It Is an Oxidation Process in Which Complex Molecules Such As Carbohydrates and Fats Are

Respiration

“It is an oxidation process in which complex molecules such as carbohydrates and fats are broken down into CO2. In this process energy is liberated in the form of ATP”

OR

“It is an oxidation-reduction process in which organic complexes are oxidized to CO2 and O2 absorbed is reduced to form H2O”

General Equation of Respiration

C6H12O6 + 6O2 6CO2 + 6H2O + 2900 kJ mol-1

This release of energy is not spontaneous but involves 50 or more reactions after which CO2 and H2O are formed.

At the average 36 ATP (30.5 x 36 = 2900 kJ) are produced in plants during the whole process.

The energy released by the oxidation of one mole of glucose will be 36 x 30.5 = 1089 kJ (useable form of energy).

Respiratory Substrates

Most important reparatory substrates are Carbohydrates, Fats, Proteins and Organic acids (Malic acid, Fumaric Acids, Glycolic Acid etc.) etc.

Types of Respiration

–Aerobic respiration

–Anaerobic respiration

Aerobic Respiration

A biological process by which reduced organic compounds are mobilized and subsequently oxidized in a controlled manner.

Released energy can be transiently stored as ATP and can be readily utilized for metabolic processes

Both cytosol and mitochondria are involved

Anaerobic Respiration

“The respiration taking place in the absence of oxygen is called anaerobic respiration”

In anaerobic respiration, there is no involvement of mitochondria

It takes place only in cytosol

Mechanism of Respiration

Aerobic Respiration

Glycolysis: One glucose molecule is converted into 2 molecules of PA. This step has no involvement of oxygen

Oxidative decarboxylation: PA is converted into Acetyl Co. A.

Citric acid cycle (Krebs cycle):In Krebs cycle, CO2 is released from Acetyl Co. A.

Electron Transport and Oxidative Phosphorylation:In this step, NADH2 and FADH2 produced in glycolysis and Krebs cycle are oxidized to NAD+ and FAD+ to give rise the available form of energy (ATP)

Anaerobic Respiration

Glycolysis:One glucose molecule is converted into 2 molecules of PA. This step has no involvement of oxygen

Fermentation: In this step PA is converted either to lactate (in animals) or to ethanol (in pants and bacteria)

Different Steps of Respiration and their Site of Occurrence

Glycolysis:In the cytosol and chloroplast

Oxidative decarboxylation: In the matrix of mitochondrion

Krebs Cycle: In the matrix of mitochondrion

Electron Transport Chain: On the inner membrane of the mitochondrion

Glycolysis (Embden Meyrhof-Parnass or Hexose Diphosphate Pathway)

Derived from Greek word Glykos = sugar + Lysis = splitting or breakdown)

Glycolysis is Lysis of sugar or lysis of glycogen

It starts with glycogen in animals and with glucose, sucrose or starch in plants

All steps of glycolysis takes place in the cytosol

Glycolysis is independent of the presence of oxygen

Although in glycolysis CO2 is not released from the substrate however, ATPs are certainly produced

Glycolysis or Sucrolysis: Which term is better?

Some botanists prefer the term sucrolysis because in plants major sugar is sucrose, but animal physiologists prefer glycolysis because major storage carbohydrate in animals is glycogen

New Discoveries about Glycolysis

PPi-PFK is present along with ATP-PFK in plants only

F-2,6-bisphosphate is present in plants like other organisms. It acts as an activator of PPi-PFk, thereby favoring formation of F-1,6-bisphosphate

Conversion of F-6-P into F-1,6-bisphosphate through PPi-PFK pathway saves one ATP molecule

Glycolysis:

Energy Balance of Glycolysis

Importance of Glycolysis

Converts one hexose molecule into two molecules of PA and thus partial oxidation of hexose occurs. Thus it is a catabolic anaerobic pathway

Production of ATP and NADH (Reductants)

Many intermediate compounds can be used in other metabolic processes

It provides PA to mitochondria where it undergoes further oxidation to yield large amount of ATP

It can act as amphibolic pathway i.e. gluconeogenesis

Conversion of glucose or fructose to F-1,6- bisphosphate requires 2 ATPs if the ATP-PFK route is used and only one ATP if the PPi-PFK route is used

In mature cells, conversion of F,6-P to F-1,6- bisP is mainly carried out by ATP-PFK enzyme, but in young growing cells, the reaction is regulated by PPi-PFK, so there are two routes of this reaction

Glycolysis is a primitive process since it occurs in almost every living cell. It is thought that it arose before the advent of eukaryotic cells and perhaps even before O2 was a prominent component in the atmosphere

Oxidative Decarboxylation (General Equation)

PA dehydrogenase complex

PA + Co. A. + NAD+ ------ Acetyl Co. A. + CO2 + NADH+H+

These reactions are catalyzed by PA dehydrogenase complex, which is composed of 3 enzymes and requires 5 cofactors

Oxidation of PA in mitochondrion involves oxidation of the acetyl Co. A. to form CO2 and H2O

Components of PA dehydrogenase Complex

PA lipoamide oxidoreductase (PA dehydrogenase)

Acetyl Co. A. dihydrolipoamide S-acetyl transferase (Dihydrolipoyl transferase)

NADH lipoamide oxidoreductase(Dihydrolipoyl dehydrogenase)

Cofactors involved in PA dehydrogenase Complex

TPP Thiamine pyrophosphate, Mg2+ ions, NAD+, Co. A, Lipoic acid

Oxidative decarboxylation:

Krebs cycle

It is also known as TCA or Citric Acid Cycle

Discovered by Hans Krebs in 1937 in the flight muscles of pigeon

Krebs was awarded Nobel Prize in 1954 in medicine

In the presence of O2 PA is completely oxidized to CO2 and H2O

An Overview of TCA cycle

Energy Balance of Krebs Cycle

Total ATPs Produced After Complete Oxidation of One mole of Hexose

ATP produced by glycolysis = 8

ATP produced by Krebs cycle = 3

Total ATP produced = 8 + 30 = 38

Total ATP utilized =

- i) Through ATP-PFK = 2(both in animals and plants)

- ii) Through PPi-PFK = 1 ( in plants only)

Net Gain

–i) Through ATP-PFK = 30 + 6 = 36 (both in animals and plants)

–ii) Through PPi-PFK = 30 + 7 = 37 ( in plants only)

Control of Respiration in Mitochondria (Krebs cycle, ETS & Oxidative Phosphorylation)

• Major controlling factor in mitochondria is the concentration of ADP. If the concentration of ADP is high than oxidative phosphorylation is high, ETS functions vary fast and Krebs cycle also runs fast. This will stimulate cytosolic glycolysis (Bottom up)
• Regulation of the first step in Krebs cycle is oxidation of PA by PA dehydrogenase complex. This complex consists of one regulatory enzyme kinase which uses ATP to phosphorylate the OH group of various threonine amino acid residues in certain part of the PA dehydrogenase enzyme. This phosphorylation quickly inactivates the enzyme so Krebs cycle stops
• The second regulatory enzyme is a Phosphatase which hydrolyses P from threonine and reactivates the PA hydrogenase so Krebs cycle reactivates
• If much more PA in mitochondria, Krebs cycle is active
• If much more ATP in mitochondria, Krebs cycle is slow

Electron Transport Chain and Oxidative Phosphorylation

Energy is conserved during Krebs cycle in the form of NADH and FADH2

It must be converted into ATP to perform useful work in the cell

This O2 dependent process is called as oxidative phosphorylation, which occurs in the inner mitochondrial membrane

Neither NADH or FADH2 can react directly with O2 but they are transferred via several intermediates before water is produced

There are several thousands of ETS on the mitochondrial membrane

Electron Transport Chain Brings About

Oxidation of NADH and FADH2

Generation of electrochemical proton gradient

Model of ETS by Moller, 1997in Taiz & Zeiger, 2003

Features of ETC

Complex I (NADH dehydrogenase)

Complex II (Succinate dehydrogenase)

Complex III (Cytochrome bc1 complex)

Complex IV (Cytochrome c oxidase)

Cytochromes and Fe-S proteins can receive or transfer 1é only at a time but not H+

Ubiquinone (UQ) and FMN can receive or transfer 2 é and 2 H+ at a time

This is important for pH gradient generation, 4 H+ cause the synthesis of one ATP

Features of ETC

In prokaryotic cells, the oxidation of NADH and FADH2 is carried out by enzymes located on plasma membrane

Whereas

In eukaryotes the necessary catalysts are in the inner membrane of mitochondria

Complex I (NADH dehydrogenase)

–Electrons are tightly bound to NADH cofactor (FMN) and several Fe-S proteins

–Electrons are transferred to ubiquinone

–Four protons are pumped from the matrix to intermembrane space for every electron pair passing through the complex I

–Ubiquinone is a small lipid soluble electron and a proton carrier and is associated with a protein

Complex II (Succinate dehydrogenase)

–Oxidation of succinate in Krebs cycle is catalyzed by this complex

–Reducing equivalents are transferred via FADH2 and group of Fe-S proteins into the ubiquinone pool

–This complex does not pump the proton

Complex III (Cytochrome bc1 complex)

–It oxidizes reduced ubiquinone (ubiquinol)

–It transfer the electron via an Fe-S center, two b-type cytochromes (b565and b560) and a membrane bound cytochrome c1to cytochrome c

–Cytochrome is a small protein loosely attached to the outer surface of the inner membrane and serves as a mobile carrier to transfer electrons between complexes III and IV

–Four protons are pumped from the matrix to intermembrane space for every electron pair passing through the complex III

Complex IV (Cytochrome c oxidase)

–It contains two copper (CuA and CuB) centers and cytochromes a and a3

–It is a terminal oxidase and brings about the reduction of O2 to form two molecules of water

–Two protons are pumped out per electron pair

ATP Synthesis

Mitochondrial electron transport is associated with net transfer of protons from the mitochondrial matrix to intermembrane space

Inner mitochondrial membrane is impermeable to H+

Free energy associated with the formation of an electrochemical proton gradient is made up of an electric trans-membrane potential (E) component and a chemical potential component (pH)

∆E results from asymmetric distribution of charged species across the membrane

∆pH is due to the proton concentration difference across the membrane

FoF1-ATP synthase consists of F1 (peripheral membrane complex of 5 subunits) and Fo (integral membrane complex, three different types of subunits)

Structure of ATP Synthase

The passage of protons through the Fo channel is coupled to the catalytic cycle of F1, allowing the synthesis of ATP and simultaneous utilization of electrochemical gradient

Components of Fo

Fo is an integral membrane protein complex that consists of three different polypeptides (a, b, c10) that form channel through which protons can move into the matrix

This movement of H+ through the Fo is coupled with the synthesis of ATPs

Components of F1

F1 is water-soluble. It consists of five different types of polypeptide subunits

These subunits are

–α (three subunits)

–β (three subunits)

–γ (one subunit)

–δ (one subunit)

–ε (one subunit)

Alternative Electron Pathways in Plants

Lambers (1990) gave a comprehensive list of inhibitors with their sites of action

Unlike the animals, plant contains several alternative electron transport pathways

–External NADH dehydrogenase

–Rotenone insensitive NADH dehydrogenase

–Alternative oxidase

–Uncoupling proteins

External NAD(P)H Dehydrogenase

Its activity depends upon on Ca2+ (0.1–0.5 µM) and therefore its activity is inhibited by EDTA

Three NAD(P)H dehydrogenases on the inner and matrix surface of the inner membrane

It probably acts as an overflow mechanism

None of the external dehydrogenases pump protons

2 ATPs are produced

Rotenone-insensitive NAD(P)H dehydrogenase

Reduction of UQ by complex I is sensitive to to inhibition by rotenone and amytal

Rotenone is isoflavonoid isolated from roots of Derris and commonly used as an insecticide. (It is also toxic to fish and people in past use Derris root to harvest fish).

Piercidine is also inhibitor

Plants possess another NADH dehydrogenase that is insensitive to both of these

Inhibitors of ETC called rotenone-insensitive dehydrogenase (NDin)

Alternative oxidase

It is activated by pyruvate –it means it operates only when there is sufficient amount of sugars

It is a quinol-oxygen oxidoreductase and it does not pump protons. Therefore, energy conservation in the form of ATP is much smaller when the AOX is active

It receives electrons directly from ubiquinone and transfers to oxygen and generates water as the end product of the reaction, so four electrons must be transferred to oxygen

It is inhibited by SHAM (salicylhydroxamic acid).

Its active site consists of Fe, like other such enzymes (Berthold et al. 2000, Siedow and Umbach 2000

The functional form of the enzyme is a dimer, with the two polypeptides either covalently or non-covalently bound to each other

Anaerobic Respiration

Independent of presence of Oxygen

It comprises two steps

–Glycolysis (Same as in aerobic respiration)

–Fermentation (PA produced in glycolysis is either converted to lactate (in animals) or ethanol (in plants and bacteria)

Both steps takes place in cytosol

No involvement of mitochondria

Fermentation in Plants and Bacteria

In plants fermentation takes place in two steps

(i) In the first step PA is converted into Acetaldehyde and CO2 is released / (ii) In the second step Acetaldehyde is converted into Ethanol by the utilization of one NADH2 molecule

Energy balance of anaerobic respiration

Energetics of Fermentation

•Fermentation does not liberate all the energy and remains in reduced by-product lactate or ethanol

•Release of energy from alcoholic fermentation is 210 kJ mol-1

•Release of energy from lactate fermentation is 150 kJ mol-1 (4% efficiency)

•Entamoeba histolytica can produce 5ATP under anaerobic conditions

Respiratory Quotient

•It is the moles of CO2 produced after a period of time to the moles of O2 consumed during respiration

RQ = CO2 produced/O2 consumed

•RQ is used to know the nature of respiratory substrate

RQ for different substrates

•In case of glucose as respiratory substrate

C6H12O6 + 6CO2 6CO2 + 6H2O

RQ = 6 moles of CO2 produced/ 6 moles of O2 consumed = 1

•In case of malic acid as respiratory substrate

C4H6O5 + 3CO2 4CO2 + 3H2O

RQ = 4 moles of CO2 produced/ 3 moles of O2 consumed = 1.33

•In case of palmitic acid (a fatty acid) converted to sucrose via glycolytic cycle during germination

C16H32O2 + 11CO2 C12H22O11 + 4CO2 + 5H2O

RQ = 4 moles of CO2 produced/ 11 moles of O2 consumed = 0.36

•The complete oxidation of succinic acid has an RQ of about 1.14

2C4H6O4 + 7CO2 8CO2 + 6H2O