Allosteric Enzymes: Functions, Structure and Kinetics

The Allosteric enzymes Are organic chemical substances that are composed with a structure of four molecules, reason why its structure is said to be quaternary.

In sum, allosteric enzymes have more than one polypeptide chain and contain units in which catalysis is performed. These, in turn, also have the site of activity, ie chemical exchange, and for this reason they perform a substrate recognition.

Allosteric enzymes

In other words, allosteric enzymes are characterized by having more than two polypeptide chains, whose subunits have different properties: one isosteric, which is the active site itself, and one allosteric, where enzymatic regulation is performed.

The latter has no catalysis activity, but it can be linked to a modulation molecule which can act as a stimulus or impediment for the performance of enzyme activity.

Brief Introduction to Allosteric Enzymes

Allosteric enzymes have the important task of making digestion easier. As they penetrate the nucleus of molecules, these enzymes have the power to intervene in the metabolism of the organism, so they have the ability to cause it to absorb and excrete according to the biochemical needs that arise.

For this to be feasible, allosteric enzymes need to move the mechanisms through which their regulatory process is performed.

These enzymes are classified into two slopes: K and V. In both it is usually seen that their saturation curve is not typically that of a hyperbole, but that of an irregular shape that mimics the sigma of the Greek alphabet.

This of course means that its structure and kinetics are not at all equal to that of the michaelian enzymes and much less to that of the non-allosteric enzymes, since its substrate causes relevant variations and differences in the speed of the reaction.

The structure and kinetics of allosteric enzymes are directly associated with cooperative interactions, specifically those that are not covalent.

This assumption is based on the premise that the sigmoid curve, which is drawn as the substrate concentration rises, is related to the structural changes that occur with the enzymes.

However, this correlation is not always absolute and lends itself to misunderstandings in which certain peculiarities are omitted in this system.


Globally, allosteric enzymes are referred to as those molecules of organic origin, in which they can affect biochemical bonds between proteins and enzymes.

The action of these allosteric enzymes develops through an infiltration into the molecular nucleus, so within the organism it is responsible for digestive catalysis. Thanks to it, various processes related to the gastrointestinal tract are extended, especially in the management of metabolism.

Therefore, the primary function of allosteric enzymes is to facilitate digestion in the body. This happens because the process of links to which they are submitted allows to favor both the assimilation of nutrients and the elimination of waste in the structure of the organism.

For this reason, the catalysis of the digestive system is continuously developed in a balanced environment in which each modulator has a specific allosteric site.

In addition, allosteric enzymes are, from a metabolic perspective, those that get the enzymatic activity is controlled through fluctuations that are perceived at the stratum level.

The smaller the changes in the concentration of that substrate, the greater the changes in the activity of the enzymes, and vice versa.

On the other hand, the values ​​of the allosteric enzymes K can be increased with a minimal dose of the inhibition modulator.

It may happen that in their performance, allosteric enzymes are inhibited at the end of the metabolic process, something that happens in some multienzymatic systems (they have many types of enzymes), being much more if the cellular capacities are exceeded.

When this happens, allosteric enzymes are ensured that catalytic activity is decreased; Otherwise, the substrate causes enzyme activity to be activated rather than regulated.

Allosteric regulation

It is known as those cellular processes in which the enzymatic activity is regulated by an adjustment process. This is possible thanks to feedback that can be positive (ie activation) or negative (inhibition).

The regulation may occur in various ways, either at the organic scale (supracellular, above the cell), by signal transduction and by covalent modification of the enzymes.

Fixation of the substrate can normally occur in the active site when the inhibitor is not present.

However, if that allosteric center is occupied by the inhibitor, this first element changes in its structure and therefore the substrate can not be fixed.

The presence of sigmoid kinetics may suggest that there is a cooperative relationship in the substrate, but this is not always the rule, with exceptions (see section"Alosterism and Cooperativity: synonyms?"Below).

Structure and kinetics

Several of the polypeptides of the allosteric enzymes lack catalysis. In any case, they also have strategic and very specific sites in which a union and recognition of the modulator is made, so that a complex modulation enzyme can result.

This is due to the fact that its greater or lesser catalysis activity depends on the polarity of the modulator, ie depending on whether the modulator is negative (inhibition) or positive (activation) poles.

The place where this biochemical exchange, or rather the enzymatic interaction with the modulator, is properly known as an allosteric site.

It is here where their properties are maintained without the modulator undergoing alterations at the chemical level. However, the bond that the modulator has with the enzyme is not irreversible, rather the reverse; Can be undone. Consequently, it can be said that this process of allosteric enzymes is not immovable.

One characteristic that highlights the allosteric enzymes is that they do not correspond to the kinetic patterns that comply with the Michaelis-Menten principles.

In other words, experiments to date have shown that the bond between an allosteric enzyme and the modulators (regardless of their polarity) has a saturation curve that has no regular shape, but sigmoid, with curvature similar to Greek letter of sigma.

The differences in that sigmoid form are few, regardless of whether modulators were used (positive or negative) or not used at all.

In all cases, the rate of reactions of allosteric enzymes show a series of dramatic modifications whose substrate concentrations are lower in the case of negative modulators and higher in positive ones. In turn, they have intermediate values ​​when there are no modulators linked to the enzymes.

The kinetic behavior of allosteric enzymes can be described with two models: symmetric and sequential.

Symmetric model

In this model, an allosteric enzyme can be presented according to the conformations, which are tense and relaxed.

The subunits may be at one end or the other, since there is a balance that moves between the two states in which the negative modulators approach the tense conformation, while the relaxed one joins the substrates and activators.

Sequential model

This model has a different paradigm. Here there are also two conformations, but each can act independently, separately.

At this point there may be a rise or fall in the affinities of the biochemical bonds of the enzymes, with levels of cooperativity that may be activation or inhibition.

Structural changes are passed successively from one subunit to another, with a definite order.

Both the symmetric and the sequential model operate on their own, according to their own norms. However, both models can work together, so they are not mutually exclusive.

In these cases intermediate states are observed in which the conformations, that is to say the tense and the relaxed, participate in a process of cooperation in which the biochemical interactions of the allosteric enzymes are fused.

Alosterism and cooperativity: synonyms?

It has been believed that allosterism is the same as cooperativism, but this is not so. The confusion of both terms, it seems, comes from their functions.

However, it should be noted that this similarity is not enough for allosterism and cooperativism to be used as equivalent words. Both have subtle nuances to which attention must be paid before falling into misguided generalizations and categorizations.

It should be remembered that allosteric enzymes, by binding modulators, take a variety of forms. Positive modulators activate, while negative modulators inhibit.

In either case, a substantial change in the enzymatic structure occurs, at the active site, which in turn becomes the change of the same active site.

One of the most practical examples of this is seen in noncompetitive inhibition, in which the negative modulator binds to an enzyme other than the substrate.

However, the affinity of this enzyme in relation to the substrate may be diminished by that negative modulator of allosteric enzymes, so it can become a competitive inhibition regardless of whether the structure of the substrate is different from the structure of the enzyme.

Likewise, there may be an increase of said affinity or that instead of an inhibiting effect there is an inverse effect, ie an activation effect.

The phenomenon of cooperativism occurs in many of the allosteric enzymes, but this only comes to be classified as such when the enzymes have several places in which they manage to join the substrate, so they are called oligomeric enzymes.

In addition, the affinities are produced according to the level of concentration of the effector, and in them the positive modulators, the negatives and even the substrate itself act in a varied way throughout this process.

In order to produce this effect, several sites capable of binding to the substrate must be present, and the result appears graphically in the scientific studies as sigmoid curves, already referred to.

This is where the entanglement occurs because it tends to associate that if there is a sigmoid curve in the enzymatic analysis it is because the observed allosteric enzyme has to be necessarily cooperative.

In addition, one of the factors contributing to this entanglement is that the degree of cooperativity in the system is operated by allosteric effectors.

Their level may increase with the presence of the inhibitors, while it tends to decrease when the activators are present.

However, the kinetic only leaves its condition of sigmoidea when it becomes michaeliana in which the concentrations of the activator are elevated.

It is therefore clear that sigmoid curves may be antonyms of allosteric enzymes. Although most of these enzymes, when this substrate is saturated, have such a signal, it is false that there is an allosteric interaction only because a sigmoid kinetic curvature can be seen on the graph.

To suppose the reverse is also fallacious; The sigmoideo does not imply that one is faced with an express and unequivocal manifestation of allosterism.

A unique allosterism: hemoglobin

Hemoglobin is considered as a classic example of what happens with allosteric systems. In this component of the Red blood cells A substrate corresponding to the sigmoid type is fixed.

This binding can be inhibited through effectors in which there is no action on the active site, which is none other than the heme group. The michaelian kinetics, on the other hand, is presented in an isolated way in the subunits that participate in the fixation of oxygen.


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