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Troponin is a complex of three regulatory proteins that is integral to muscle contraction
Muscle contraction

Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may #Eccentric contraction, #Concentric contraction or #Isometric contraction....
 in skeletal
Skeletal muscle

They generally contract voluntarily , although they can contract involuntarily through Reflex action. The whole muscle is wrapped in a special type of connective tissue, epimysium....
 and cardiac muscle
Cardiac muscle

Cardiac muscle is a type of involuntary sarcomere muscle found in the walls of the heart, specifically the wikt:myocardium. Cardiac muscle cells are known as cardiac myocytes ....
, but not smooth muscle
Smooth muscle

Smooth muscle is a type of non-striated muscle, found within the tunica media layer of large and small arteries and veins, the urinary bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, the ciliary muscle, and iris of the eye....
.

Discussions of troponin often pertain to its functional characteristics and/or to its usefulness as a diagnostic marker for various heart disorders.

onin is attached to the protein tropomyosin
Tropomyosin

Tropomyosin is an actin-binding protein that regulates actin mechanics. It is important, among other things, for muscle contraction. Tropomyosin, along with the troponin complex, associate with actin in muscle fibers and regulate muscle contraction by regulating the binding of myosin....
 and lies within the groove between actin
Actin

Actin is a Globular_protein, roughly 42-kDa protein found in all Eukaryote where it may be present at concentrations of over 100 ?M. It is also one of the most highly-Conservation proteins, differing by no more than 20% in species as diverse as algae and humans....
 filaments in muscle tissue.






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Troponin is a complex of three regulatory proteins that is integral to muscle contraction
Muscle contraction

Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may #Eccentric contraction, #Concentric contraction or #Isometric contraction....
 in skeletal
Skeletal muscle

They generally contract voluntarily , although they can contract involuntarily through Reflex action. The whole muscle is wrapped in a special type of connective tissue, epimysium....
 and cardiac muscle
Cardiac muscle

Cardiac muscle is a type of involuntary sarcomere muscle found in the walls of the heart, specifically the wikt:myocardium. Cardiac muscle cells are known as cardiac myocytes ....
, but not smooth muscle
Smooth muscle

Smooth muscle is a type of non-striated muscle, found within the tunica media layer of large and small arteries and veins, the urinary bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, the ciliary muscle, and iris of the eye....
.

Discussions of troponin often pertain to its functional characteristics and/or to its usefulness as a diagnostic marker for various heart disorders.

Function

Troponin is attached to the protein tropomyosin
Tropomyosin

Tropomyosin is an actin-binding protein that regulates actin mechanics. It is important, among other things, for muscle contraction. Tropomyosin, along with the troponin complex, associate with actin in muscle fibers and regulate muscle contraction by regulating the binding of myosin....
 and lies within the groove between actin
Actin

Actin is a Globular_protein, roughly 42-kDa protein found in all Eukaryote where it may be present at concentrations of over 100 ?M. It is also one of the most highly-Conservation proteins, differing by no more than 20% in species as diverse as algae and humans....
 filaments in muscle tissue. In a relaxed muscle, tropomyosin blocks the attachment site for the myosin
Myosin

Myosins are a large family of motor proteins found in eukaryotic Biological tissue. They are responsible for actin-based motility.Following the discovery, by Pollard and Korn, of enzymes with myosin-like function in Acanthamoeba, a large number of divergent myosin genes have been discovered throughout eukaryotes....
 crossbridge, thus preventing contraction. When the muscle cell is stimulated to contract by an action potential
Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
, calcium channels open in the sarcoplasmic reticulum and release calcium into the sarcoplasm
Sarcoplasm

The Sarcoplasm of a muscle fiber is comparable to the cytoplasm of other cells fluid of the muscle, but it houses unusually large amounts of glycosomes and significant amounts of myoglobin, an oxygen binding protein....
. Some of this calcium attaches to troponin, causing a conformational change that moves tropomyosin out of the way so that the cross bridges can attach to actin and produce muscle contraction.

Troponin is found in both skeletal muscle
Skeletal muscle

They generally contract voluntarily , although they can contract involuntarily through Reflex action. The whole muscle is wrapped in a special type of connective tissue, epimysium....
 and cardiac muscle, but the specific versions of troponin differ between types of muscle. The main difference is that the TnC subunit of troponin in skeletal muscle has four calcium ion binding sites, whereas in cardiac muscle there are only three.

Physiology

Both cardiac and skeletal muscles are controlled by changes in the intracellular calcium
Calcium in biology

Calcium plays a pivotal role in the physiology and biochemistry of organisms and the cell . It plays an important role in signal transduction pathways, where it acts as a second messenger, in neurotransmitter release from neurons, contraction of all muscle cell types, and fertilization....
 concentration. When calcium rises, the muscles contract, and when calcium falls the muscles relax.

Troponin is a component of thin filaments (along with actin
Actin

Actin is a Globular_protein, roughly 42-kDa protein found in all Eukaryote where it may be present at concentrations of over 100 ?M. It is also one of the most highly-Conservation proteins, differing by no more than 20% in species as diverse as algae and humans....
 and tropomyosin
Tropomyosin

Tropomyosin is an actin-binding protein that regulates actin mechanics. It is important, among other things, for muscle contraction. Tropomyosin, along with the troponin complex, associate with actin in muscle fibers and regulate muscle contraction by regulating the binding of myosin....
), and is the protein to which calcium binds to accomplish this regulation. Troponin has three subunits, TnC, TnI, and TnT. When calcium is bound to specific sites on TnC, tropomyosin
Tropomyosin

Tropomyosin is an actin-binding protein that regulates actin mechanics. It is important, among other things, for muscle contraction. Tropomyosin, along with the troponin complex, associate with actin in muscle fibers and regulate muscle contraction by regulating the binding of myosin....
 rolls out of the way of the actin filament active sites, so that myosin
Myosin

Myosins are a large family of motor proteins found in eukaryotic Biological tissue. They are responsible for actin-based motility.Following the discovery, by Pollard and Korn, of enzymes with myosin-like function in Acanthamoeba, a large number of divergent myosin genes have been discovered throughout eukaryotes....
 (a molecular motor organized in muscle thick filaments) can attach to the thin filament and produce force and/or movement. In the absence of calcium, tropomyosin interferes with this action of myosin, and therefore muscles remain relaxed.

Troponin I has also been shown to inhibit angiogenesis
Angiogenesis

Angiogenesis is a physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over this, vasculogenesis is the term used for spontaneous blood-vessel formation, and Intussusception is the term for new blood vessel formation by splitting off existing ones....
 in vivo
In vivo

In vivo means that which takes place inside an organism. In science, in vivo refers to experimentation done in or on the living tissue of a whole, living organism as opposed to a partial or dead one or a in vitro....
 and in vitro
In vitro

In vitro refers to the technique of performing a given procedure in a controlled environment outside of a living organism. Some may argue that in vitro refers to a process that is created in a "test tube"; however, Robert Kail and John Cavanaugh on page 58 in the 4th edition of Human Development: A Life-Span View cite that in fact th...
.

Individual subunits serve different functions:
  • Troponin C
    Troponin C

    Troponin C is a part of the troponin complex. It contains four calcium-binding EF hand. It contains an N lobe and a C lobe. The C lobe serves a structural purpose and binds to the N domain of TnI....
     binds to calcium ions to produce a conformational change in TnI
  • Troponin T
    Troponin T

    Troponin T is a part of the troponin complex. It binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex.The tissue specific subtypes are:...
     binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex
  • Troponin I
    Troponin I

    Troponin I is a part of the troponin complex. It binds to actin in thin myofilaments to hold the troponin-tropomyosin complex in place. The letter I is given due to its inhibitory character....
     binds to actin in thin myofilaments to hold the troponin-tropomyosin complex in place


Diagnostic use

Troponin levels can be used as a test of several different heart disorders, including myocardial infarction
Myocardial infarction

Myocardial infarction , commonly known as a heart attack, occurs when the Blood flow to part of the heart is interrupted. This is most commonly due to occlusion of a coronary artery following the rupture of a Vulnerable plaque, which is an unstable collection of lipids and white blood cells in the wall of an artery....
.

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