Stages and energetics of the process
Globally, tissue respiration can be divided into two closely related components:
- Oxidation of substrates. The process is triggered by dehydrogenation reactions, i.e., the removal of hydrogen atoms from organic molecules. This is followed by a multi-step electron transport cascade, the ultimate destination of which is oxygen.
- Energy transformation. As electrons move along the respiratory chain, their free energy level decreases. The released energy is utilized in two ways: a portion is inevitably lost as heat, and approximately 40% goes toward oxidative phosphorylation. This is a crucial mechanism for synthesizing the cell's energy currency—ATP—from ADP and inorganic phosphate.
First-line enzymes: dehydrogenases
The very first stage of tissue respiration is the dehydrogenation of substrates generated during catabolism. This task is carried out by enzymes from the dehydrogenase class, which are localized predominantly in the mitochondrial matrix.
Depending on the coenzyme incorporated into their structure, they are divided into two large groups: NAD-dependent and FAD-dependent. Their mechanisms of action and chemical structures differ fundamentally.
Features of NAD-dependent dehydrogenases
In this group of catalysts, the non-protein moiety is nicotinamide adenine dinucleotide ($NAD^+$). The molecule is a dinucleotide consisting of adenylyl (adenine, ribose, phosphate) and nicotinamide nucleotides. Their phosphate groups are linked together.
Key characteristics:
- Binding to the protein: the coenzyme is loosely bound to the apoenzyme. Upon being reduced to $NADH$, it dissociates from the protein moiety and serves as a hydrogen donor for subsequent enzymes.
- Active center: the nicotinamide ring, which is a derivative of vitamin PP (B3).
- Reaction chemistry: in the oxidized state ($NAD^+$), the ring is aromatic, and the nitrogen atom carries a positive charge while forming four bonds. Upon reduction, a hydride ion ($H^-$) adds to the carbon in the para position. At this point, aromaticity is disrupted, the double bonds are redistributed, and the nitrogen charge is neutralized. It is important to note that the second proton ($H^+$) is not incorporated into the molecule but remains in the medium.
A classic example is the conversion of malate to oxaloacetate by malate dehydrogenase. The equation is: Malate + $NAD^+ \rightarrow$ Oxaloacetate + $NADH + H^+$
Features of FAD-dependent dehydrogenases
The second group uses flavin adenine dinucleotide (FAD). Its structure is more complex: it consists of adenine, ribose, pyrophosphate, and riboflavin (composed of the ribitol sugar alcohol and an isoalloxazine ring).
Key characteristics:
- Binding to the protein: covalent, extremely tight. The coenzyme cannot dissociate from the apoenzyme.
- Active center: a triple-ring system — the isoalloxazine ring (a derivative of vitamin B2).
- Reaction chemistry: in the oxidized state, the ring has a quinoid structure, where two nitrogen atoms have double bonds. Upon reduction to $FADH_2$, two hydrogen atoms ($2H^+ + 2e^-$) add to these nitrogen atoms. The double bonds break, forming N-H bonds.
Because FAD is rigidly fixed to the enzyme, the reaction requires a second substrate—a mobile hydrogen acceptor—to proceed. For all flavin enzymes, this role is performed by ubiquinone (coenzyme Q).
An example is the oxidation of succinate to fumarate by succinate dehydrogenase. First, succinate is oxidized, and FAD is reduced to $FADH_2$. Then, $FADH_2$ transfers hydrogen to ubiquinone, forming $QH_2$.
Briefly: answers to topical questions
Which enzyme complexes form the mitochondrial respiratory chain?
The mitochondrial electron transport chain includes:
- Complex I — NADH dehydrogenase (NADH-ubiquinone oxidoreductase), containing the FMN coenzyme.
- Complex II — succinate dehydrogenase, containing the FAD coenzyme.
- Complex III — $QH_2$ dehydrogenase ($QH_2$-cytochrome c oxidoreductase).
- Complex IV — cytochrome c oxidase (cytochrome c oxidase).
Intermediary carriers: coenzyme Q (ubiquinone) and cytochrome c.
What are the mechanisms of coupling oxidation and phosphorylation according to Mitchell's chemiosmotic theory?
According to Peter Mitchell's chemiosmotic theory, the coupling of oxidation and phosphorylation occurs through the generation of a transmembrane electrochemical proton gradient.
- The energy of electron transport along the transport chain is used by complexes I, III, and IV to pump protons ($H^+$) from the matrix into the intermembrane space.
- This results in the formation of an electrochemical proton gradient (ΔμH⁺).
- Protons return to the matrix through a specialized channel in the ATP synthase enzyme (Complex V).
- The energy of this proton current is used to synthesize ATP from ADP and inorganic phosphate on the matrix side of the inner membrane.
What is the P/O ratio, and what are its values for the oxidation of NADH and FADH2?
The oxidative phosphorylation ratio (P/O) is the ratio of the amount of inorganic phosphate consumed ($H_3PO_4$) to the number of atoms of absorbed oxygen reduced to $H_2O$. It indicates the efficiency of ATP synthesis.
| Hydrogen donor | P/O ratio | Involved coupling complexes |
|---|---|---|
| NADH | 3 | I, III, IV |
| $FADH_2$ | 2 | III, IV |
Actual P/O values are somewhat lower than theoretical ones because part of the energy is dissipated as heat.
Which specific inhibitors block various complexes of the respiratory chain?
Various substances inhibit specific enzyme complexes of the respiratory chain:
- Barbiturates (including sodium amobarbital) and rotenone block NADH dehydrogenase (Complex I), interrupting electron transfer between iron-sulfur clusters and ubiquinone.
- Antimycin A inhibits $QH_2$ dehydrogenase (Complex III).
- CO, $H_2S$, and cyanide inhibit cytochrome c oxidase (Complex IV).
To avoid confusing coenzyme mechanisms, remember: FAD is bound to the enzyme "tightly" (covalently), so it strictly requires a mobile courier—ubiquinone (coenzyme Q). NAD, on the other hand, is "free" and can transport hydrogen independently.
Class and exam questions
What is oxidative phosphorylation?
This is the process of synthesizing ATP molecules from ADP and inorganic phosphate, driven by the energy released during the stages of tissue respiration.
Where does the second proton go during NAD reduction?
During the reaction, only a single hydride ion ($H^-$) is added to the nicotinamide ring. The second proton ($H^+$) does not bind to the coenzyme molecule but remains free in the surrounding environment.
Why do FAD-dependent dehydrogenases require a second substrate?
The FAD coenzyme is covalently bound to the protein moiety and cannot dissociate after reduction. To transfer hydrogen further down the chain, it requires an external mobile acceptor, which is ubiquinone.
What else to study on this topic
- The process of hydrogen removal in dehydrogenation reactions
- Structure and functions of ubiquinone (coenzyme Q)
- The relationship between vitamin PP structure and NAD dehydrogenase activity
- Mechanism of the succinate dehydrogenase reaction
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