Respiratory Chain Is the Most Productive Pathway to Make ATP

Organic chemistry and Biological chemistry for Health Sciences

59-191

Lecture 21

Respiratory chain is the most productive pathway to make ATP. It uses chemicals that are synthesized by another pathway called citric acid cycle (tricarboxylic acid cycle/krebs cycle). The chief purpose of the citric acid cycle is to supply chemical needs of respiratory chain.

Citric acid cycle cannot run unless acetyl group is fed into the cycle through an enzyme cofactor, acetyl coenzyme A. The catabolism of molecules from all three major foods-carbohydrate, protein and lipids-produces acetyl coenzyme A. Acetyl coenzyme A or acetyl CoA, is the fuel for citric acid cycle.

Fatty acids are major source of acetyl CoA. A series of reactions called b-oxidation pathway breaks fatty acids into acetyl units. Most amino acids can be catabolized to acetyl units or to intermediates of citric acid cycle.

The end product of glycolysis, the catabolic pathway of carbohydrate, is pyruvate ion. Pyruvate ion is converted to acetyl CoA through a short pathway.

Glycolysis makes some ATP also by substrate level phosphorylation under low oxygen conditions. Glycolysis is thus a backup source for ATP at low oxygen environment condition. In the absence of sufficient oxygen the end product of glycolysis is lactate instead of pyruvate.

Once the cell is resupplied with oxygen, lactate is converted back to pyruvate. Thus glycolysis can end up either in pyruvate or lactate, depending on oxygen supply.

The full sequences of oxygen consuming reactions from glucose to pyruvate ion to acetyl CoA and on through the citric acid cycle and the respiratory chain is the aerobic sequence of glucose catabolism.

When glycolysis is run without oxygen, from glucose only as far as to lactate ion, the series of reactions is called the anaerobic sequence of glucose catabolism.

CITRIC ACID CYCLE:

The two carbons in acetyl group is broken down to CO2 in a series of reactions and their hydrogens are sent to the respiratory chain as units of H:- and H+ in citric acid cycle. The hydrogen eventually form water by reacting with oxygen. Citric acid cycle occurs in the innermost part of a mitochodrion.

The sequence of reactions in the citric acid cycle is cyclical. Before an acetyl group can enter the citric acid cycle, it must be attached to conenzyme A, CoASH. So acetyl CoA is the carrier of acetyl units in the citric acid cycle. In acetyl CoA, the acetyl residue replaces the H on SH of CoASH.

Synthesis of acety CoA from pyruvate involves two steps-decarboxylation and oxidation. The actual synthesis occurs in five steps that are catalyzed by a multienzyme complex called pyruvate dehydrogenase. Three B vitamins-thiamine, nicotinic acid (niacin), and riboflavin-are among the compounds needed to make the coenzymes for the complex.

The enzymes in the citric acid cycle exist in mitochondrion in close proximity to those of the respiratory chain.

In summary, to begin a turn of the cycle acetyl coenzyme A donates its acetyl group to four carbon compound oxaloacetate ion to form the six carbon citrate. Citrate is then transformed to isocitrate, also a six carbon molecule, which is dehydrogenated with loss of CO2 to ultimately yield the five carbon compound a-ketogluterate. The latter (a-ketogluterate) undergoes loss of CO2 and ultimately yields four carbon compound succinate and a second molecule of CO2. Succinate is then enzymatically converted in three steps into four carbon oxaloacetate, with which the cycle begun; thus oxaloacetate is ready to react with another molecule of acetyl CoA to start a second turn.

The citric acid cycle has eight steps.

1. Citrate is dehydrated to give the double bond of cis-acotinate. The enzyme involved in acotinase.

2. Cis-acotinate is hydrated to give an isomer of citrate called isocitrate. Acotinase also catalyzes this step.

3. The 20alcohol group of isocitrate is dehydrogenated by isocitrate dehydrogenase to give the keto group of oxalosuccinate. The alcohol system gives up both H:- and H+. An NAD+ coenzyme accepts H:- while H+ is forced across a membrane within the mitochondrion. The NAD+ thereby changes to NADH.

4. Oxalosuccinate still bound to isocitrate dehydrogenase loses a carboxyl group to give a-ketogluterate.

5. a-ketogluterate undergoes a very complicated series of reactions all catalyzed by one team of enzymes called a-ketogluterate dehydrogenase. A carboxyl group is lost and another dehydrogenation occurs. The same kind of fate occurs to H:- and H+ here as happened in step 3.

The product is the coenzymeA derivative of succinic acid. It’s converted by the action of the enzyme succinoyl-CoA synthetase to the succinate ion. The reaction also generates guanosine triphosphate, GTP from its diphosphate. GTP is another high energy triphosphate similar to ATP. But GTP is able to transfer one of its phosphate units to ADP and so make one ATP. So one ATP is synthesized through substrate level phosphorylation in citric acid cycle.

6. Succinate dehydrogenates to form fumerate ion. Succinate dehydrogenase catalyzes this reaction. Both H:- and H+ are accepted by FAD, which becomes FADH2. FADH2 also intersects with the respiratory chain.

7. Fumerate is hydrated (water adds to the double bond) to form malate. This reaction is catalyzed by fumerase.

8. The 20 alcohol group in malate is oxidized by malate dehydrogenase to make oxaloacetate. NADH forms as the alcohol group gives up H:- to NAD+. The chemical energy of NADH is used to form three more ATP molecules by the respiratory chain.

In each turn of the cycle, one acetyl group enters as acetyl CoA and two molecules of CO2 leave. In each turn one molecule of oxaloacetate is used to form citrate but-after a series of reactions-the oxaloacetate is regenerated. Therefore no net removal of oxaloacetate occurs; one molecule of oxaloacetate can theoretically suffice to bring about oxidation of infinite number of acetyl group. Four of the eight steps in the process involve dehydrogenation, another name for oxidation. The dehydrogenation steps provide fuel for the respiratory chain, and the synthesis of ATP. The other steps simply set molecules up for dehydrogenation.

RESPIRATORY CHAIN:

The mitochondrial respiratory chain consists of a series of electron carriers, most of which are integral membrane proteins, with prosthetic groups capable of accepting and donating either one or two electrons. Each component of the chain can accept electrons from the previous carrier and can transfer them to the following one, in a specific sequence.

The respiratory chain involves a long series of oxidation-reduction reactions. The electron flow is down an energy hill all the way to oxygen, and its irreversible. Other events simultaneously take place to synthesize ATP from ADP and Pi.

Respiratory chain and ATP synthesis occurs in mitochondria. A mitochondrion has two important membranes, one outer and one inner. The space between the two membranes is called intermembrane space. The inside of the inner membrane is called matrix, which is filled with enzymes, cofactors, inorganic ions and substrates.

In the inner membrane there are four immobile enzyme complexes designated as I, II, III and IV. Inner membrane also has enzymes capable to move from one complex to another named coenzyme Q and cytochrome c.

The transfer of electrons along the respiratory chain is accompanied by outward pumping of protons across the inner mitochondrial membrane to the intermembrane space compartment. This establishes an unstable proton gradient between the intermembrane region (more H+) and the matrix (less H+). Eventually the fluid of the intermembrane space will have a pH 1.4 unit lower (more acidic) than matrix.

The protons can only return to the matrix through certain locations, where the returning proton activates an enzyme that makes and releases ATP. This is how cell connects respiratory chain with the synthesis of ATP. English scientist Peter Michell first formulated an outline of this understanding, and it is called chemiosmotic theory.

How respiratory chain works?

One entry to the respiratory chain is the synthesis of NADH. If MH2 represents any metabolite that can donate H:- to NAD+ , the dehydrogenation of that metabolite can be written as follows

MH2 + NAD+ M + NADH + H+

NADH interacts with the complex I of the respiratory chain. FMN in Complex I accepts the electron pair (carried by H:-) from NADH. It starts the flow of electron towards oxygen. The H:- unit of NADH transfers to FMN according to the following equation.

NADH + H+ + FMN FMNH2 + NAD+

This also restores the NAD+ enzyme and moves the electron pair one more step.