Cytochrome c Protein
Cytochrome c Protein (CYCS)
<div class="infobox infobox-protein">
<div class="infobox-header">Cytochrome c Protein</div>
<div class="infobox-row"><span class="infobox-label">Protein Name</span><span class="infobox-value">Cytochrome c, somatic</span></div>
<div class="infobox-row"><span class="infobox-label">Gene</span><span class="infobox-value">[CYCS](/genes/cycs)</span></div>
<div class="infobox-row"><span class="infobox-label">UniProt ID</span><span class="infobox-value">[P99999](https://www.uniprot.org/uniprot/P99999)</span></div>
<div class="infobox-row"><span class="infobox-label">PDB ID</span><span class="infobox-value">1HRC, 3ZCF</span></div>
<div class="infobox-row"><span class="infobox-label">Molecular Weight</span><span class="infobox-value">~11.7 kDa (104 amino acids)</span></div>
<div class="infobox-row"><span class="infobox-label">Subcellular Localization</span><span class="infobox-value">Mitochondrial intermembrane space</span></div>
<div class="infobox-row"><span class="infobox-label">Protein Family</span><span class="infobox-value">Cytochrome c family</span></div>
<div class="infobox-row"><span class="infobox-label">Cofactor</span><span class="infobox-value">Heme c (covalently bound)</span></div>
</div>
Overview
Cytochrome c is a small, highly conserved heme protein that serves a dual role in cellular physiology: as an essential electron carrier in the mitochondrial [electron transport chain](/mechanisms/mitochondrial-dysfunction) (Complex III → Complex IV) and as a critical trigger of intrinsic [apoptosis](/mechanisms/apoptosis-neurodegeneration) when released into the cytosol[@huttemann2011]. In [neurons](/entities/neurons), which are exquisitely sensitive to both energy failure and inappropriate cell death, cytochrome c sits at the nexus of mitochondrial bioenergetics and survival signaling. Its release from mitochondria is a pivotal, often irreversible, commitment step in neuronal death across [Alzheimer's disease](/diseases/alzheimers-disease), [Parkinson's disease](/diseases/parkinsons-disease), stroke, and traumatic brain injury[@jiang2004].
Structure
Cytochrome c is a 104-amino-acid globular protein with a tightly packed alpha-helical fold surrounding a single heme c prosthetic group[@huttemann2011]:
- Heme attachment: The iron-porphyrin ring is covalently linked via thioether bonds to Cys14 and Cys17 through a conserved CXXCH motif. His18 and Met80 serve as axial ligands to the central iron atom.
- Redox-active iron: Cycles between Fe²⁺ (reduced, ferrocytochrome c) and Fe³⁺ (oxidized, ferricytochrome c) during electron transfer, with a midpoint potential of +260 mV.
- Lysine-rich surface: A ring of positively charged lysine residues (Lys13, Lys72, Lys73, Lys79, Lys86, Lys87) mediates electrostatic interactions with the negatively charged surfaces of Complex III, Complex IV, and [Apaf-1](/entities/apaf1).
- Cardiolipin binding site: A hydrophobic patch and a secondary lipid-binding site anchor cytochrome c to the inner mitochondrial membrane via [cardiolipin](/entities/cardiolipin), a phospholipid critical for its retention in the intermembrane space[@kagan2005].
- Structural compactness: The protein lacks significant disordered regions, contributing to its extreme stability (Tm > 80 °C) and evolutionary conservation (>95 % identity between human and horse sequences).
Normal Function
Electron Transport Chain
In its primary bioenergetic role, cytochrome c is a mobile electron shuttle in the mitochondrial intermembrane space[@huttemann2011]:
Accepts electrons from Complex III (cytochrome bc1): the Rieske iron-sulfur subunit reduces cytochrome c via the Q-cycle mechanism
Delivers electrons to Complex IV (cytochrome c oxidase): binds to subunit II, transferring one electron at a time to the CuA center
Rate-limiting step: The diffusion of cytochrome c between Complexes III and IV can become rate-limiting for oxidative phosphorylation under conditions of membrane crowding or cardiolipin depletionIn neurons, which derive ~95 % of ATP from oxidative phosphorylation, even modest reductions in cytochrome c availability or function impair synaptic transmission and axonal transport.
Apoptosis Initiation
Upon mitochondrial outer membrane permeabilisation (MOMP), cytochrome c is released into the cytosol where it triggers the caspase activation cascade[@jiang2004]:
MOMP trigger: Pro-apoptotic [BCL-2](/entities/bcl2) family members (BAX, BAK) oligomerise to form pores in the outer mitochondrial membrane
Cardiolipin oxidation: [ROS](/entities/reactive-oxygen-species)-mediated peroxidation of cardiolipin weakens cytochrome c tethering, facilitating release[@kagan2005]
Apoptosome assembly: Cytosolic cytochrome c binds [Apaf-1](/entities/apaf1) in a dATP-dependent manner, triggering Apaf-1 oligomerisation into the heptameric apoptosome
Caspase-9 activation: The apoptosome recruits and activates procaspase-9, which then cleaves and activates executioner caspases-3 and -7
Point of no return: Once sufficient cytochrome c is released, the process is considered irreversible in most cell types, though neurons have evolved partial resistance through XIAP and other IAP proteins[@bhatt2017]ROS Scavenging and Cardiolipin Peroxidase Activity
Beyond electron transfer and [apoptosis](/entities/apoptosis), cytochrome c possesses intrinsic peroxidase activity when its Met80-Fe bond is disrupted[@kagan2005]:
- In complex with cardiolipin, cytochrome c gains peroxidase activity, selectively oxidising cardiolipin
- This creates a positive feedback loop: cardiolipin peroxidation → weakened cytochrome c binding → more cytochrome c release → more peroxidase activity
- Under physiological conditions, reduced cytochrome c scavenges superoxide (O₂⁻), acting as a mitochondrial antioxidant
Role in Neurodegeneration
Alzheimer's Disease
Cytochrome c release is a well-documented feature of [amyloid-beta (Aβ)](/amyloid-beta-))))))))))-induced neuronal death[@lustbader2004]:
- [Aβ](/proteins/amyloid-beta) oligomers trigger mitochondrial membrane depolarisation and BAX translocation
- Cytochrome c is released from hippocampal and cortical neuron mitochondria in AD brain tissue (post-mortem studies show elevated cytosolic cytochrome c)[@lustbader2004]
- Aβ directly interacts with mitochondrial membranes, disrupting the electron transport chain at the cytochrome c–Complex IV interface
- Caspase-3 activation downstream of cytochrome c release contributes to [tau](/proteins/tau) cleavage at Asp421, generating a pro-aggregation tau fragment that accelerates tangle formation[@rissman2004]
Parkinson's Disease
In PD, cytochrome c release is central to dopaminergic neuron vulnerability[@perier2007]:
- [Alpha-synuclein](/proteins/alpha-synuclein) oligomers bind cardiolipin on the inner mitochondrial membrane, displacing cytochrome c and directly promoting its release
- Complex I inhibition (by MPTP/rotenone) increases ROS, oxidises cardiolipin, and triggers cytochrome c release → caspase-dependent death of substantia nigra neurons
- [PINK1](/genes/pink1)/[Parkin](/genes/parkin) pathway deficiency leads to accumulation of damaged mitochondria with impaired cytochrome c retention
- Post-mortem PD substantia nigra shows elevated cytosolic cytochrome c and activated caspase-9[@perier2007]
Stroke and Excitotoxicity
Ischaemia–reperfusion injury causes massive cytochrome c release due to calcium overload, ROS burst, and mitochondrial permeability transition pore (mPTP) opening[@bhatt2002]:
- Glutamate excitotoxicity via NMDA receptors drives mitochondrial calcium uptake → mPTP opening → cytochrome c release
- The penumbral region shows a gradient of cytochrome c release correlating with distance from the ischaemic core
- Hypothermia neuroprotection partly works by preserving cytochrome c in the intermembrane space
Traumatic Brain Injury
Mechanical disruption of neuronal mitochondria in TBI causes immediate and delayed cytochrome c release, with CSF cytochrome c levels serving as a biomarker of injury severity[@pineda2007].
Therapeutic Implications
Anti-Apoptotic Strategies
- BCL-2 family modulators: Overexpression of BCL-XL prevents BAX pore formation and cytochrome c release in AD and PD models
- mPTP inhibitors: Cyclosporin A and its non-immunosuppressive analogue Debio 025 block mPTP-mediated cytochrome c release
- Caspase inhibitors: Z-VAD-FMK and minocycline (a broad caspase inhibitor) are neuroprotective downstream of cytochrome c release[@bhatt2017]
Cardiolipin Stabilisation
- SS-31 (elamipretide): A mitochondria-targeted peptide that stabilises cardiolipin–cytochrome c interactions, preventing cytochrome c detachment and peroxidase activation; Phase II trials in mitochondrial myopathy and heart failure[@birk2013]
- Cardiolipin-targeted antioxidants: MitoQ and other mitochondria-targeted compounds reduce cardiolipin peroxidation
Biomarker Potential
Cytochrome c levels in cerebrospinal fluid and plasma are elevated in stroke, TBI, and fulminant hepatic failure, and are being evaluated as a rapid biomarker for mitochondrial injury severity in neurological emergencies[@pineda2007].
See Also
- [Mitochondrial Dysfunction in Neurodegeneration](/mechanisms/mitochondrial-dysfunction)
- [Apoptosis in Neurodegeneration](/mechanisms/apoptosis-neurodegeneration)
- [Cardiolipin](/entities/cardiolipin)
- [Apaf-1](/entities/apaf1)
- [BCL-2 Family](/entities/bcl2)
- [Caspase-3](/proteins/caspase-3)
- [Complex IV — Cytochrome c Oxidase](/entities/complex-iv)
External Links
- [UniProt: P99999](https://www.uniprot.org/uniprot/P99999)
- [PDB: 1HRC](https://www.rcsb.org/structure/1HRC)
- [OMIM: 123970 — CYCS](https://omim.org/entry/123970)
References
[Huttemann M et al, The multiple functions of cytochrome c and their regulation in life and death decisions of the mammalian cell (2011)](https://pubmed.ncbi.nlm.nih.gov/21742514/)
[Jiang X, Wang X, Cytochrome c-mediated apoptosis (2004)](https://pubmed.ncbi.nlm.nih.gov/14744438/)
[Kagan VE et al, Cytochrome c acts as a cardiolipin oxygenase required for release of proapoptotic factors (2005)](https://pubmed.ncbi.nlm.nih.gov/16186509/)
[Bhatt P, bhatt S, Caspase inhibitors as neuroprotective agents (2017)](https://pubmed.ncbi.nlm.nih.gov/28159545/)
[Lustbader JW et al, ABAD directly links Abeta to mitochondrial toxicity in Alzheimer's disease (2004)](https://pubmed.ncbi.nlm.nih.gov/15066283/)
[Rissman RA et al, Caspase-cleavage of tau is an early event in Alzheimer disease tangle pathology (2004)](https://pubmed.ncbi.nlm.nih.gov/14745001/)
[Perier C et al, Two molecular pathways initiate mitochondria-dependent dopaminergic neurodegeneration in experimental Parkinson's disease (2007)](https://pubmed.ncbi.nlm.nih.gov/17553990/)
[Bhatt P et al, Cytochrome c release and apoptosome formation in brain after focal cerebral ischemia (2002)](https://pubmed.ncbi.nlm.nih.gov/12106063/)
[Pineda JA et al, Clinical significance of plasma cytochrome c levels after traumatic brain injury (2007)](https://pubmed.ncbi.nlm.nih.gov/17417084/)
[Birk AV et al, The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin (2013)](https://pubmed.ncbi.nlm.nih.gov/24068936/)