Cooperativity

Cooperativity, a term used in biochemistry, characterized the function of transport proteins ( such as receptors), and enzymes, which consist of several sub-units ( so called " oligomeric protein ").

Definition

Proteins from several similar subunits often show the phenomenon of cooperativity: the binding strength of a ligand depends on how many of the remaining subunits are already bearing a ligand. If the bond increasingly stronger results in the well-studied phenomenon of positive cooperativity. The ligands interfere with each other, so that the last binding sites are occupied with low affinity, follows the lesser known ( but equally frequent ) phenomenon of negative cooperativity. The first case acts as a switch (proteins with ligands at all binding sites and proteins without ligands dominate the scene, partially saturated proteins are under-represented ). In the second case, the degree of binding of the proteins is more uniform and is dependent on the ligand concentration.

Prototype of an allosteric protein with positive cooperativity is composed of four subunits ( tetrameric ) hemoglobin. In contrast to the monomeric oxygen - carrier protein myoglobin, hemoglobin binds except oxygen ( O2) or protons (H ), carbon dioxide ( CO2) and chloride ions. The binding properties of oxygen to the tetramer ( which is shown in the figure simply as dimer) are modulated varied:

The binding of O2 favors the binding of other O2 molecules to the same hemoglobin molecule;
The bond strength of O2 is pH-dependent. Addition to an increasing concentration of hydrogen ions is also an increasing CO2 concentration promotes O2 delivery. Exactly those signals used by the muscle to signal its own oxygen
Finally, the release of oxygen or by a special regulator molecule - in people, this is 2,3- bisphosphoglycerate (2,3- BPG; circuit on the T- state) - promoted. The regulation of oxygen supply by 2,3- BPG is for the supply of a fetus and for the supply of man under the conditions of high altitudes ( Bergsteiger! ) of paramount importance

To enforce all of these ligands (up to O2 itself) the delivery of oxygen, that is, the transition of the high-affinity R- conformation in the low-affinity T conformation. The following figure shows the principle of this regulation: the conformational transition of T ( tense ) to R ( relaxed ) is triggered by the oxygen pulls the central iron atom ( Fe , brown circle) in the heme (red bar ) in the plane. Other groups of proteins amino acid residues follow this movement, which hydrogen bonds broken and the so-called " Bohr protons " (H ) and CO2 are released.

Detection and description

Cooperative binding proteins ( carriers, receptors, enzymes ) does not follow the principle of Sättigungshyperbel: they have either a " sigmoid " (positive cooperativity ) or a " pseudohyperboles " binding behavior (negative cooperativity ) on. Was only due to the inconspicuous, resembling a hyperbola characteristic and the negative cooperativity often overlooked.

The binding curves

The phenomenon of cooperativity can be determined by Adair described by functions, which two Km values , K ( 1) and K (2 ), are responsible. These complex functions can be turn as a transition between two Sättigungshyperbeln (1 and 2, ie black curves in the figure below ) to understand:

Positive cooperativity: the ground state ( T) bond initially according to the hyperbola 1 (low affinity). Then bind the transition happens in the high-affinity state ( R ), so that other ligands according to the hyperbola 2. The transition between these states corresponds to the red line, which has a substantially sigmoid shape and is easy to diagnose. The curve shown corresponds to a Hill coefficient of 1.92;
Negative cooperativity: to understand than the reverse transition between a state of high affinity ( Km small value; hyperbola 2) to such a low affinity ( high Km value; hyperbola 1). The transition between these states corresponds to the blue line, which seems a hyperbola to the same, but is different therefrom: it is characterized by a Hill coefficient nH = 0.63.

Negative cooperativity was first discovered for the binding of NAD by glyceraldehyde 3- phosphate dehydrogenase ( GAPDH).

Halbseitenreaktivität (English half- of-the -sites reactivity ) extreme form of negative cooperativity, wherein an oligomeric enzyme with 2N binding sites ( almost ) exclusively at n binding site (s ) reacts with the substrate.

Linearizations

Clear instructions on cooperativity is found when subjecting the binding curves described in " Enzyme Kinetics " " linearization method ": arise here as characteristic deviations from a straight line - particularly evident in the case of the Scatchard plot. These deviations are most likely leveled at Hill - diagram but would have end- branches of 45 ° ( slope = 1) when the upper and lower range of substrate concentrations were available measurement points. Only in the case nH = n ( maximum cooperativity ) this would account:

The Hill coefficient, nH, describes the degree of cooperativity and can not exceed the number of interacting subunits ( n ). For hemoglobin ( n = 4) is nH = 2.9, derived from the slope at the zero crossing.
For negative cooperativity is nH <1

The best determination of cooperativity takes place today in the way of " nonlinear regression " using

The Hill equation, i.e. an extended Michaelis -Menten equation: parameters nH and K50 (substrate concentration is reached at 50 % saturation);
The Adair equation parameters and KR KT ( dissociation constant for the "T" and the "R- state"). The examples below saturation functions were for the values