CYTOCHROME C OXIDASE: STRUCTURE, FUNCTIONS, INHIBITORS - BIOLOGY - 2025

The cytochrome c oxidase is a complex of enzymatic proteins that can cross the lipid bilayer of the cell membrane. It is hemosoluble and is mainly associated with the inner membrane of the mitochondria, being found both in prokaryotic organisms (bacteria) and in eukaryotes (unicellular and multicellular).

Also called complex IV, this enzyme is vital in the aerobic metabolic functions of organisms, as it is essential in the electron transport chain in which the cell burns sugars and captures part of the energy released to store adenosine triphosphate or ATP.

Ball-and-stick model of heme, of a molecule found in the crystalline structure of cytochrome c oxidase in the bovine heart. Taken and edited from: Benjah-bmm27.

The name cytochrome means "cellular pigments." These are blood proteins that carry electrons. Cytochromes were discovered by the Irish physician Charles Alexander MacMunn in 1884. MacMunn pioneered the discovery of the respiratory pigment in the blood, today called cytochrome 1.

In the 1920s, the Russian entomologist and parasitologist David Keilin rediscovered and characterized respiratory pigments and was the one who named them cytochromes. Although MacMunn had discovered them in 1884, the scientific community had forgotten him and some had even misinterpreted his work.

General characteristics

In general, respiratory pigments have characteristic visible light spectra. It is known that there are at least four integral membrane protein complexes in which there are 5 different types of cytochromes: a, a3, b, c1 and c, classified according to the wavelengths of the spectral absorption maxima.

They are generally found on the inner membrane of the mitochondria. However, they have also been observed in the endoplasmic reticulum and chloroplasts, in eukaryotic and prokaryotic organisms.

They present the heme prosthetic group containing iron (Fe). Each of the known cytochromes acts in multienzyme complexes in the transport of electrons in the respiratory process or chain.

Cytochromes have the function of participating in oxidation-reduction reactions. Reduction reactions, where they accept electrons, occur differently for each type of cytochrome, and their value is determined by the flow of electrons in the respiratory chain.

-Cytochrome c

Four classes of cytochrome c are known, which are as follows.

Class I

Within this class are the soluble low spin cytochromes c (lowspin), present in bacteria and mitochondria. They are octahedral. The heme binding site is at the N-terminus of histidine and the sixth ligand is supplied by a methionine residue at the C-terminus.

Of this class, several subclasses can even be recognized, whose three-dimensional structure has been determined.

Class II

High spin cytochrome c and some low spin cytochromes are found in this class. Those with high turn have the binding site close to the C-terminus, and in those with low turn the sixth ligand appears to be a methionine residue near the N end (N-terminus in English). They are pentacoordinated with fifth ligand histidine.

Class III

This class is characterized by presenting cytochromes c with multiple heme (c3 and c7) and a low oxidation-reducing potential, with only 30 amino acid residues per heme group. In representatives of this class, heme c groups have non-equivalent structure and functions, in addition to presenting different redox potentials. They are octahedral.

Class IV

According to some authors, this class was created only to include complex proteins that other prosthetic groups present, as well as heme c, or flavocytochrome c, among others.

Cytochrome c oxidase or Complex IV

Cytochrome c oxidase is a mitochondrial enzyme that carries out the final phase of electron transport in cellular respiration. This enzyme catalyzes the transport of electrons from reduced cytochrome c to oxygen.

Some chemical compounds such as cyanide, carbon dioxide and azide, can inhibit the functioning of this enzyme, causing the so-called cellular chemical asphyxia. Other forms of inhibition of complex IV are genetic mutations.

From an evolutionary point of view, cytochrome c oxidase is only found in aerobic organisms, and several groups of scientists suggest that the presence of this protein indicates evolutionary relationships where plants, fungi, and also animals shared a common ancestor.

Structure

Cytochrome c oxidase forms a homodimeric complex, that is, composed of two similar monomers in the inner membrane of the mitochondria. The enzyme complex consists of 3 to 4 subunits in prokaryotic organisms and up to a maximum of 13 (some suggest 14) polypeptides in organisms such as mammals.

In these organisms 3 polypeptides are of mitochondrial origin and the rest originate in the nucleus. Each monomer has 28 transmembrane helices that separate hydrophilic domains against the membrane matrix and the intermembrane space.

It has a single catalytic unit, which is found in all enzymes that catalyze oxidation / reduction reactions, using molecular oxygen (oxidases, especially heme-copper). The complex contains cytochromes a and a3 linked by subunit I and two copper centers.

It has one or more heme c groups linked to the surrounding protein structure by one or more (generally two) thioether bonds. Other authors suggest that there is a single heme C group covalently linked to the protein between the porphyrin ring and two cysteine ​​residues.

The only heme c group mentioned above is surrounded by hydrophobic residues and is hexacoordinated, with histidine at position 18 of the polypeptide chain and methionine at position 80.

Cytochrome c oxidase subunit F. Taken and edited from: Jawahar Swaminathan and the MSD staff at the European Institute of Bioinformatics

Features

Cytochrome c oxidases are protagonists in three main physiological mechanisms, which we will see below.

Apoptosis or programmed cell death

Apoptosis is programmed cell destruction or death, caused by the organism itself and whose purpose is to control growth, development, elimination of damaged tissues and the regulation of the immune system. In this physiological process, cytochrome c oxidase participates as an intermediate.

This protein, released by the mitochondria, leads to an interaction with the endoplasmic reticulum, which causes the secretion or release of calcium. The progressive increase in calcium triggers a massive release of cytochrome c oxidase, until cytotoxic levels of calcium are reached.

Cytotoxic levels of calcium and the release of cytochromes c cause the cascade activation of several caspase enzymes, which are responsible for the destruction of cells.

Cell or tissue regeneration

Several studies indicate that when cytochrome c oxidase is exposed to wavelengths of 670 nanometers, it participates in a functional complex, which penetrates damaged or injured tissue and increases the rate of cell regeneration.

Energy metabolism

This is perhaps the best known and the most relevant function of cytochrome c oxidase. It is precisely the oxidase complex (of the respiratory chain) that is responsible for collecting the electrons from cytochrome c and transferring them to the oxygen molecule, reducing it to two water molecules.

Connected to this process, a proton translocation occurs through the membrane, resulting in the generation of an electrochemical gradient that the ATP synthetase complex uses to produce or synthesize ATP (adenosine triphosphate).

Inhibitors

Cytochrome c oxidase is inhibited by various chemical compounds and processes. The way it occurs can arise as a natural way to regulate the production or action of the enzyme or it can occur accidentally due to poisoning.

In the presence of azide, cyanide or carbon monoxide, cytochrome c oxidase binds to these and the functioning of the protein complex is inhibited. This causes a disruption in the cellular respiratory process and thus causes the chemical suffocation of the cells.

Other compounds such as nitric oxide, hydrogen sulfide, methanol, and some methylated alcohols, also cause inhibition of cytochrome c oxidase.

Deficiency

Cytochrome c oxidase is an enzyme that is controlled by genes in both the nucleus and the mitochondria. There are genetic alterations or mutations that can lead to a deficiency of cytochrome c oxidase.

These mutations disturb the functionality of the enzyme, since they alter its enzymatic structure, bringing with it metabolic disorders during embryonic development (according to human studies), which will later affect the organism in its first years of life.

Cytochrome c oxidase deficiency affects tissues with high energy demand, such as the heart, liver, brain, and muscles. The symptoms of these mutations are reflected before the two years of life and can manifest as strong or mild conditions.

Mild symptoms can be seen even soon after the first year of age, and individuals with them usually only have decreased muscle tension (hypotonia) and muscle atrophy (myopathy).

On the other hand, individuals with stronger symptoms may have muscle atrophy and encephalomyopathy. Other conditions caused by the absence of cytochrome c oxidase are hypertrophic cardiomyopathy, pathological enlargement of the liver, Leigh syndrome, and lactic acidosis.

Uses in phylogeny

Phylogeny is the science that is responsible for the studies of the origin, formation and evolutionary development from the ancestor-descendant point of view, of organisms. In the last decades, phylogeny studies with molecular analysis have been more and more frequent, yielding a lot of information and solving taxonomic problems.

In this sense, some phylogenetic studies indicate that the use of cytochrome c oxidases can help establish evolutionary relationships. This is because this protein complex is highly conserved and is present in a wide variety of organisms, ranging from unicellular protists to large vertebrates.

Examples of this are the tests carried out with humans, chimpanzees (Pan paniscus) and Rhesus macaques (Macaca mulatta). Such tests revealed that the human and chimpanzee cytochrome c oxidase molecules were identical.

It also showed that the cytochrome c oxidase molecules of the Rhesus macaque differed by one amino acid from those of the first two, consequently reaffirming ancestor-descendant relationships between chimpanzees and humans.

References

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