Biocatalysts refer to proteins that drive all non-spontaneous chemical reactions in any biological system.
Also known as enzymes, they are sensitive to temperature and pH, and act only on their specific reactants, unlike inorganic catalysts. However, their activities can be modulated and controlled. Since enzymes are also active in vitro and are organic in nature, they have been isolated, modified, and used in many industries.
What do Catalysts do?
A non-spontaneous chemical reaction can only happen when the reactant molecules have obtained a sufficient amount of energy, termed activation energy. The sooner the reactants acquire the activation energy, the faster they can turn into their unstable transition state and begin to transform into (a) product(s).
Thus, chemical reactions possessing high activation energy will require a tremendous amount of energy for their reactants to transform into products. In such cases, extra heat (thermal energy) or pressure (kinetic energy) is often applied so that the expected product(s) are generated in time.
Rather than putting in more energy to meet the activation energy, catalysts can be added to certain chemical reactions so that their activation energy is lowered. At the initiation of a catalyzed reaction, catalysts spontaneously interact with molecules of the reactants and encourage their transformation into products.
Towards the end of the reaction, catalysts are released from the catalyst-product complex and are reused in the next round. When the reaction reaches the reactant-product equilibrium, the amount of the catalyst that remains in the system is not different from the start of the reaction.
In other words, catalysts guide the reactants to take a detour to avoid the high hill of activation energy. The detour is typically more complex, but the time it takes to reach the destination is far shorter.
Enzymes are Natural Catalysts with High Turnover Rate
Catalysts exist in many forms and are either readily available in nature or man-made. They can be in the same phase as the reactant (homogeneous catalysis) or a different phase (heterogeneous catalysis). They can exist as small molecules such as ions, radicals, metal atoms, organometallic compounds, and complexes, or as large and complex molecules like proteins, metal crystals, or porous solid surfaces.
Biocatalysts refer to enzymes, which are proteins that catalyze any chemical reaction that takes place in a living cell of any organism. Similar to inorganic catalysts, enzymes expedite the rate of chemical reactions by reducing the activation energy but at a higher turnover rate.
Because enzymes are fundamentally proteins that function in living organisms, most of their activities are optimal in the aqueous phase, in a condition that resembles the organism’s natural state, which is sensitive to changes in temperature and pH.
Biocatalysis activity is specifically induced and inhibited
One of the most unique features of biocatalysts is the specificity in the initiation and inhibition of their catalytic activities. This unique feature stems from the three-dimensional structure of enzymes, which are built up from intramolecular interactions between the amino acid species that are assembled into proteins.
Most amino acids in enzymes contribute to their geometric form, while only a few serve as residues at the catalytic center. Also known as the active site, the catalytic center is a small site on the enzyme where enzyme-reactant and enzyme-product interactions take place.
The shape of the active site resembles a small pocket or a narrow channel, which allows only reactants, referred to as substrates, that are complementary to the active site to access. Thus, only the intended reactant(s) can interact with the functional residues at the active site to initiate the enzyme’s catalytic activity.
Enzymes are said to be absolute specific when they recognize only a particular substrate and group-specific when they recognize molecules that have a specific functional group.
Similar to the initiation of biocatalysis, enzymatic activity can be inhibited when a molecule with a similar conformation to the substrate competes with the substrate and successfully binds to the active site.
Reactions catalyzed by biocatalysts can be regulated
Other than substrate specificity, the regulation of biocatalytic activity is another hallmark of biocatalysts. Again, this unique feature is also due to the structural dimension of proteins.
For example, certain enzymes may require interactions with a specific cofactor or coenzyme for their activity. When bound to a cofactor or coenzyme, the enzyme alters its structure into an active form so that catalytic activity can be achieved.
Along the same line, some enzymes can specifically bind to a small molecule, termed ligand, at their non-catalytic binding site, the allosteric site. Enzyme-ligand binding influences the enzyme conformation, which can result in catalytic enhancement if the ligand is an activator or the suppression of its catalytic activity if the ligand is an inhibitor.
The followings compare the main differences between enzymes and synthetic catalysts:
|1. Natural enzymes/proteins||1. Non-enzymatic catalysts, e.g., ions, metal atoms, or solid surface|
|2. Sensitive to temperature and pH changes||2. Are less sensitive to temperature and pH change|
|3. Catalyze only when interacting with a specific reactant (substrate)||3. Catalyze various reactants|
|4. Can be specifically regulated||4. Cannot be regulated|
Biocatalysts use one or more of the following mechanisms to catalyze chemical reactions:
1. Catalysis Through Proximity and Orientation Effects
Biocatalysts can facilitate enzymatic reactions when they form weak and transient bonds with their substrates. The formation of an enzyme-substrate bond brings all substrates in the reaction into contact, especially if the reaction is composed of more than one. The enzyme-substrate bond also arranges the substrates in the right order and the orientation that they have the most reactive chirality.
2. Acid-Base Catalysis
In acid-base catalysis, the functional group in the active site can act as catalytic acids or bases. As catalytic acids in a general acid-catalyzed reaction, the enzyme’s catalytic group donates protons to the substrate, while in general base catalysis, the catalytic group receives protons from the substrate.
Both acid and base catalysis enhance the reactivity of the substrate functional group, stabilize the transition state and facilitate bond cleavage. Many enzymes contain many functional residues which act as acids at one side of the substrate and simultaneously as bases at another. Such acid-base catalysis is termed concerted acid-base catalysis.
3. Covalent Catalysis
Also known as nucleophilic catalysis, covalent catalysis involves the formation of a transient covalent bond between the enzyme and its substrate. A nucleophilic group of the enzyme binds with an electrophilic group of the substrate, which results in an unstable enzyme-substrate complex that possesses an electrophilic catalytic center. After the electrons are withdrawn from the substrate, it’s transformed into a product, and the covalent bond between the enzyme and product is eliminated.
4. Metal Ion Catalysis
Metalloenzyme is a group of enzymes that predominantly use metal ion catalysis. They contain metal ion cofactors and are only active when they are bound to their cofactors. The positive charges in metal ions provide them with the ability to behave similarly to acids. Nevertheless, metal ion charges can be higher than +1 and do not shift the pH of the system regardless of their concentration. These characteristics allow metal ions to enhance the nucleophilicity of the system when they act as Lewis acids and accept electrons from the substrate even at neutral or basic pH.
Another way that metal ions participate in catalyzed reactions is to shield substrates or stabilize intermediates that are negatively charged. The shielding and stabilization of negatively charged species adjust the orientation and minimize any repulsing force from the substrates or intermediates. As a result, the activation energy of the reaction is reduced, and the transformation of substrates to products is favored, or vice versa.
5. Electrostatic Catalysis
The restricted shape of the enzyme active binding site creates an enclosed setting, to which only its complementary substrates and other highly similar molecules can gain access. In electrostatic catalyzed reactions, the binding of the enzyme to its substrate brings about charge distribution around the active site, which stabilizes the transition state or the intermediates of the reaction.
6. Preferential Binding of the Transition State Complex
In this type of catalysis, biocatalysts react more favorably to the transition state than their substrates or product(s). In essence, the enzymes have the highest affinity to the transition state, higher than compared to the substrates and products. This drives the reaction towards an increase in the concentration of the transition state, which in turn enhances the rate of the catalyzed reaction.
Examples of Biocatalysts Applications
Biocatalysts are active outside of living cells, given that all reaction components are present, and the temperature and pH condition is suitable for their catalytic activities. Due to enzyme specificity and high turnover rate, they have been applied across many industries. For instances:
- In consumer products, it’s estimated that the largest use of isolated enzymes goes to the use of ɑ-amylase and glucoamylase for the liquefaction and saccharification of starch. The two processes result in glucose, which is used as commodities in other products. For example, glucose is the substrate for glucose isomerase, which is used in the production of high-fructose corn syrup.
- In the pharmaceutical industry, the use of biocatalysts to acquire large-scale chiral compounds is one of the industry’s main focuses. Chirality is one of the essential determinants of whether a compound will be active or toxic. Since enzymes are specific by nature, they are capable of stereoselectivity. Lipases and transaminases are among the most frequently used biocatalysts to synthesize chiral compounds in the pharmaceutical industry.
- In research, one notable application of biocatalysts is in Polymerase Chain Reactions (PCR), regarded as the cornerstone of molecular biology. With PCR, millions of DNA fragments are synthesized in laboratories using DNA polymerase from the thermophilic bacteria, Thermus aquaticus. The technique has been credited as one of the breakthroughs that propel the biotechnology revolution.
All things considered; enzymes are similar to typical catalysts in that they expedite chemical reactions by redirecting them to the path with lower activation energy.
However, like proteins, enzymes are three-dimensional ‘natural’ catalysts, whose structure affords them selectivity and regulatory means, unlike any inorganic catalyst.
Enzymes as biocatalysts are highly discriminatory, in terms of their reactants and products, and can catalyze complex reactions with a high turnover rate, which makes them highly applicable in many industries.
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