Cytochromes are iron-containing proteins found in nearly every living cell on Earth, from soil bacteria to human muscle tissue, where they shuttle electrons in the chemical reactions that power life. The name literally means “cell color,” a nod to the vivid pigments that first drew scientists’ attention in the late 1800s. But cytochromes do far more than add color to tissue samples. Different members of this protein family drive energy production in your mitochondria, trigger programmed cell death when something goes wrong, metabolize the majority of prescription drugs you will ever take, and even help plants carry out photosynthesis.
How Cytochromes Were Found
The story begins in 1886, when a physician named Charles A. MacMunn noticed distinctive pigments in animal tissues that seemed to change color depending on whether they were exposed to oxygen. His observations were largely dismissed. It took nearly four decades before David Keilin, working at Cambridge in 1925, independently rediscovered these pigments and demonstrated their role in cellular respiration.1PubMed. Two faces of cytochrome c Keilin named them cytochromes and classified them into types a, b, and c based on how they absorbed light. That classification system, rooted in spectroscopy, still forms the backbone of how researchers categorize the family today.
The Heme Group and What Makes a Cytochrome
All cytochromes share a common feature: a heme group, which is essentially an iron atom sitting inside a flat ring of carbon and nitrogen. The iron cycles between two charge states as it accepts and donates electrons, and that cycling is what makes cytochromes useful as electron carriers. Beyond that shared core, the family is remarkably diverse. C-type cytochromes, for instance, have the heme permanently bonded to the protein through two chemical links involving cysteine residues in a specific sequence pattern.2PubMed Central. C-type cytochromes: diverse structures and biogenesis systems pose evolutionary problems Other types attach the heme more loosely. This structural variety allows different cytochromes to operate at different voltages, so to speak, enabling the stepwise transfer of electrons through complex chains rather than one big, wasteful dump of energy.
Powering Your Cells Through the Electron Transport Chain
The most familiar job for cytochromes is in the mitochondria, where they form critical links in the electron transport chain. This is the process that converts the food you eat into the molecular fuel your cells actually use. Cytochrome c is a small, water-soluble protein that acts as a mobile shuttle, ferrying electrons from one large protein complex (complex III) to the next one (complex IV, also called cytochrome c oxidase).3Journal of Biological Chemistry. Structure of Complex III with Bound Cytochrome c in Reduced State and Definition of a Minimal Core Interface for Electron Transfer Without this shuttle service, the chain breaks down and aerobic energy production grinds to a halt.4PubMed Central. Cytochrome c phosphorylation: Control of mitochondrial electron transport chain flux and apoptosis
At the end of the line sits cytochrome c oxidase, the terminal enzyme of the chain. Its job is to hand those accumulated electrons off to oxygen, producing water. In the process, it pumps protons across the inner mitochondrial membrane, creating a gradient that ultimately drives the production of ATP. Four protons get pumped for every oxygen molecule reduced.5PubMed. The mechanism of proton pumping by cytochrome c oxidase The enzyme also appears to be remarkably careful about how it handles oxygen. Its structural design allows it to receive multiple electrons in a coordinated way, minimizing the production of harmful reactive oxygen species that could damage the cell.6PubMed Central. Bovine cytochrome c oxidase structures enable O2 reduction with minimization of reactive oxygens and provide a proton-pumping gate
The Death Switch Inside Cytochrome c
Cytochrome c leads a double life. Under normal conditions, it peacefully shuttles electrons in the mitochondria. But when a cell is severely damaged, infected, or receives specific signals telling it to die, cytochrome c escapes from the mitochondria into the main body of the cell. Once free, it triggers a cascade of protein-cutting enzymes called caspases that dismantle the cell from the inside out.7PubMed. Cytochrome C-mediated apoptosis This programmed cell death, called apoptosis, is a normal and essential process. Your body uses it to eliminate about 50 to 70 billion cells every day, clearing out old, defective, or potentially cancerous cells.
The mechanism involves cytochrome c binding to a protein called Apaf-1, which then assembles into a wheel-shaped structure that activates the caspase chain reaction.8PubMed. Mitochondrial control of apoptosis: the role of cytochrome c But even before this step, cytochrome c plays a less well-known role that researchers have been piecing together. Inside the mitochondria, it pairs with a specific fat molecule called cardiolipin to form a complex that works as a peroxidase, meaning it can catalyze oxidation reactions. This cytochrome c-cardiolipin complex oxidizes cardiolipin itself, and that oxidized cardiolipin is required for the release of pro-death factors from the mitochondria.9PubMed. Cytochrome c acts as a cardiolipin oxygenase required for release of proapoptotic factors So cytochrome c does not merely escape from the mitochondria during apoptosis; it actively helps unlock the door.
The peroxidase activity of the cytochrome c-cardiolipin complex increases dramatically under certain conditions, with activity jumping more than 50-fold when the two molecules interact strongly.10PubMed Central. Peroxidase activity and structural transitions of cytochrome c bound to cardiolipin-containing membranes This catalytic boost is accompanied by partial unfolding of cytochrome c, which is a striking transformation: the same protein that was neatly folded and carrying electrons moments earlier physically changes shape to become a destruction enzyme. Among all the phospholipids in mitochondria, cardiolipin is the only one that undergoes this early oxidation during apoptosis, making the relationship between the two molecules a genuine “kiss of death” for the cell.11PubMed Central. Cytochrome c/cardiolipin relations in mitochondria: a kiss of death
Cytochrome P450 and Why Your Liver Cares
The cytochrome P450 superfamily is an enormous group of enzymes found primarily in the liver, though they also appear in the gut, kidneys, lungs, and other tissues. Their name comes from a quirk of spectroscopy: when bound to carbon monoxide, they absorb light most strongly at 450 nanometers. Despite sharing a name with the cytochromes in your mitochondria, the P450s have a very different day job. They specialize in chemically modifying molecules, particularly foreign substances like drugs, toxins, and environmental chemicals. Roughly 80% of oxidative drug metabolism and about half of the overall elimination of common medications can be attributed to P450 enzymes from just three gene families.12PubMed Central. Cytochrome P450 Enzymes and Drug Metabolism in Humans
The liver has been the best-studied site for drug-metabolizing P450s, and the major forms responsible for breaking down medications belong to those same three gene families.13PubMed. Drug metabolism by cytochromes P450 in the liver and small bowel But P450s are not just a detox system. They also carry out essential housekeeping reactions, including steps in the biosynthesis of steroid hormones like cortisol, estrogen, and testosterone.14PubMed Central. Diverse reactions catalyzed by cytochrome P450 and biosynthesis of steroid hormone Other P450 enzymes in the liver control bile acid production from cholesterol, a process regulated by multiple signals including cholesterol levels, thyroid hormones, and even time of day.15PubMed. Regulation of liver-specific steroid metabolizing cytochromes P450: cholesterol 7alpha-hydroxylase, bile acid 6beta-hydroxylase, and growth hormone-responsive steroid hormone hydroxylases Plants have their own massive P450 repertoire, where these enzymes drive the production of pigments, fragrances, defensive chemicals, and other specialized metabolites that have shaped plant evolution.16Molecular Plant. Plant cytochrome P450 plasticity and evolution
Why the Same Drug Hits People Differently
One of the most practical consequences of the P450 system is that genetic variation in these enzymes creates real differences in how people respond to medications. There are remarkable differences among patients in the catalytic activity of each P450, and this variability accounts, at least in part, for why some people respond well to a drug while others experience side effects or get no benefit at all.17Gastroenterology Clinics of North America. DRUG METABOLISM BY CYTOCHROMES P450 IN THE LIVER AND SMALL BOWEL
The clinical stakes can be high. People who carry loss-of-function versions of the CYP2D6 gene, for example, face an increased risk of dangerous heart rhythm problems if given standard doses of certain antipsychotic medications. Carriers of particular CYP2C9 variants often need lower doses of warfarin, a blood thinner, to avoid excessive bleeding. The flip side is equally concerning: when a drug needs to be converted by a P450 enzyme into its active form (a “prodrug”), people who lack that enzyme get little therapeutic effect. Codeine, for instance, relies on CYP2D6 to become morphine in the body. Someone with no working copies of that gene gets almost no pain relief from codeine, while someone with extra copies of the gene can convert codeine too quickly, risking opioid toxicity from a normal dose.18PubMed Central. Role of cytochrome P450 genotype in the steps toward personalized drug therapy This is the basis of pharmacogenomics, where a genetic test before prescribing can guide the choice of drug or dose.
The Grapefruit Problem
The most famous P450 interaction in everyday life involves grapefruit juice. Compounds in grapefruit called furanocoumarins bind to CYP3A4, a major drug-metabolizing enzyme in the intestinal wall, and permanently inactivate it. Because the enzyme is knocked out rather than just temporarily blocked, the effect persists until the body manufactures replacement enzymes, which takes time. The result is that drugs normally broken down during absorption pass through in much higher quantities, sometimes dangerously so.19PubMed. Grapefruit-drug interactions
Recent research has found that the problem is broader than initially appreciated. Repeated grapefruit juice intake does not just inhibit CYP3A4; it also causes clinically meaningful inhibition of CYP2B6, CYP2C9, and CYP2C19, revealing previously underappreciated interaction risks with a wider range of medications than those metabolized by CYP3A4 alone.20PubMed Central. Repeated Intake of Grapefruit Juice Inhibits CYP2B6, CYP2C9, CYP2C19, and CYP3A4 while Lingonberry Powder Does Not Induce Major CYP Enzymes in Humans For people taking statins, certain blood pressure medications, or immunosuppressants, a glass of grapefruit juice can effectively multiply the dose they are absorbing. The warnings on prescription labels are not overly cautious: they reflect real biochemistry.
How Poisons Target Cytochromes
Cyanide and carbon monoxide are two of the most infamous poisons in human history, and both kill by targeting the same cytochrome. Cyanide binds tightly to the iron in cytochrome c oxidase, the terminal enzyme of the mitochondrial electron transport chain, blocking the final handoff of electrons to oxygen. Carbon monoxide does the same.21PubMed. Diagnosis of cyanide intoxication by measurement of cytochrome c oxidase activity With cytochrome c oxidase disabled, cells cannot produce ATP aerobically, even though oxygen may be plentiful in the blood. The cell effectively suffocates internally.
In acute carbon monoxide poisoning, the inhibition of cytochrome c oxidase can be severe. Measurements in poisoned patients showed about 76% inhibition of the enzyme’s activity during the acute phase, with activity still reduced by roughly half three days later and not returning to normal until about day 12.22PubMed. Mitochondrial cytochrome c oxidase inhibition during acute carbon monoxide poisoning This lingering inhibition helps explain something clinicians have long observed: CO poisoning symptoms often persist well after the classic marker (carboxyhemoglobin in the blood) has returned to normal. The damage at the mitochondrial level outlasts the damage at the hemoglobin level.
Cytochromes in Photosynthesis
Cytochromes are not exclusively an animal affair. In plants, algae, and cyanobacteria, a cytochrome complex called cytochrome b6f sits at the center of the photosynthetic light reactions. It links the two photosystems (PSII and PSI) by accepting electrons from one carrier and passing them to another, functioning as a hub that controls the flow of photosynthetic electron transport.23Biochimica et Biophysica Acta (BBA) – Bioenergetics. Cytochrome b6f – Orchestrator of photosynthetic electron transfer Like cytochrome c oxidase in mitochondria, the b6f complex also pumps protons across a membrane, generating the gradient that drives ATP synthesis in chloroplasts.24PubMed. The cytochrome b(6)f complex: plastoquinol oxidation and regulation of electron transport in chloroplasts The parallel between mitochondrial and chloroplast electron transport chains is no coincidence; both systems share deep evolutionary roots.
Bacteria That Breathe Rocks
Some of the most unusual cytochrome biology belongs to microorganisms that have figured out how to breathe things other than oxygen. Bacteria in the genus Geobacter, for instance, can transfer electrons all the way out of their cells and onto solid mineral surfaces, metal particles, or even electrodes. This process, called extracellular electron transfer, depends heavily on c-type cytochromes arranged along the cell surface and along electrically conductive hair-like structures called e-pili.25PubMed Central. Cytochromes in Extracellular Electron Transfer in Geobacter
Geobacter and a related genus, Shewanella, were the first bacteria characterized with the ability to couple their growth to the reduction of iron or manganese oxide minerals located outside the cell.26PubMed. Electron Transfer Beyond the Outer Membrane: Putting Electrons to Rest In the decade or so since their discovery, researchers have worked out how conserved strategies conduct electrons from the inner membrane to the outer surface. These organisms have attracted interest for applications including microbial fuel cells, bioremediation of contaminated sites, and biosensors, all powered by cytochrome-mediated electron transfer to external materials.
A Molecular Clock for Evolution
Cytochrome c has played an outsized role in the history of evolutionary biology because of how conserved its sequence is across species. In the 1960s and 1970s, Richard Dickerson compared protein sequences across species and noticed that the number of amino acid differences increased in a roughly linear fashion with divergence time. Cytochrome c evolved slowly compared to other proteins, while hemoglobin changed at an intermediate rate and fibrinopeptides changed quickly.27PubMed. Richard Dickerson, Molecular Clocks, and Rates of Protein Evolution The slow rate of cytochrome c evolution reflects severe functional constraints: because the protein must interact precisely with its binding partners in the electron transport chain and the apoptosis machinery, most mutations are harmful and get weeded out.
Even so, cytochrome c has not been entirely static. Studies of primate cytochrome c genes have uncovered episodes of positive selection and gene silencing across the lineage, suggesting that the protein has occasionally been under evolutionary pressure to change rather than stay the same.28PLoS ONE. Silencing, Positive Selection and Parallel Evolution: Busy History of Primate Cytochromes c These bursts of change, set against an otherwise extremely conserved background, offer clues about how cytochrome c’s dual functions in energy production and cell death may have co-evolved.
Seeing Cytochromes in Action
The original classification of cytochromes by Keilin relied on spectroscopy, and the technique remains central to studying them. Different cytochromes absorb light at characteristic wavelengths, producing signature peaks in their absorption spectra. Cytochrome c oxidase, for example, shows distinct spectral features in the Soret region (around 420 to 450 nanometers) and the alpha-band region (around 560 to 630 nanometers) that shift depending on the enzyme’s activity state.29PubMed Central. Spectroscopic identification of the catalytic intermediates of cytochrome c oxidase in respiring heart mitochondria By analyzing these spectral changes in intact, respiring mitochondria, researchers can watch the enzyme cycle through its catalytic intermediates in real time, revealing that more than half the enzyme population is actively working at any given moment.
Advanced methods like second-derivative absorption spectroscopy have allowed researchers to tease apart the individual contributions of the two heme groups in cytochrome c oxidase (called heme a and heme a3), even though their absorption peaks overlap heavily in a standard spectrum.30PubMed. Resolution of the electronic transitions of cytochrome c oxidase: evidence for two conformational states of ferrous cytochrome alpha This kind of spectral dissection has been essential for understanding how the enzyme moves electrons internally and how it avoids producing toxic oxygen intermediates.
Engineering Cytochromes for Industry
The catalytic versatility of P450 enzymes has not gone unnoticed by chemists looking for greener ways to manufacture drugs and chemicals. P450s can selectively add oxygen atoms to otherwise unreactive carbon-hydrogen bonds, a task that is extremely difficult with conventional chemistry. Through protein engineering techniques, including directed evolution and rational design, researchers have been reshaping P450 enzymes for industrial applications, expanding their catalytic repertoire to include reactions that do not occur naturally.31PubMed. Use of engineered cytochromes P450 for accelerating drug discovery and development Frances Arnold won the 2018 Nobel Prize in Chemistry in part for pioneering directed evolution of enzymes, and P450s were among the showcase examples.
Another approach sidesteps the protein entirely and builds synthetic mimics. Metalloporphyrins, which share the flat iron-ring architecture of the P450 active site, can be designed to replicate the enzyme’s oxidation chemistry in a test tube.32PubMed. Review on the application of metalloporphyrin catalysts as cytochrome P450 enzyme mimics in biomimetic modeling Some of these synthetic porphyrins have been incorporated into porous frameworks that provide a scaffold resembling the protein’s protective pocket, improving their stability and selectivity.33PubMed Central. Chemical reactions catalyzed by metalloporphyrin-based metal-organic frameworks Others use thiolate ligands to more faithfully copy the electronic environment at the P450 active center.34Coordination Chemistry Reviews. Synthetic models for the active site of cytochrome P450 These biomimetic catalysts are finding uses in drug development, environmental chemistry, and the synthesis of fine chemicals where selectivity matters.
Red Blood Cells and Cytochrome b5
One cytochrome with a narrowly focused but essential role is cytochrome b5 reductase in red blood cells. Hemoglobin, the oxygen-carrying protein in blood, occasionally gets oxidized from its functional form into methemoglobin, which cannot carry oxygen. Cytochrome b5 reductase uses electrons from NADH to reduce the iron in methemoglobin back to its working state, restoring the protein’s ability to bind oxygen.35Journal of Biological Chemistry. Kinetic studies on methemoglobin reduction by human red cell NADH cytochrome b5 reductase People born with deficiencies in this enzyme develop methemoglobinemia, a condition where enough hemoglobin accumulates in its non-functional form to turn the skin visibly blue and reduce the blood’s oxygen-carrying capacity. It is a rare condition, but it illustrates how a single cytochrome operating behind the scenes can have outsized consequences when it fails.