Biorecognition elements, "In the search for the perfect match"

Hi all and welcome to a new FoodSmartphone post. You may remember that in my last blog I have introduced biosensors definition as well as some applications related with these powerful devices. This time i will in explain in detail some essential components of biosensors that were briefly mentioned in the last opportunity. I’m sure that you have heard about antibodies, DNA and Enzymes, but how these biological elements can be adapted to create reliable devices? 

To start this description is necessary to remember that biosensors are analytical devices that convert biological responses into electrical signals. The fundamental components on them are the biorecognition elements and the transducers, that’s why we will dedicate this blog to discuss about the most common bioelements used for the selective recognition event, to understand the working principles behind them.  

Principal components of a Biosensor

One of the most challenging aspects when develop biosensors, is that they must be able to discriminate a single analyte from a complex sample. The recognition event will take place using a specific bioreceptor (a biomolecule capable of recognizing the target analyte). For this purpose, plenty of different bioelements are used, like enzymes, antibodies, DNA/RNA and aptamers. It’s important to know that all of them present intrinsic advantages and disadvantages, that are adaptable to different approaches considering the type of matrix, analyte, transducer.

Enzymes

To start, enzymes were one of the first bioelements reported for sensing applications. These proteins with catalytic activity are capable to bind and react chemically with specific target molecules. Sometimes changes in their conformation (as a result of the activity) are used to observe the interaction events. The analyte is assessed directly by monitoring the formation of a product, or the transformation of a regent in the course of the enzymatic reaction.  It can be indirectly related to the signal generated by an optical indicator (optochemical transducer) witch responds to changes in the biocatalytic process.  A variety of enzymes belonging to families of oxido-reductases, hydrolases, and lyases have been coupled with different transducers to create biosensors.

Mechanism of action of enzymes

Antibodies

In the next group we find antibodies, probably the most widely used bioreceptors due to their high specificity and versatility. A complete and clear description about these macromolecules was given by our partner (ESR8) a few blogs ago, if you want to refresh some ideas. (https://foodsmartphone.blog/2019/10/18/where-do-antibodies-come-from/).

Also known as immunoglobulins, antibodies are proteins folded into well defined structures synthesized by living organisms like, humans, mices, rats and rabbits or even cells, as response of the presence of foreign substances. The immunogen is the molecule capable of eliciting an immune response by an organism immune system. Antigen is the molecule that is able to bind the product of that immune response: the antibody. These proteins can be produced against plenty of different targets, to recognize with high specificity the selected analyte in a complex matrix.

Basic structure of an antibody

Nucleic acids

DNA is the basic hereditary material in all cells and contains the information necessary to build proteins. This macromolecule is a linear polymer made up of nucleotides. There are two type of nucleic acids DNA and RNA differing in one nucleotide that in the first case is Thymine and in the other Uracil, and the presence of deoxiribose for DNA and ribose for RNA. Hybridization is the reaction that can occur between any single stranded nucleic acid chain: DNA/DNA, DNA/RNA, RNA/RNA.

These sequences can be attached to different type of lables, like fluorophores, dyes, intercalators to directly monitor the binding events. This use applies if the target analyte is a complementary nucleic acid molecule.  Another application is by molecular beacons, that are nucleic acid probes with a fluorescent moiety at one end and a quencher at the other and adopts either a hairpin conformation in the absence of the target or an extended conformation when hybridized to the target (Tyagi & Kramer, 1996).

Biomimetics

And last but not least the biomimetics, where we find Aptamers and Molecularly Imprinted Polymers (MIP’s). On one hand,  Aptamers are oligonucleotide molecules (ssDNA or RNA) that can bind with high selectivity and specificity to a wide range of other kind of molecules like peptides, proteins, drugs, inorganic molecules and also cells without the necessity of interaction with other nucleic acid.  They can be described as linear sequences of oligonucleotides with 30-40 nucleotides long. This short chains can adopt, complex three dimensional structures due to intramolecular base pairing interactions, to produce loops or bulges to bind the surface of the target molecule.

On the other hand, molecularly imprinted polymers corresponds to other group of biomimetics, these are defined as synthetic polymers with cavities of pre-determined selectivity that can be tailored to mimic the molecular recognition ability of biomolecules (antibodies, enzymes, etc). In order to produce them several steps are required, starting with the template molecule selection, followed by exposing it to functional monomers that will interact with the template fixing it during polymerization. The last step is to eliminate the template from the formed polymer, obtaining complementary binding sites in shape to the template structure.

To sum up, it’s clear that the remarkable advances in molecular and synthetic biology are leading to the progress in development of biotechnologies. In biosensors field, the main challenge remains the preservation and stability of biological components over time. As we could see a wide range of options are available to create biosensors, but with some information and a little of imagination, the possibilities are enormous.

Have a nice weekend,

Julian

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