Types of Enzymes: An Introduction
But first, what is an enzyme? An enzyme is a substance – usually a protein – created by a living organism that acts as a catalyst to bring about life-sustaining biochemical reactions. Different types of enzymes have different classifications based on the kind of reactions they catalyze. Every organism – from the one-celled bacterium to the trillion-celled elephant – has many enzymes at work. Learning more about how enzymes work will help you understand basic chemistry and biology, which you can learn more about right here on Udemy.
The word “enzyme” was coined in 1878 by German scientist Wilhelm Kühne. It is derived from the Greek words for “leavened.” The root zyme means to leaven, as with dough or bread. (It also gives us the word zymurgy, which is not only often the last word in the dictionary but also the branch of chemistry that deals with wine-making and brewing.) Scientists in the 19th century, like Louis Pasteur, observed that some chemical reactions occurred with the addition of ferments, or living organisms like yeast. But they also noticed that some reactions occurred without the living organisms themselves but instead just some of their extracts. What catalysts made these reactions happen without the actions of a living cell? This lead to the discovery of enzymes.
The basic role of any enzyme is to increase the rate of biochemical reactions. Most reactions inside cells occur almost one million times faster with the aid of enzymes thanks to the lowered level of activation energy required. Enzymes typically react with just one substrate – that is, one specific molecule type. There can be thousands of enzymes at work in one organism, each one with a vital role.
For instance, when the enzyme rennin is added to milk curds form. Milk is made of the protein casein. Rennin acts on the casein. The larger peptide molecule in the casein is broken down into two polypeptides, thanks to the rennin. This breakdown is vital to young mammals, who need to derive their nutrition from their mother’s milk. Without this breakdown the milk would be digested too swiftly and the nourishing ability would be lost. The semi-solid curds sit in the stomach longer. This reaction of rennin and casein is how we get cheese!
Since enzymes are catalysts, they are not consumed by the reactions they bring about. Enzymes, however, are much more specifically targeted than most other catalysts. They may work only on specific substrates. Enzymes catalyze some 4000 known biochemical reactions.
All these known enzymes are classified according to six basic groups. These categories are organized according to how the enzyme works on a molecular level. They are important to your understanding of organic chemistry (learn more with this course). These six types of enzymes are as follows: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. Hydrolases are the most common type, followed by oxioreductases and transferases. They account for over half of the known enzymes.
Oxidoreductases catalyze oxidation or reduction reactions. These reactions involve the transfer of electrons from one molecule (the reductant) to another (the oxidant). These reactions are vital to life for their role in essential metabolic processes like glycolysis, which occurs in nearly every organism on the planet.
The transferase enzymes catalyze the transfer of a functional group (such as methyl) from one molecule to another. The first molecule is called the donor and the second molecule is called the acceptor. These transfer processes are some of the most basic and vital reactions in life.
The hydrolases bring about hydrolysis: this is the breaking of chemical bonds with the addition water. There is a wide variety of identified hydrolases, over 200 of them, from those that break down proteins to those that cleave ester bonds and more. Exohydrolase enzymes cut the molecules at the end of the chain, and endohydrolase enzymes do so in the middle of the chain.
Lysis reactions – those that generate a double bond – are brought about by lyase enzymes. Lysis reactions are the kind of elimination reactions that are not hydrolytic or oxidative. The lyases are also sometimes called synthase enzymes. A Michael addition – the reverse reaction – is also possible. However, two substrates are required for the reverse reaction to happen, whereas one substrate is required for the lysis reaction. This makes lyases unique among enzymes.
The isomerase enzymes catalyze structural changes within a molecule – this just brings about a change in shape since there is only one substrate and one product with nothing gained or lost. Within this category, there are a few sub-categories depending upon their effect. There are geometric, structural, enantiomer, and stereoisomer isomerases.
Ligation is brought about by ligase enzymes. Ligation occurs when two substrates are joined together. Chemical potential energy is usually required for this reaction to occur, so it is often paired with the hydrolysis of a diphosphate bond. DNA ligase – which catalyzes the ligation or repair of breaks in DNA – is an example of a vital enzyme in this category.
The work that enzymes do in making cellular activity – and all life – possible is one of the key concepts of biochemistry. Each living cell contains a multitude of biochemical actions. The chemical and physical changes that go on categorize something as organic life. The creation of new tissue – the replacement of old tissue – the conversion of food into energy – the disposal of waste – reproduction: these are all the characteristics of life.
Catalysis makes the crucial metabolic processes of life possible. Catalysts are substances that bring about the acceleration of chemical reactions but themselves remain unchanged. Enzymes are the catalysts that make the speed and processes of life possible.
For instance, the oxidation of a fatty acid into carbon dioxide and water is not easy to achieve in a test tube. Extreme levels of pH, high temperatures, and corrosive chemicals are required to make the reaction in a laboratory. The fatty acid protein in the lab is boiled for 24 hours in a hydrogen chloride solution to achieve the breakdown.
However, in the body these reactions happen regularly, and easily, without any of these extreme elements. This is thanks to enzymes. The breakdown takes about four hours without the pH extreme or high temperatures. This is how life is possible on planet Earth.
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