Apoenzyme: what it is, characteristics and chemistry of how it works

From a biochemical point of view, a metabolic pathway or metabolic pathway is a succession of chemical reactions in which a substrate is transformed and gives rise to a final product (s), through a series of intermediate metabolites. Metabolic pathways are continually occurring in every cell in our body because, for example, glycolysis is an essential chemical process for cells to obtain energy.

Metabolic pathways are essential for maintaining our homeostatic balance, which means that each of our cells can survive by maintaining a stable internal composition. Enzymes, biochemical catalysts that accelerate the reactions that take place inside living beings, play an essential role here.

Enzymes act on substrates, which eventually become single or multi-step products. Although the functionality of these organic molecules is widely discussed, it is interesting to know that there is more than one type of enzyme, each with its own chemical and functional peculiarities. Here we tell you all about apoenzymes.

    What is an enzyme

    According to the Oxford Language Dictionary, an enzyme is defined as a soluble protein produced by the cells of the body, which promotes and regulates chemical reactions in living things.

    Although these molecules are protein in most cases, we must not forget that there are others produced based on RNA, ribozymes, the peculiarities that we leave for another occasion.

    Enzymes are biocatalysts, And we must stop succinctly in this term to continue with the concept which belongs to us here. A chemical reaction can be observed from both a thermodynamic and kinetic point of view, but in any case the most immediate is to indicate the change in free energy that occurs when the reaction takes place.

    A + B → C + D

    In order for A + B to turn into products, there must be activation energy, that is, the minimum amount of energy that a system needs before it can start a certain process, this “barrier”. that must be overcome. To achieve state activation (EA). Biocatalysts such as enzymes reduce this energy required for these reactions to occur through two mechanisms:

    • By attaching to the substrate (the initial substance), which weakens its chemical bonds and facilitates its rupture to end up giving rise to products.
    • Attract reactive compounds to its surface, which facilitates the general process.

    Therefore, enzymes act with both substrate specificity and action to know who to bind to or what type of reaction to initiate, respectively. It is clear that we have presented the term concisely, as the structure and functionality of enzymes provide enough information to write several books on the subject.

    What is an apoenzyme

    Once we have succinctly circumscribed the term enzyme, we are ready to move on to what is ours here. Enzymes are generally globular proteins composed of a concatenation of amino acids (almost all enzymes are larger than the substrates but only 3 to 4 specific amino acids are involved in the catalysis) soluble in water but, depending of their composition, two are distinguished.

    The enzymes used are made up of one or more protein chains, also called polypeptide chains. On another side, Holoenzymes are chemically more complex because they are made up of a protein part called an apoenzyme and a non-protein part called a cofactor..

    Holoenzyme = Apoenzyme + cofactor

    Let’s take a look at each of these components in detail below.

    1. Apoenzyme

    Therefore, an apoenzyme can be defined as the protein part of an enzyme (holoenzyme) which, to be active, must be linked to the corresponding cofactor, Also known as a coenzyme when it is an organic cofactor of a non-protein nature. It is also called an apoprotein.

    The apoenzyme, like the enzymes themselves, is a globular protein formed exclusively by an ordered sequence of amino acids, their simplest subunits. These amino acids exert the enzymatic capacity of the biomolecule but, as we said, only a few are involved in the catalysis on their own. Depending on their function, we can distinguish 4 types of amino acids in the apoenzyme:

    • Nonessential: they are not involved in catalysis by being, but are part of the structure of the apoenzyme. If they are removed, it does not lose its catalytic ability.
    • Structural: they are responsible for the three-dimensional structure of the apoenzyme.
    • Bonding or fixing: they establish volatile bonds with the substrate and guide it so that the reaction can take place.
    • Catalytic: the 3 or 4 amino acids of the whole structure with the catalytic function itself. They bind to the substrate via a covalent bond and weaken its structure, facilitating the reaction.

    These last two types of amino acids, catalytic and binder, form the active site, that is, the area of ​​the enzyme where the substrate binds to be catalyzed.. Enzyme-substrate coupling is such as Hermann Emil Fischer, a renowned German chemist of the 20th century, describes such a union as follows: “the substrate adapts to the active or catalytic center of an enzyme like the key to a lock” .

    Through this comparison, the key-lock complex has been used to explain enzyme action historically in schools and institutes, although in reality it is a much more versatile and adaptable mechanism.

      2. Cofactor

      To fully understand the holoenzyme (and therefore the apoenzyme), it is necessary to describe the cofactor, the non-protein component of it. There are basically two types of cofactors: metal ions and organic molecules, also called enzymes.. In the group of metal ions we find representatives such as Fe2 +, Cu2 +, K +, Mn2 +, Mg2 + and many others. Primarily, these ions generally act as the catalytic center itself or as stabilizers for the conformation of the holoenzyme.

      On the other hand, coenzymes are organic non-protein cofactors (because if they were composed of amino acids, they would be part of the chain of the enzyme itself). They are thermostable biological compounds which together with the apoenzyme make up the entire holoenzyme.It should be noted that the coenzyme-apoenzyme binding is not specific, as these organic cofactors can be different types of apoenzymes and the binding is usually temporary. Some examples of coenzymes are FAD (flavin adenine dinucleotide), coenzyme A and coenzyme Q. Sure, some of them look like you, don’t they?

      Finally, we emphasize that the basic mechanism of action of coenzymes can be summarized in the following points:

      • The coenzyme binds to the apoenzyme, forming the functional holoenzyme.
      • The enzyme captures its specific substrate, that is to say the “base” which will give rise to the products sought after the metabolic reaction.
      • The holoenzyme attacks the substrate, resulting in a compound with weak bonds which ultimately results in an unstable substance.
      • The enzyme transfers electrons from the substrate to the coenzyme during the formation of this unstable compound.
      • The coenzyme accepts these electrons and breaks away from the apoenzyme and moves to “leave” these electrons, thus returning to their initial state.

      Of course, these steps are stated in the most reductionist way possible, but the general idea is clear: 1 apoenzyme and cofactor, whether organic or inorganic, give rise to a holoenzyme, A biocatalyst that allows metabolic reactions in our body to occur more quickly.

      General summary

      So, we can summarize that the apoenzyme is the protein part of a holoenzyme, which makes up most of its three-dimensional chemical structure. In general, enzymes can be designed as biomolecules essential to life, due to their specificity, reversibility, efficiency, great catalytic power and permanence over time, they are capable of accelerating multiple chemical reactions which, without them, would be much slower and more expensive.

      With all this conglomerate of terminology, we want to put special emphasis on the fact that in order to know the functionality of a molecule, it is also necessary to know its chemical structure and its components. necessary for operation. Without the apoenzyme, the concept of a complex enzyme formed by compounds beyond proteins could not be understood.

      Bibliographical references:

      • Aguado Esteban, C. (2008). Research on specific therapies for inherited metabolic mutation: response to cofactors and antisense therapy.
      • Briceño, K. apoenzyme: characteristics, functions and examples.
      • Briceño, K. holoenzima: characteristics, functions and examples.
      • Monguí Aponte, LY, Hernández Guzmán, TD, and González Gómez, LF (2020). Teaching the learning of coenzyme and apoenzyme concepts associated with the study of enzyme activity: an overview of the problem-based learning model using the flipped classroom methodology.
      • Moreno, JCV (2016). Pyridoxal phosphate: mechanism of inhibition of paper as a cofactor and synthesis (Doctoral thesis, Complutense University).
      • Racó, LEC and Muñoz, LMM (2005). Soil enzymes: health and quality indicators. Colombian Biological Register, 10 (1), 5-18.
      • Soler-González, AS (1994). Basic topology of HMGCoA reductase by computer analysis of its sequences and study of the holoenzyme reconstituted in phospholipid vesicles (Doctoral thesis, University of Granada).
      • Theme 5: enzymes. Collected January 16 from http://www.edu.xunta.gal/centros/iespuntacandieira/system/files/05_Enzimas.pdf.

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