerate-1,3-biphosphate to ADP:

8) The interconversion of 3-phosphoglycerate and 2-phosphoglycerate

- In the first step of this conversion- phosphoglycerate mutase catalyzes the conversion of C-3 phosphorylated compound to a C-2 phosphorylated compound thru a 2-step addition/elimination cycle.

- Glycerate-3-phosphate has a low phosphoryl group transfer potential.

- It is a poor candidate for further ATP synthesis

- Cells convert glycerate-3-phosphate with its energy-poor phosphate ester to phosphoenolpyruvate (PEP)- has high phosphoryl group transfer potential

9) Dehydration of 2-phosphoglycerate

- Enolase catalyzes the dehydration of glycerate-2-phosphate to form PEP

- PEP has a higher phosphoryl group transfer potential than glycerate-2-phosphate- it contains an enol-phosphate group instead of a simple phosphate ester.

10) Synthesis of pyruvate- In the final reaction of glycolysis, pyruvate

kinase catalyzes the transfer of a phosphoryl group from PEP to ADP

- 2 molecules of ATP are formed for each molecule of glucose.

FATES OF PYRUVATE

Glycolysis- produced 2 molecules of ATP and 2 molecules of NADH per molecule of glucose

Pyruvate is the product of glycolysis that has a high-energy which can produce a substantial amount of ATP. Before this can happen however, an intermediate molecule called acetyl-CoA is formed thru decarboxylation.

Acetyl-CoA- substrate for TCA cycle. TCA cycle- an amphibolic pathway that

completely oxidizes 2 Carbons to CO2 and NADH

In the presence of O2, this cycle is kept going.

The electrons of NADH (and FADH2, another electron carrier) produced in TCA cycle are delivered to O2 via the electron transport system to produce water.

Electron transport system- a linked series of oxidation-reduction reactions that transfer electrons from donor, NADH to acceptor, O2.

Under anaerobic conditions, further oxidation of pyruvate is impeded.

A no of cells and organisms compensate by converting this molecule to a more reduced formed and regenerating NAD+ required for glycolysis to continue.

The regeneration of NAD+ is referred as fermentation. Muscle cells and certain bacteria produce NAD+ by

transforming pyruvate into lactate. In rapidly contracting muscle cells the demand for

energy is high.After the O2 supply is depleted, lactic acid fermentation

provides sufficient NAD+ to allow glycolysis to continue.

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Microorganisms that use lactic acid fermentation to generate energy can be seperated into 2 groups:

- Homolactic fermenters- produce only lactate- Heterolactic/mixed acid fermenters- produce

several organic acids. Eg occurs in rumen of cattle.

- Symbiotic organisms, digest cellulose and synthesize organic acids (lactic, acetic, propionic acids).

- The organic acids are absorbed from the rumen and used as nutrients.

Alcoholic fermentation- in yeast and several bacterial species

In yeast, pyruvate is decarboxylated to form acetaldehyde, which is then reduced by NADH to form ethanol.

Alcoholic fermentation by several yeasts is used commercially to produce beer, wine and bread

Certain bacterial species capable of producing alcohol other than ethanol

Eg Clostridium acetobutylicum- produces butanol.

Butanol- detergents and synthetic fibres.

Fates of Pyruvate