Cytochrome c Protein

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 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]:

In 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]:

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