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Structural View of Biology >> Enzymes >> Lyases - Breaking Chemical Bonds without Water

Structural View of Biology

Enzymes are Nature's chemists, performing all of the chemical transformations needed for life. Enzymes catalyze chemical reactions by bringing together all of the necessary chemical tools in the proper place. They typically have an "active site" that captures the chemicals that will be modified, holding them in the perfect orientation to perform the chemical change. Researchers have separated the many types of enzymes into a few functional classes, based on the reactions that they perform. Click on any of the sub-categories below to explore a few examples of each enzyme class. You can also explore many other enzymes in the other functional categories in "Structural View of Biology."

Lyases break molecules into two pieces, but without using water like the hydrolases. Often, the reactions performed by lyases form complicated products, with new double bonds or rings.

Scroll to a Molecule of the Month Feature in this subcategory:

  • Aconitase and Iron Regulatory Protein 1

    Aconitase and Iron Regulatory Protein 1

    Aconitase is an essential enzyme in the tricarboxylic acid cycle and iron regulatory protein 1 interacts with messenger RNA to control the levels of iron inside cells. You might ask: what do these two proteins have in common? They were discovered and studied by different researchers, who gave them names that described their two very different functions. But surprisingly, when they looked at the amino acid sequence of these proteins, they turned out to be identical. The same protein is performing two very different jobs.

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  • Carbonic Anhydrase

    Carbonic Anhydrase

    Breathing is a fundamental function in life - ever wondered what really happens when we breathe? The air we breathe in has precious oxygen that fuels the breakdown of sugars and fat in our cells. In our lungs, oxygen diffuses into the blood, binds to hemoglobin and is transported to all the cells of our body (see the Molecule of the Month feature on hemoglobin). Carbon dioxide is a byproduct of sugar and fat breakdown in cells and needs to be removed from our body. Again, blood acts as a transport medium. Carbon dioxide diffuses out of cells and is transported in blood in a few different ways: less than 10% dissolves in the blood plasma, about 20% binds to hemoglobin, while the majority of it (70%) is converted to carbonic acid to be carried to the lungs. An enzyme present in red blood cells, carbonic anhydrase, aids in the conversion of carbon dioxide to carbonic acid and bicarbonate ions. When red blood cells reach the lungs, the same enzyme helps to convert the bicarbonate ions back to carbon dioxide, which we breathe out. Although these reactions can occur even without the enzyme, carbonic anhydrase can increase the rate of these conversions up to a million fold.

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  • Citrate Synthase

    Citrate Synthase

    Your body burns up a lot of food every day. However, cells don't burn food like a fireplace. Instead, food molecules are combined with oxygen molecules one-by-one, in many carefully controlled steps. In this way, the energy that is released can be captured in convenient forms, like ATP or NADH, which are then used elsewhere to power essential cellular functions. Our cells get most of their energy from a long series of reactions that combine oxygen and glucose, forming carbon dioxide and water, and creating lots of ATP and NADH in the process.

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  • Rubisco


    Carbon is essential to life. All of our molecular machines are built around a central scaffolding of organic carbon. Unfortunately, carbon in the earth and atmosphere is locked in highly oxidized forms, such as carbonate minerals and carbon dioxide gas. In order to be useful, this oxidized carbon must be "fixed" into more organic forms, rich in carbon-carbon bonds and decorated with hydrogen atoms. Powered by the energy of sunlight, plants perform this central task of carbon fixation. Inside plant cells, the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco) forms the bridge between life and the lifeless, creating organic carbon from the inorganic carbon dioxide in the air. Rubisco takes carbon dioxide and attaches it to ribulose bisphosphate, a short sugar chain with five carbon atoms. Rubisco then clips the lengthened chain into two identical phosphoglycerate pieces, each with three carbon atoms. Phosphoglycerates are familiar molecules in the cell, and many pathways are available to use it. Most of the phosphoglycerate made by rubisco is recycled to build more ribulose bisphosphate, which is needed to feed the carbon-fixing cycle. But one out of every six molecules is skimmed off and used to make sucrose (table sugar) to feed the rest of the plant, or stored away in the form of starch for later use.

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