Enzymes and catalysts both affect the rate of a reaction. In fact, all known enzymes are catalysts, but not all catalysts are enzymes. The difference between catalysts and enzymes is that enzymes are largely organic in nature and are bio-catalysts, while non-enzymatic catalysts can be inorganic compounds. Neither catalysts nor enzymes are consumed in the reactions they catalyze.
For simplicity, catalyst in this article refers to non-enzymatic catalysts to easily differentiate from enzymes.
|Function||Catalysts are substances that increase or decrease the rate of a chemical reaction but remain unchanged.||Enzymes are proteins that increase rate of chemical reactions converting substrate into product.|
|Molecular weight||Low molecular weight compounds.||High molecular weight globular proteins.|
|Types||There are two types of catalysts – positive and negative catalysts.||There are two types of enzymes – activation enzymes and inhibitory enzymes.|
|Nature||Catalysts are simple inorganic molecules.||Enzymes are complex proteins.|
|Alternate terms||Inorganic catalyst.||Organic catalyst or bio catalyst.|
|Reaction rates||Typically slower||Several times faster|
|Specificity||They are not specific and therefore end up producing residues with errors||Enzymes are highly specific producing large amount of good residues|
|Conditions||High temp, pressure||Mild conditions, physiological pH and temperature|
|C-C and C-H bonds||absent||present|
|Example||vanadium oxide||amylase, lipase|
|Activation Energy||Lowers it||Lowers it|
A Brief History of Catalysts, Enzymes and Catalysis
Catalysis reactions have been known to humans for many centuries but they were unable to explain the occurrences they were seeing all around them like, fermentation of wine to vinegar, leavening of bread etc. It was in 1812 that Russian chemist Gottlieb Sigismund Constantin Kirchhof studied the breakdown of starch into sugar or glucose in boiling water in presence of few drops of concentrated sulphuric acid. The sulphuric acid remained unchanged after the experiment and could be recovered. In 1835 Swedish chemist Jöns Jakob Berzelius proposed the name ‘catalysis’ from the Greek term, ‘kata’ meaning down and ‘lyein’ meaning loosen.
Once catalysis reactions were understood, scientists discovered many reactions that changed rates in presence of catalysts. Louis Pasteur discovered that there was some factor that catalyzed his sugar fermentation experiments and which was active only in living cells. This factor was later termed as ‘enzyme’ by German physiologist Wilhelm Kühne in 1878. Enzyme comes from Greek word meaning ‘in leaven’. In 1897, Eduard Buchner named the enzyme that fermented sucrose as zymase. His experiments also proved that enzymes could function outside a living cell. Eventually structure and function of various enzymes catalyzing important functions were discovered.
Structure of Catalysts and Enzymes
A catalyst is any substance that can cause significant alterations to the rate of a chemical reaction. Thus it could be a pure element like nickel or platinum, a pure compound like Silica, Manganese Dioxide, dissolved ions like Copper ions or even a mixture like Iron-Molybdenum. The most commonly used catalysts are proton acids in hydrolysis reaction. Redox reactions are catalyzed by transition metals and platinum is used for reactions involving hydrogen. Some catlaysts occur as precatalysts and get converted to catalysts in the course of reaction. The typical example is that of Wilkinson’s catalyst – RhCl(PPh3)3 which loses one triphenylphosphine ligand while catalyzing the reaction.
Enzymes are globular proteins and can consist of 62 amino acids (4-oxalocrotonate) to a size of 2,500 amino acids (fatty acid synthase). There also exists RNA based enzymes called ribozymes. Enzymes are substrate specific and usually are larger than their respective substrates. Only a small part of an enzyme takes part in a enzymatic reaction. The active site is where substrates bind to enzyme for facilitating the reaction. Other factors like co factors, direct products, etc also have specific binding sites on enzyme. Enzymes are made of long chains of amino acids that fold over each other giving rise to a globular structure. The amino acid sequence gives enzymes their substrate specificity. Heat and chemical can denature an enzyme.
Differences in Mechanism of Reactions
Both catalysts and enzymes lower the activation energy of a reaction thereby increasing its rate.
A catalyst can be positive (increasing reaction rate) or negative (decreasing reaction rate) in nature. They react with reactants in a chemical reaction to give rise to intermediates that eventually release the product and regenerate the catalyst. Consider a reaction where
C is a Catalyst
A and B are reactants and
P is the Product.
A typical catalytic chemical reaction would be:
A + C → AC
B + AC → ABC
ABC → PC
PC → P + C
The catalyst is regenerated in the last step even though in the intermediate steps it had integrated with reactants.
Enzymatic reactions occur in many ways:
- Lowering of activation energy and giving rise to a stable transition state usually achieved by distorting shape of substrate.
- Lowering of transition state energy without distorting substrate.
- Temporary formation of enzyme substrate complex and thereby providing an alternate pathway for reaction to proceed.
- Reducing reaction entropy.
- Increasing temperature.
The mechanism of enzymatic action follows the induced fit model as suggested by Daniel Koshland in 1958. According to this model, substrate is molded into the enzyme and there can be slight changes in shape in enzyme and substrate as the substrate binds itself at the active site of enzyme to form the enzyme substrate complex.
Examples of Catalyst- and Enzyme-aided Reactions
A catalytic converter used in cars is a device that removes gases causing pollution from car exhaust systems. Platinum and Rhodium are the catalysts used here which break down dangerous gases into harmless ones. For e.g. nitrogen oxide is converted into nitrogen and oxygen in presence of small amount of Platinum and Rhodium.
The enzyme amylase aids in digestion of conversion of complex starch into more easily digestible sucrose.
Catalysts are used in energy processing; bulk chemicals production; fine chemicals; in the production of margarine and in the environment where they play a critical role of chlorine free radicals in the breakdown of ozone.
Enzymes are used in food processing; baby foods; brewing; fruit juices; dairy production; starch, paper and bio fuel industry; make-up, contact lens cleansing; rubber and photography and molecular biology.
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