M.Prasad NaiduMSc Medical Biochemistry,Ph.D.Research Scholar

ETC is the 4th and final stage of aerobic respiration.

Through ETC, the E needed for the cellular activities is released in the form of ATP.

ETC is an O2 dependent process which occurs in the inner mitochondrial membrane.

The energy rich carbohydrates (Glu), FA and AAs undergo a series of metabolic reactions and finally get oxidized to CO2 and H20.

The reducing equivalents from various metabolic intermediates are transferred to NAD+ and FAD to produce NADH and FADH2.

The latter two reduced coenzymes pass through the ETC or respiratory chain and finally reduce O2 to H20.

The passage of electrons through the ETC is associated with the loss of free energy.

A part of this free E is utilized to generate ATP from ADP and Pi.…

This is the final common pathway in aerobic cells by which electrons derived from various substrates are transferred to oxygen.

ETC is series of highly organized oxidation-reduction enzymes.

ETC is localized in Mitochondria.MC are the centres for metabolic

oxidative reactions to generate reduced coenzymes (NADH and FADH2) which in turn, are utilized in ETC to liberate E in the form of ATP.

Hence, MC is regarded as Power House of the Cell.

5 distinct parts.1.the outer membrane2.the inner membrane3.the inter membrane space4.the cristae5.the matrix.

ETC and ATP synthesizing system are located on IMM.

IMM is rich in proteins. It is impermeable to ions(H+,K+,Na+) and

small molecules (ADP, ATP). IMM is highly folded to form Cristae. The surface area of the IMM is greatly

increased due to Cristae. The IMM Possesses specialized particles

( that look like lollipops ), the phosphorylating subunits which are the centres for ATP production.

The interior ground substance.Rich in enzymes responsible for TCA

Cycle, oxidation of FA and the oxidation of amino acids.

The IMM can be disrupted into 5 distinct enzyme complexes, denoted as Complex I, II, III, IV and V

The complex I-IV are carriers of electrons while V is responsible for ATP synthesis.

Besides these enzyme complexes, there are certain mobile e- carriers in ETC.

These include NADH, Coenzyme Q, Cytochrome C and Oxygen.

The complexes (I-IV) and the mobile carriers are collectively involved in the transport of e- which ultimately combine with O2 to produce H2O.

Most of the O2 supplied to the Body is utilized by MC for ETC.

Complex I(NADH-CoQ reductase), Complex II(Succinate Co.Q reductase), Complex III(CoQ-Cyt C reductase) Complex IV(Cyt.oxidase) & Complex V(ATP synthetase)

There are 5 distinct carriers that participate in the ETC.Viz 1.Nicotinamide nucleotides 2.Flavo proteins 3.Iron-Sulfur proteins 4.Coenzyme Q 5.Cytochromes. These carriers are sequentially arranged and are

responsible for the transfer of e- from a given substrate to ultimately combine with proton and O2 to form H2O.

Of the 2 coenzymes NAD+ and NADP+ derived from the vit. Niacin, NAD+ is more actively involved in the ETC.

NAD+ is reduced to NADH + H+ by dehydrogenases with the removal of 2H atoms from the substrate.

The substrates include Gly-3-P, Pyruvate, isocitrate, α-KG, and malate.

NADPH + H+ produced by NADP+ -dependent dehydrogenase is not usually a substrate for ETC.

NADPH is more effectively utilized for anabolic reactions Eg: FA synthesis, Cholesterol synthesis.

The enzyme NADH dehydrogenase (NADH-CoQ reductase) is a flavo protein.

FMN is the prosthetic group. FMN accepts 2e- and a proton to form FMNH2. NADH dehydrogenase is a complex enzyme closely

associated with non-heme iron proteins (NHI) or iron-sulfur proteins.

NADH+ H++ FMN--NAD+ + FMNH2 SDH(Succinate-Co.Q reductase) is an enzyme found in

the IMM. It is also a flavoprotein with FAD as the coenzyme. SDH can accept 2 H atoms(2H+ + 2e- ) from succinate. Succinate+FAD- Fumarate + FADH2

A group of quinones has been found to be present in MC namely FeS, Fe2S2, Fe4S4 and Fe3S4 etc.,

FeS proteins exist in the oxidized(Fe3+) or reduced (Fe2+).

About 6 FeS proteins connected with ETC have been identified.

The machanism of FeS proteins in ETC is not clearly understood.

One FeS participates in the transfer of electrons from FMN to Co.Q

Other FeS proteins associated with cyt.b and cyt.c1 participate in the transport of electrons.

Vit E, D and plastoquinones also involved in ETC.

FeS: It has a single Fe coordinated to the side chain-SH groups of 4 Cys.residues.

Fe2S2: It contains 2 iron atoms, 2 inorganic sulfides and 4 –SH groups. Each iron is linked to 2-SH and 2-sulfur groups.

Fe4S4: It consists of 4 iron atoms and 4 cys-SH groups and 4 inorganic sulfides. Each iron remains linked to 1-SH, 3-inorganic sulfides while each sulfide is coordinated to 3 iron atoms.

Fe3S4: It consists of 3 Fe, 4- SH and 4inorganic sulfides. Each FeS protein transfers only one e- at a time. The enzymes may have one or more of the

combinations

Also called Ubiquinone since it is ubiquitous in living system.

It is a quinone derivative with a variable isoprenoid side chain.

The mammalian tissues possess a quinone with 10 isoprenoid units which is known as coenzyme Q10.(CoQ10)

CoQ is a lipophilic e- carrier. CoQ is not found in Mitochondria Vit k performs similar function as CoQ in

these organisms.

Cytochromes are conjugated proteins. Contains Heme group. The heme group of cyt differ from that Hb

and Mb. The Iron of Heme in cyt is alternately

oxidized(Fe3+) and reduced(Fe2+), which is essential for the transport of e- in ETC.

This is in contrast to the Hb and Mb iron which remains in the Fe2+ state.

Cyt are identified by their characteristic absorption spectra.

Ferricytochromes show diffuse and non-characteristic absorption spectra.

Ferrocytochromes exhibit characteristic absorption bands called α, β and γ–soret bands.

Cytochromes are characterized into different groups according to the light wavelength at which the alpha band shows its peak(α-abs.max.)

cyt.c:- Since it is largely available , it is the best studied of the cyts.

It is a central member of ETC with an intermediate redox potential)

Water soluble-loosely bound to IMM-easy to extract. Shows characteristic absorption spectra in the reduced

form at 550,521 and 416mµ Oxidized form @530mµ and 400mµ The iron content of cyt.c. is 0.38% Heme is attached with protein by means of 2 thioester

linkages involving sulfur of 2 cys and apoprotein. Cyt.c is incapable of combining with O2/CO. a protein with 1-PPchain 104aa (mw12400-13000) NADPH-Cyt.c.reductase can readily reduce Cyt.c

Cyt.c1:like cyt.c – contains an ironprotoporphyrinIX complex-heme-c.

It has abs.maxima @554,524&418mµ Incapable of combining w O2,CO,CN- Cyt.b:also- protoporphyrinIXcomplex-(heme-b). But the

apoprotein is diff. Tightly bound to Flavo proteins and ubiquinones in the

MC. The Ferrocyt.b has an abs.max.@563mµ, 530mµ &

430mµ. It is thermostable & not easily extractable. It also does not react with O2,CO or CN- Normally its oxidation requires the presence of Cyt.c,a,&

a3.

Complex IV of the ETC. Both contain an identical type of iron

porphyrin complex Inspite of identical hemes, cyt.a & a3 differ in

e-affinity & bio.activity.This is bcos of their location of hemes

One heme is located along with one Cu ion. This heme is called heme-a

This Cyt.a functions as anaerobic oxidizing unit.

The other heme is located along with the 2nd Cu ion and is called heme-a3 (functions as aerobic reducing unit).

Cyt.a-abs.max-605,517&414;Cyt.a3600&445mµ.

Cyt.a does not react with O2,CO/CN- where as Cyt.a3 is autooxidizable and forms compounds with CO & CN-.

Heme containing enzymes. Found in bacteria, fungi and animals. On the basis of sequence similarity peroxidases are

grouped into two super families. 1.fungal, plant & bac.peroxidases 2.animals form the 2nd super family of peroxidases. P.ases use H2O2 as the e- acceptor to catalyze a no. of

oxidative reactions. P.ases contain heme group and this heme group is

responsible for carrying out the activity of peroxidases Heme consists of a protoporphyrin ring and a central

iron atom in +3 oxidation state. A protoporphyrin is made up of 4 pyrrole rings linked by

methine bridges.--- with diff. side chains.

Heme containing redox enzymes. Produced by all aerobic organisms ranging from bacteria

to man. Converts H2O2 to H2O and mole.O. Utilizes H2O2 both as an e-acceptor and an e- donor. Catalase also catalyzes RCOOH + HQOH ROH + QO + H2O where R is an

alkyl or acyl group and HQOH is a 2e- donor. Most catalases exist as tetramers of 60-75KD. Each subunit contains an active heme group buried

deep within the structure. The stable structure of catalases is resistant to PH,

thermal denaturation and proteolysis. About heme

1. Monofunctional heme containing catalases.

2. Bifunctional heme containing catalase-peroxidases that are closely related to plant peroxidases.

3. Non-heme manganese containing catalases.

Are copper containing dioxygen carriers. Responsible for Di O2 transport in molluscs and

Arthopods High mol.wts and multiple subunits. Each subunit has a mol.wt of 76000D Each subunit is made up of 3 domains. Di O2 is bound to the active site in 2nd domain. There are 2 Cu atoms with an oxidation state of +1 So a PP chain of Arthopod HeCN binds 1 O2 molecule. The structure of Molluscan HeCN is diff. from that of

Artho. in Wt, subunits structure and O2 binding capacity.

Mol.wt of HeCN is 290000D with 2 Cu atoms for every 50KD

So 1 PP chain can bind 6-8 O2 molecules.

Is a biological Di O2 carrier. Responsible for Di O2 transport in marine invertebrates. The 4 diff.phyla of invertebrates are Sipuniculids,

priapulids, Brachiopods & annelid worm magelona HeEy is found as an oligomer. Blood contains an octameric form & tissues contain

trimeric or tetrameric Octameric HeEy consists of 8 subunits which are very

similar to 40 structure to MyoHeEy. Although diff oligomers are known, all of them share a

DiIron active site. The 2 iron atoms are 3.25-3.30Ao apart. The 2 Fe atoms are bound to 5 His.residues.

Study of model compound involves the structure determination, physical measurements and reactions of simple co-ordination compounds.

A model can give only a partial view of real system and provide valuable evidence for the study of the real systems.

ESSENTIAL & TRACE ELEMENTS Essential for life of animals, plants & microbes. They include Na, K(alkali metals), Ca, Mg (alkali earth

metals) & transition metals (V,Cr,Mn,Fe,Co,Zn, Mo and Cd.)

These elements are required for biological processes and are called essential elements.

Trace metals----occurs low conc in animal and plant cells. They are a part of good nutrition.

In high doses they may be toxic to the body or produce deficiencies in other trace metals.

For Eg. High levels of Zn can result in the def. of Cu.

Regulatory axn is exercised by Na+, K+,Mg2+ & Ca2+ ions. As cellular regulators they are involved in nerve

transmission, Maintanence of cell membrane permeability and Regulation of osmotic pressure Ca regulates muscle contraction, cell division and

growth, & enzyme activities. Also – blood coagulation system Mg,Ca, and Zn ions have structural role. Ca is a component of bones, teeth and animal shells. Zn – structural role in fingures Mg helps to stabilize 3D-structure of RNA& DNA.

Metallo enzymes catalyze several biological reactions. Metal ions are at the active site of these enzymes. Imp.metal enzymes – CP(Zn), Urease (Ni) & vit.B12(Co) Metal ions play imp.role in diO2 tpt and storage. A diO2 carrier protein contains diO2 binding site. This

active site is a complex of Fe/Cu. The 3 imp.DiO2 carriers are Hb, HeEy & HeCN Hb:- found in RBC----respiration---- the active site of Hb

consists of iron- porphyrin( heme) group. HeEy:- found in marine invertebrates. The O2 binding

site contains a pair of Fe atoms. HeCN:- Cu containing diO2 carriers found in molluscs and

arthopods.

Mb stores O2 in muscles. It contains a heme group

Metal ions play an imp. Role in e- transfer agents include ferredoxin, rubredoxin and cytochromes.

Ferredoxin and rebredoxin contain Fe-S sites. These sites are involved in e- transfer Cytochromes serve as e- carriers in both

plants and animals.

2nd most abundant transition element in the hu. body. About 2 gms of Zn and requires a daily intake of (RDA)

8-13mg. Stimulates the activity of 100 enzymes.Eg: CA, CP Plays structural role in proteins called zinc fingures. Also required in plants for leaf formation and synthesis

of auxin. Zn ion is good lewis acid in biochemical systems. Zn2+ can be 4,5,6- coordinate. Zn2+ complexes show easily 4 to 5 coordinate

interconversion. If the interconversion is fast, catalysis is also fast.

Zn complexes are labile than Ni2+/ Mg2+ complexes

Ca has a structural role. Chief component of bones, teeth and animal

shells. Imp. In cellular messenger system. Muscle contraction, secretion, ion transport,

cell division and growth and blood clotting. Ca and P are imp for bone formation. 99% of Ca is stored in bones Ca is necessary for the growth of children.

Humans contain about 4 gms of iron. Functions as the principal e- carrier in

biological oxidation & reduction reactions. Fe-S proteins are present in all forms of life. Fe-S sites occur in ferredoxins and

rubredoxins. They are involved in intra protein and inter

protein e- transfer.