Energy Yielding Processes

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Veronica Summers
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1 Energy Yielding Processes

2 Oxida3on- Reduc3on Reac3on This is a key processes in energy metabolism. The synthesis of ATP from ADP and Pi involves the transfer of energy from energy- yielding compounds (carbohydrate, fat, protein, and alcohol). This process uses oxida3on- reduc3on reac3ons, in which electrons (along with hydrogen ions) are transferred in a series of reac3ons from energy- yielding compounds eventually to oxygen. These reac3ons form water and release much energy, which can be used to produce ATP.

3 A substance is oxidized when it loses 1 or more electrons. For example, copper is oxidized when it loses an electron: A substance is reduced when it gains 1 or more electrons. For example, iron is reduced when it gains an electron: The movement of electrons governs oxida3on- reduc3on processes. If 1 substance loses electrons (is oxidized), another substance must gain electrons (is reduced). Thus these processes go together one cannot occur without the other.

4 The chemical reac3ons involved in flow of energy for various ac3vi3es collec3vely form the process known as cellular metabolism. Cellular metabolism consists of a complex network of chemical reac3ons that capture energy and raw materials from the environment and allows them to be changed into forms that are used to sustain cells. During this cellular metabolism, ATP and proton mo3ve force (PMF) is being generated and u3lized, thus the focus of the metabolism. Heterotrophic or chemoorganotrophic metabolism requires supply of preformed energy rich organic substances for produc3on of cellular cons3tuents and as a source of the chemical energy used to generate ATP. This reac3on essen3ally involves the oxida3on of energy rich reduced organic molecules to oxidized end products via a metabolic pathway that releases sufficient energy to be coupled to the transforma3on of ATP. Thus it means an external molecule in highly oxidized state is required to act as the final electron acceptor whose reduc3on balances the oxida3on of the ini3al organic substrate molecule

5 There are three types of energy genera3on metabolic pathways depending on the nature of electron acceptors. Aerobic respira3on Anaerobic respira3on Fermenta3on

6 Fueling Processes Respira3on is a process involving exogenous (external) acceptors in the electron transport chain and can be aerobic or anerobic respira3on depending on the acceptor. When the final electron acceptor Final electron acceptor is oxygen, the process is termed aerobic respira3on Final electron acceptor is not oxygen, the process is termed anaerobic respira3on The electron transport system uses the donated electron to make ATP via oxida3ve phosphoryla3on When the electron transport system is not used but endogenous (internal) acceptors are employed, the process is termed fermenta3on and ATP is formed via substrate level phosphoryla3on

7 The two major types of glucose catabolism are: Carbohydrate Catabolism Respiration, in which glucose is completely broken down To CO 2 and H 2 O - aerobic respiration To NO 2, N 2, H 2 S, CH 4 and H 2 O anaerobic respiration Fermentation, in which glucose is partially broken down (organic molecule)

8 Aerobic Respira6on Aerobic respira3on pathway begins with an organic substrate molecule and combines with oxygen in an oxida3on- reduc3on process then end with the forma3on of CO 2 and H 2 O in addi3on to substan3al amount of ATP generated. It is a universal energy genera3ng process in majority of prokaryotes and almost all eukaryotes. Bacteria exhibit a varia3on in aerobic respira3on as they have ability to process and channel a wide range of organic substrates into central metabolic pathway to produce energy and precursors intermediates..

9 Aerobic Respira6on Aerobic catabolism can be divided into three different stages: Stage I Larger molecules are broken down into their cons3tuent parts with lizle energy released Stage II Can either be an aerobic or anaerobic process whereby even simpler molecules are produced as is ATP as well as NADH and/or FADH 2 Stage III Complete oxida3on of molecules under aerobic condi3ons to form CO 2 as well ATP, NADH, and FADH 2 (the lazer two molecules generate even more ATP through the electron transport system Some of the pathways in metabolism are amphibolic, i.e., they func3on both catabolically and anabolically.

10 The Pathways of Catabolism Converge to a Few End Products Figure 17.7 The three stages of catabolism. Stage 1: Proteins, polysaccharides, and lipids are broken down into their component building blocks. Stage 2: The building blocks are degraded into the common product, the acetyl groups of acetyl-coa. Stage 3: Catabolism converges to three principal end products: water, carbon dioxide, and ammonia.

11 GLYCOLYSIS (glyco = sugar or sweet; Lysis = breakdown) This is the first phase of aerobic respira3on during which organic molecule is par3ally oxidized to smaller molecules usually with the genera3on of some ATP and reduced co enzymes. In the case of carbohydrates, smaller molecules such as glucose is ini3ally broken down to pyruvate.

12 Microorganisms breakdown sugars to pyruvate and similar intermediates by 3 routes Embden- Meyerhof- Parnas pathway Pentose phosphate pathway/hexose monophosphate shunt Entner- Doudoroff pathway

13 Embden- Meyerhof- Parnas Pathway (EMP pathway). This pathway is usually referred to glycolysis, glycoly3c pathway or hexose biphosphate pathway. It is the most common pathway for glucose degrada3on to pyruvate. It is common in all major groups of microorganisms except in archaebacteria and func3ons in the presence or absence of oxygen. In this pathway glucose is first converted in a series of reac3ons to fructose- 6- phosphate which is in turn cleaved to two interconvertable 3- carbon sugars that enter a common set of catabolic process to form 2 pyruvates. The 3 carbon molecules formed are glyceraldehyde- 3- phosphate and dihydroxyacetone phosphate (DHAP) by the ac3on of aldolase. DHAP though not directly involved in glycoly3c pathway can be converted to glyceraldehyde- 3- phosphate. Aber the forma3on of glyceraldehyde- 3- phosphate, it is followed by usage of energy stored in this compound to drive the synthesis of ATP. The overall reac3on for glycolysis by EMP pathway is as follows Glucose + 2ADP+2Pi+2NAD + 2 pyruvate +2NADH 2 +2H + + 2ATP The oxida3on of glucose by this pathway releases ATP and also forma3on of two reduced coenzyme (NADH 2 ) molecules. Glycoly3c pathway does not only oxidize carbohydrates to pyruvate and to phosphorylated ADP, it also provides precursor metabolites for many other pathways. Glucose- 6- phosphate is a precursor to polysaccharides, pentose phosphates and aroma3c amino acids; Fructose 6 phosphate is a precursor to amino sugars; DHAP is a precursor to phospholipids as well as amino acids such as glycine, serine and cysteine. Phospho enol pyruvate (PEP) is a also a precursor aroma3c amino acids and to the lactyl por3on of muramic acid

15 Glycoly3c pathway

16 Overview of glycolysis Six carbon stage This stage involves the stepwise phosphoryla3on of glucose (6 carbon [C6] molecule) twice and converted to fructose 1,6- bisphosphate [C6]. This process require 2molecules of ATP and does not generate any form of energy. Three carbon stage This is the catabolism of fructose 1,6- bisphosphate [C6] formed in the previous stage to two molecules of pyruvate [C3]. For each pyruvate generated one molecule of NADH and two molecules of ATP by substrate- level phosphoryla3on Substrate- level phosphoryla3on is the synthesis of ATP by coupling ADP phosphoryla3on with an exergonic reac3on Overall in the glycolysis pathway, 4 molecules of ATP is generated however since two molecules of ATP is used up in the phosphoryla3on process the overall ATP generated is therefore 2 molecules. In addi3on 2 molecules of NADH is also generated.

17 Catabolism Metabolic Pathways 1 +ATP 2 +ATP 3 +ATP

18 Pentose Phosphate (PP) Pathway (Hexose monophosphate pathway [HMP]). This pathway may be used simultaneously with glycoly3c and Entner- Doudoroff pathway. It operates under both aerobic or anaerobic condi3ons but does not require oxygen to take place HMP is important in both catabolism and biosynthesis. The pathway begins with oxida3on of glucose- 6- phosphate to 6- phosphogluconate. It is followed by the oxida3on of the 6- phosphate to release CO 2. A pentose sugar, ribulose 5- phosphate formed is converted to a mixture of 3 to 7 carbon sugar phosphates. In this pathway, NADP is the electron acceptor to yield NADPH which serves as a source of electrons for reduc3on of molecules during biosynthesis.

ü It is an alternate route for the oxidation of glucose without

19 Overview ü The pentose phosphate pathway is also called Hexose Monophosphate Shunt or Phosphogluconate Pathway. ü It is an alternate route for the oxidation of glucose without direct consumption or generation of ATP. ü It takes place entirely in the cytoplasm.

20 The pentose pathway is a shunt. The pathway begins with the glycoly3c intermediate glucose 6- P. It reconnects with glycolysis because two of the end products of the pentose pathway are glyceraldehyde 3- P and fructose 6- P; two intermediates further down in the glycoly3c pathway. It is for this reason that the pentose pathway is oben referred to as a shunt.

21 It s a shunt

22 Overall the PP pathway converts 3 glucose- 6- phosphates to two fructose- 6- phosphates, 3 CO 2 molecules and 6 molecules of NADPH. The intermediates produced are used in two ways: i) Fructose- 6- phosphate can be changed back to glucose- 6- phosphate while glyceraldehyde- 3- phosphate is converted to pyruvate by glycoly3c enzymes ii) The glyceraldehyde- 3- phophate can be returned to the PP pathway through glucose- 6- phosphate forma3on. This results in the complete degrada3on of glucose- 6- phosphate to CO 2 and the produc3on of NADPH.

23 Importance of PP Pathway PP pathway has several catabolic and anabolic func3ons viz i) The pathway may be used to generate ATP. ii) The intermediates five carbon sugars produced can be used to synthesis of pentoses and hexoses. iii) NADPH produced serves as source of electrons for the reduc3on of molecules during biosynthesis. iv) The four and five carbon sugars serve as precursors for various biosynthe3c pathways. Erythrose 4- phosphate (4C) is used to synthesize aroma3c amino acids. Also ribose 5- phosphate is a major component of nucleic acid. Ribulose 5- phosphate is a primary acceptor in photosynthesis.

24 Importance of pentose phosphate pathway : ü Generation of NADPH - mainly used for reductive synthesis of fatty acids, cholesterol and steroid hormones. - hydroxylation reaction in metabolism of phenylalanine and tryptophan. - production of reduced glutathione in erythrocytes and other cells. ü Production of ribose residues - used for nucleotide, nucleic acid, and coenzyme biosynthesis ü Serves as an entry into Glycolysis for both 5- carbon & 6- carbon sugars.

The PPP is divided into two phases ü Oxidative non- reversible phase - generates NAPDH - Glucose 6- p undergoes dehydrogenation and decarboxylation to give a pentose, ribulose 5- p, which is

25 The PPP is divided into two phases ü Oxidative non- reversible phase - generates NAPDH - Glucose 6- p undergoes dehydrogenation and decarboxylation to give a pentose, ribulose 5- p, which is converted to its isomer, D- ribose 5- p. - Overall equation of 1st phase: Glucose 6- p + 2 NADP + + H 2 O ribose 5- p + CO NADPH + 2 H + ü Non- oxidative reversible phase - ribose 5- P is converted back to Glucose 6- p by a series of reactions involving especially two enzymes 1. Transketolase :Transfer of the 2- C fragment 2. Transaldolase :Transfer of the 3- C fragment

26 Pentose monophosphate shunt

27 Glucose Pyruvate via E- DP Two stage linear pathway like Glycolysis; First stage unique; second stage iden3cal. KDPG the unique intermediate. (KDPG) Yields 1 ATP, 1 NADH, 1NADPH. Note one pyruvate is generated at each stage of the pathway. Found in some Gram nega3ve bacteria instead of Glycolysis (Rhizobium, Agrobacterium, Azotobacter, Pseudomonas)

28 Methyl glyoxal Pathway The methyl glyoxal pathway is an alterna3ve pathway to the EMP pathway in some bacteria like E. coli, Clostridium and Pseudomonas species when the cell experiences low phosphate concentra3on. During this condi3on, then phosphate becomes the rate limi3ng step. DHAP formed is converted to methyl glyoxal and then to pyruvate. This step bypasses phosphoryla3on step that converts glyceraldehyde- 3- phosphate to 1,3- bisphosphoglycerate. The pyruvate produced is further metabolized to generate ATP. The pathway help to release the stress of elevated sugar phosphate concentra3on. The pathway is however a toxic pathway because less energy is produced and methyl glyoxal formed is toxic. In summary, this pathway u3lizes 2 ATP molecules rather than genera3ng ATP

30 Glycoly3c pathway

31 Glycoly3c Pathways used by various Bacteria Bacterium E-M PPP E-D Acetobacter aceti Bacillus subtilis major minor - E. coli Lactobacillus acidophilus Pseudomonas aeruginosa Vibrio cholera minor - major

Carbohydrate Catabolism Most of a cell s energy is produced from the oxidation of carbohydrates. Glycolysis - the most common pathway for the oxidation of glucose.

32 Carbohydrate Catabolism Most of a cell s energy is produced from the oxidation of carbohydrates. Glycolysis - the most common pathway for the oxidation of glucose. Glucose is the most commonly used carbohydrate. One glucose molecule. End-product - Pyruvic acid 2 ATP and 2 NADH molecules are produced Alternatives to Glycolysis The pentose phosphate pathway Used to metabolize five-carbon sugars; One ATP and 12 NADPH molecules are produced from one glucose molecule. The Entner-Doudoroff pathway One ATP and two NADPH molecules from one glucose molecule. Does not involve glycolysis Pseudomonas, Rhizobium, Agrobacterium

33 Respiration - Intermediate Step Pyruvic acid (from glycolysis) is oxidized and decarboyxlated 2 Pyruvic acid 2 NADH Figure

34 Fates of Pyruvate

35 Tricarboxylic Acid (TCA) Cycle This is also known as Krebs cycle or citric acid cycle and it is the second phase of respiratory metabolism of glucose. Acetyl CoA an intermediates produced from reac3on of pyruvate with CoA by pyruvate dehydrogenase is fed into the TCA cycle to generate the produc3on of carbon dioxide, water, reduced CoA and ATP.

37 TCA involves 5 steps i) Oxida3on of pyruvate [C3] to acetyl- coenzyme A (AcCoA) [C2] with the release of CO 2 and the forma3on of NADH ii) AcCoA [C2] condenses with oxaloacetate (OAA) [C4] to form citric acid (citrate) [C6], the first compound of the 6- carbon stage. iii) Through a series of reac3ons in the six- carbon stage, citric acid [C6] loses a carbon as CO2 to form α- ketoglutarate [C5] while genera3ng one NADH molecule iv) In the five- carbon stage, α- ketoglutarate [C5] loses a carbon as CO2 to form succinyl coenzyme A (succinyl- CoA) [C4] while genera3ng one NADH molecule. v) Through a series of reac3ons in the four- carbon stage, succinyl- CoA [C4] is converted to OAA [C4] while genera3ng one NADH molecule, one FADH2 molecule, and one GTP molecule (equivalent to ATP; produced via substrate- level phosphoryla3on)

38 Prokaryotes and some bacteria synthesize GTP from GDP and Pi rather than ATP. GTP is the energy source used in some specific cellular reac3ons such protein synthesis at the ribosomes. The energy level of GTP is equivalent to ATP and in addi3on the high energy phosphate group can be transferred from GTP to ADP to form ATP. At the end of the TCA cycle, all the carbon from the original glucose molecule has been converted to carbon dioxide.

39 Importance of TCA cycle The TCA cycle is also important in the flow of carbon through the cell because it is an amphibolic reac3on in aerobic organisms. 1) TCA cycle is important in the In oxida3ve catabolism of carbohydrates, fazy acids and amino acids, 2) It provides precursors for many biosynthe3c pathways through reac3ons that serve the same purpose in anaerobic ancestors. a) Ketoglutaric acid and oxaloacetate can serve as precursors of the amino acids, aspartate and glutamate by simple transamina3on. These two amino acids can further be u3lized with carbons of oxaloacetate and ketoglutaric acid to build other amino acids as well as purines and pyrimidine nucleo3des. b) Oxaloacetate can be converted to glucose in gluconeogenesis. c) Succinyl coa is an important intermediates in the synthesis of porphyrin ring of heme groups which serves as oxygen carriers in hemoglobin and myoglobin or electron carriers in cytochromes. d) Citrate is released by some commercially exploited microorganisms

40 Anaplero3c Reac3ons These are reac3ons that replace cycle intermediates and are reported in microorganisms, plants and animals. It usually involves the addi3on of carbon dioxide to an acceptor molecule such as pyruvate and phosphoenol pyruvate (PEP) to form oxaloacetate, the cyclic intermediates with the help of pyruvate carboxylase or PEP Caboxylase. Typically found in Arthrobacter globiformis and yeasts or E. coli and S. typhimurium.

41 The Glyoxylate cycle This occur in microorganisms, plants and invertebrates but not in vertebrates It involves the conversion of acetate into energy rich fuel and as a source of PEP for carbohydrates synthesis. Each turn of this cycle consumes two molecules of acetyl coa and produces one molecule of succinate which is available for biosynthe3c purposes by conver3ng succinate to oxaloacetate through fumarate or malate which can be converted to PEP and further to glucose by gluconeogenesis.

42 Total Yield Per Glucose Molecule ATP NADH NADPH FADH Glycolysis HMP shunt ED pathway TCA cycle 2 (GTP) 8-2

Ch. 9 Cellular Respira,on BIOL 222

Ch. 9 Cellular Respira,on BIOL Energy Arrives as sunlight Photosynthesis Energy ECOSYSTEM Light energy Plants capture sunlight organic molecules and generates O Carbs used in cellular respira@on CO + H