Cytochrome C is a heme family protein that is generally located within the space between the inner mitochondrial membrane and outer mitochondrial membrane. It is a vital part of the respiratory cycle taking a key role in the transfer of electrons in order to form water. It transfers the electrons between the complex 3 and complex 4 of the respiratory chain or electron transport chain. Cytochrome C is also one of the initiation proteins for apoptosis or cell death. One method of apoptosis is completely reliant on the release of cytochrome C into the cytosol in order to initiate apoptosis. Different conformations of Cytochrome C cause it to have different functions overall. The composition of Cytochrome C is relatively quite simple in comparison to other major proteins since it has approximately 20% of its residues being Lysine (The Journal of Biochemistry). Cytochrome C, since it is so simple, is one of the most experimented on proteins out there. The structure being easy to map out and capable of being edited makes it a perfect beginner protein to experiment with. However simple, it is a very important protein for overall function in all Eukaryotes (New Journal of Science). It is also an ancient protein that established itself in the earliest stages of life but was not discovered until 1886 by Charles A. Macmunn. Cytochrome C was also rediscovered in 1925 by Charles Keilin. Since then, many have experimented with the inhibition of cytochrome C release which has shown promising results in therapeutic potential for Huntington’s disease. Others have used Cytochrome C in cancer research using it for its apoptosis function. The relatively small protein has a diverse job description causing it to be one of the most versatile experimental proteins known to this day
Structure
Cytochrome C is a heme protein (or a part of the heme family) which means that it has a
heme prosthetic group. This heme prosthetic group is covalently bonded using thioether bonds to
cysteine residues. This heme prosthetic is eight cyclic structures forming a circle around a central
iron atom. They can form different compounds by having different attachments around the 8
cyclic ring structure. Two conformations of cytochrome C exist naturally. In the monoheme
form, the other axial position is usually left empty however, it can be occupied by other
molecules such as histidine or lysine. Leaving the location empty prevents steric hindrance and
allows for easier attachment. The other forms contain anywhere from one to seven methionine
groups on what we perceive the left side of the heme group. When drawn out, the structure of
Cytochrome C looks vertically and horizontally symmetrical due to the central heme group prior
to adding side chains. The side chains which determine overall function are branched off of the
central heme group and vary depending on the proteins location in the cell. They can have one
form of side chain branching off at multiple locations, like a methionine attaching at multiple
locations, or it can have different types of attachments, like one methionine at a location and then
a lysine or histidine at another location. (Three-Dimensional Structure of Cytochrome c' )
Function
Cytochrome C function is dependent on the conformation of the structure it is portraying
at the time which is primarily determined by the location of the cytochrome c protein within the
cell. Monoheme cytochrome C, which is primarily found in the mitochondria of the cell,
functions in eukaryotes and prokaryotes during the electron transport chain. They are an electron
transfer protein during the bc1 complex of the electron transport chain. (See below for more
detailed information) Involving identical structure to the cytochrome C protein in mitochondria,
one conformation of cytochrome C is also a member of the electron transport chain in
photosynthesis in plants and cyanobacteria. (PDB101: Molecule of the Month: Cytochrome c.)
You can also find it in a Heme C form which is a membrane bound protein that converts O2 into
two water molecules using the electrons. Cytochrome C is also a main signaling factor for
apoptosis of cells. In the intrinsic pathway of apoptosis, Cytochrome C plays a key role in the
initiation of cell death. Without Cytochrome C, the cell could not release the protein into the
cytosol which at high volumes leads to intrinsic apoptosis. (see below for more detailed
information on this function)
Role in Apoptosis
Apoptosis is one form of programmed cell death in multicellular organisms. There are
multiple tags that are on a cell that signal for it to go to the apoptotic pathway. Once tagged, cells
go through a biochemical pathway that changes the cells morphology and leads to the “suicide”
or self death of the cell. A cell can go through either an extrinsic or an intrinsic pathway in order
to perform apoptosis. During the extrinsic pathway, an immune response is initiated by killer
lymphocytes which cause an apoptotic cascade. (Apoptosis: a Review of Programmed Cell
Death) Cytochrome C takes play in the intrinsic pathway. This is when a stimulus causes
Cytochrome C to be released into the Cytosol. Once cytochrome C is in the cytosol, it is recognized and bound to apoptotic factors which are then activated forming the apoptosome complex. Then caspases join in and are activated which result in a caspase cascade forcing
apoptosis. (Cytochrome c: Functions beyond Respiration.) Also over time while a cell is getting
old, it has degradation of its membranes. This degradation also leads to the release of
Cytochrome C which would signal that the cell is old and ready to be killed off. Without
Cytochrome C, intrinsic apoptosis would not be possible because the apoptotic factors would
never be activated. Same as if there are mutations in cytochrome C causing it to be unable to
permeate through the membrane, or if there is a mutation that increases the permeability of it
through the membrane, the apoptotic pathway would be accelerated or inhibited. (Cytochrome C
Proteopedia)
Purpose in ETC and Photosynthesis
Cytochrome C also plays a key role in the Electron Transport Chain in mitochondria. It is one of
the many electron carriers in the electron transport chain but quite a vital one. The heme group
portion of Cytochrome C accepts the electrons from the bc1 complex and then carries the
electrons to complex IV. Once at complex IV, the cytochrome C release their electron that they
are carrying and it is given to the Cytochrome C Oxidase enzyme. This enzyme collects 4
electrons from 4 different Cytochrome C transport proteins and transfers them to one dioxygen
molecule in order to make two molecules of water. It is also found within the thylakoid
membrane in the chloroplast of plants, cyanobacteria, and green algae. In photosynthesis,
Cytochrome C is one of the steps that transfers electrons from photosystem II to photosystem I.
Concurrently, it pumps protons across the thylakoid membrane adding in its own contribution
into creating an electrochemical gradient. Later in the cycle, the electrochemical gradient will
then be used in order to synthesize ATP from ADP. (The Multiple Functions of Cytochrome c)
Medical/Research Purposes
A proposal by many research scientists has been to regulate mitochondrial energy
production and ROS production through the phosphorylation of cytochrome C. It has been
observed that Tyr48Glu phosphomimetic mutant Cytochrome c reacts with CcO, but it is
partially inhibited which leads to controlled respiration. (The Multiple Functions of Cytochrome
c) They are proposing that “this effect plays an essential role in the prevention of ROS under
healthy conditions.” There is evidence when cellular stress is happening, Cytochrome C then
becomes phosphorylated. Once dephosphorylated, controlled respiration ceases which then sets
up Cytochrome C to initiate apoptosis. They report that the cellular stress causes mitochondrial
membrane potential differences and it needs to be taken into account to be able to determine the
risks behind changes in OxPhos activity. The study focuses mainly on the phosphorylation of
Cytochrome C, but acknowledges the fact that other factors may also be affected through their
actions. Others have began to focus their research on major diseases such as Huntington’s
disease or diverse forms of cancer. In a post by the New Journal of Science, they report that the
closest that anyone has come to a universal cure for cancers has been with the use of the
apoptotic function of cytochrome C. They went on to explain that tricking the body into
believing these cancerous cells are ready to die, they could negate the effects of the ineffective
p53 gene.
