Apoenzyme: Characteristics, Functions And Examples

An apoenzyme is the protein part of an enzyme, which is why it is also known as an apoprotein. The apoenzyme is inactive, that is, it cannot perform its function of carrying out a certain biochemical reaction, and it is incomplete until it binds to other molecules known as cofactors.

The protein part (apoenzyme) together with a cofactor form a complete enzyme (holoenzyme). Enzymes are proteins that can increase the speed of biochemical processes. Some enzymes need their cofactors to carry out catalysis, while others do not.

Apoenzyme

Main features

They are protein structures

Apoenzymes correspond to the protein part of an enzyme, which are molecules whose function is to act as catalysts for certain chemical reactions in the body.

They are part of the conjugated enzymes

Enzymes that do not require cofactors are known as simple ones, such as pepsin, trypsin, and urease. Instead, enzymes that require a particular cofactor are known as conjugated enzymes. These are made up of two main components: the cofactor, which is the non-protein structure; and the apoenzyme, the protein structure.

The cofactor can be an organic compound (eg, a vitamin) or an inorganic compound (eg, a metal ion). The organic cofactor can be a coenzyme or a prosthetic group. A coenzyme is a cofactor that is loosely bound to the enzyme and therefore can be easily released from the active site of the enzyme.

They admit a variety of cofactors

There are many cofactors that bind with apoenzymes to produce holoenzymes. Common coenzymes are NAD +, FAD, coenzyme A, vitamin B, and vitamin C. Common metal ions that bind with apoenzymes are iron, copper, calcium, zinc, and magnesium, among others.

The cofactors bind tightly or loosely with the apoenzyme to convert the apoenzyme to a holoenzyme. Once the cofactor is removed from the holoenzyme it is converted back to apoenzyme, which is inactive and incomplete.

Apoenzyme functions

Create holoenzymes

The main function of apoenzymes is to give rise to holoenzymes: apoenzymes bind with a cofactor and from this link a holoenzyme is generated.

Lead to catalytic action

Catalysis refers to the process through which some chemical reactions can be accelerated. Thanks to apoenzymes, holoenzymes are completed and are able to activate their catalytic action.

Examples

Carbonic anhydrase

Carbonic anhydrase is a crucial enzyme in animal cells, plant cells, and in the environment to stabilize carbon dioxide concentrations.

Without this enzyme, the conversion of carbon dioxide to bicarbonate – and vice versa – would be extremely slow, making it almost impossible to carry out vital processes, such as photosynthesis in plants and exhalation during respiration.

Hemoglobin

Hemoglobin is a globular protein present in the red blood cells of vertebrates and in the plasma of many invertebrates , whose function is to transport oxygen and carbon dioxide.

The binding of oxygen and carbon dioxide to the enzyme occurs at a site called the heme group, which is responsible for giving vertebrate blood its red color.

Cytochrome oxidase

Cytochrome oxidase is an enzyme that is present in most cells. It contains iron and a porphyrin.

This oxidizing enzyme is very important for energy production processes. It is found in the mitochondrial membrane where it catalyzes the transfer of electrons from cytochrome to oxygen, which ultimately leads to the formation of water and ATP (energy molecule).

Alcohol dehydrogenase

Alcohol dehydrogenase is an enzyme found primarily in the liver and stomach. This apoenzyme catalyzes the first step in alcohol metabolism; that is, the oxidation of ethanol and other alcohols. In this way, it transforms them into acetaldehyde.

Its name indicates the mechanism of action in this process: the prefix “des” means “no”, and “hydro” refers to a hydrogen atom. Thus, the function of alcohol dehydrogenase is to remove a hydrogen atom from the alcohol.

Pyruvate kinase

Pyruvate kinase is the apoenzyme that catalyzes the final step in the cellular process of glucose breakdown ( glycolysis ).

Its function is to accelerate the transfer of a phosphate group from phosphoenolpyruvate to adenosine diphosphate, producing one molecule of pyruvate and one of ATP.

Pyruvate kinase has 4 different forms (isoenzymes) in the different tissues of animals, each of which possesses particular kinetic properties necessary to match the metabolic requirements of these tissues.

Pyruvate carboxylase

Pyruvate carboxylase is the enzyme that catalyzes carboxylation; that is, the transfer of a carboxyl group to a pyruvate molecule to form oxaloacetate.

It catalyzes specifically in different tissues, for example: in liver and kidney it accelerates the initial reactions for the synthesis of glucose, while in adipose tissue and brain it promotes the synthesis of lipids from pyruvate.

It is also involved in other reactions that are part of carbohydrate biosynthesis.

Acetyl Coenzyme A carboxylase

Acetyl-CoA carboxylase is an important enzyme in the metabolism of fatty acids. It is a protein that is found in both animals and plants, presenting various subunits that catalyze different reactions.

Its function is basically to transfer a carboxyl group to acetyl-CoA to convert it into malonyl coenzyme A (malonyl-CoA).

It has 2 isoforms, called ACC1 and ACC2, which differ in their function and in their distribution in mammalian tissues.

Monoamine oxidase

Monoamine oxidase is an enzyme that is present in nervous tissues where it performs important functions for the inactivation of certain neurotransmitters, such as serotonin , melatonin and epinephrine.

Participates in biochemical degradation reactions of various monoamines in the brain . In these oxidative reactions, the enzyme uses oxygen to remove an amino group from a molecule and produce an aldehyde (or ketone), and the corresponding ammonia.

Lactate dehydrogenase

Lactate dehydrogenase is an enzyme found in the cells of animals, plants, and prokaryotes. Its function is to promote the conversion of lactate to pyruvic acid, and vice versa.

This enzyme is important in cellular respiration during which glucose, from food, is degraded to obtain useful energy for cells.

Although lactate dehydrogenase is abundant in tissues, levels of this enzyme are low in the blood. However, when there is an injury or illness, many molecules are released into the bloodstream. Thus, lactate dehydrogenase is an indicator of certain injuries and diseases, such as heart attacks, anemia, cancer, HIV, among others.

Catalase

Catalase is found in all organisms that live in the presence of oxygen. It is an enzyme that speeds up the reaction by which hydrogen peroxide breaks down into water and oxygen. In this way it prevents the accumulation of toxic compounds.

Thus, it helps protect organs and tissues from damage caused by peroxide, a compound that is continuously produced in numerous metabolic reactions. In mammals it is predominantly found in the liver.

References

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