Enzymes: How They Work and What Affects Them

Enzymes are usually named after their substrate with the ending -ase, while sugars end in -ose. Lactase splits lactose, the sugar in milk, into glucose and galactose.

Lipase breaks down lipids (fats and oils), sucrase breaks down sucrose (table sugar), protease breaks down proteins such as gluten, and cellulase breaks down cellulose, the fiber in cotton and paper. Starch has no pair: it is broken down by amylase, which is not on the list.

The substrate first has to reach the active site. Once it is held there, its bonds are strained, it is turned into products, and the products leave, freeing the active site for the next substrate molecule.

The optimal temperature is where the curve reaches its peak, so the enzyme works fastest there. Above it, the rate drops quickly because the enzyme starts to lose its shape.

Cold slows enzymes down but does not destroy them, which is why food lasts longer in a fridge and vegetables are briefly boiled (blanched) before freezing. Above the optimum, the enzyme loses its shape, so the rate falls much more steeply than it rose.

Enzymes are adapted to where they work: human enzymes to body temperature, and Taq polymerase to the hot springs of Yellowstone, where Thermus aquaticus bacteria live. Because it survives near-boiling heat, it is used in PCR, a lab method that cycles up to about 95°C.

Hydrogen ions change the electric charges on amino acid side chains. The bonds that hold the active site in shape are disrupted, so the substrate no longer fits.

Proteases break down proteins: hemoglobin in blood and albumin in egg white. Olive oil needs a lipase, and red wine is colored by plant pigments, not proteins.

With little substrate, many active sites stand empty, so more substrate speeds things up. Once all active sites are busy, the enzyme is saturated: only more enzyme helps. As substrate runs out, the reaction slows down.

An enzyme only lowers the energy barrier, so the same products form sooner. The products and the total energy released depend on the reaction itself, not on the enzyme.

B vitamins are turned into coenzymes, small organic helpers that many enzymes need to work (niacin into NAD, riboflavin into FAD). They are reused over and over, so tiny amounts are enough.

This is feedback inhibition. When the product builds up, it switches off the start of its own pathway, so the cell does not waste resources; when the product runs low, the first enzyme works again.

In PKU, the enzyme that converts phenylalanine into tyrosine does not work. Phenylalanine then builds up in the blood and can damage the brain, so patients follow a low-phenylalanine diet.

Helicase unzips the double helix, DNA polymerase adds nucleotides to build each new strand, and DNA ligase seals the gaps between pieces of new DNA. Laying down short RNA starters has no pair: that is the job of primase, which is not on the list.

RuBisCO, which fixes carbon dioxide in photosynthesis, is thought to be the most abundant protein on Earth, and carbonic anhydrase is one of the fastest enzymes known. Many enzymes work outside the cells that made them, such as digestive enzymes in the gut.

While the substrate is held in the active site, enzyme and substrate form an enzyme-substrate complex. It lasts only until the products are released.