Your body runs thousands of chemical reactions every second, from digesting lunch to sending nerve signals. Most of them would barely happen at body temperature without help from specialized proteins called enzymes. Life depends less on which reactions are possible and more on how fast the body can make them go. A few important reactions run without any help, and some of these slowly damage the body over time.
Why Enzymes Make Body Chemistry Possible
Every reaction needs a starting push of energy before molecules will react.1AAT Bioquest. How do enzymes speed up a chemical reaction? For some reactions that push is so large that it acts like a wall, and they almost never happen on their own.2AAT Bioquest. How do enzymes speed up a chemical reaction? With that wall lowered by an enzyme, reactions run millions of times faster than they would otherwise.3American Chemical Society. Enzymes: Moving at the Speed of Life
An enzyme comes out of the reaction unchanged and can be used again and again. It doesn’t supply energy to the reaction, and it doesn’t change what the reaction produces. Most enzymes are proteins with a specific shape, and that shape is why each one handles only certain reactions. Heat or a large change in acidity can bend an enzyme out of shape and stop it working.
Enzymes lower the wall in three ways. Some bring the reacting molecules together instead of leaving them to bump into each other by chance.4AAT Bioquest. How do enzymes speed up a chemical reaction? Some steer the reaction down a different route that needs less energy.5AAT Bioquest. How do enzymes speed up a chemical reaction? And some hold the molecules in exactly the right position in a pocket on the enzyme.6AAT Bioquest. How do enzymes speed up a chemical reaction?
Turning Food Into Energy
The body stores the energy it gets from food as ATP, a small molecule that works as the cell’s energy currency. The first stage of getting it out of food is glycolysis. Each glucose molecule is split into two smaller molecules called pyruvate in a chain of 10 reactions.7Cold Spring Harbor Perspectives in Biology. Glycolysis This step makes 2 ATP and needs no oxygen.8Cold Spring Harbor Perspectives in Biology. Glycolysis So cells can make a little energy quickly even when oxygen runs short, as in hard exercise.
Glycolysis pulls out only a small share of the energy in glucose, because the pyruvate it produces still holds most of the original chemical energy. To get at the rest, pyruvate moves into the mitochondria, the cell’s power stations.9PubMed. Should the standard model of cellular energy metabolism be reconsidered? Possible coupling between the pentose phosphate pathway, glycolysis and extra-mitochondrial oxidative phosphorylation There it becomes acetyl-CoA, which can also come from breaking down fat.10PubMed Central. Mitochondrial electron transport chain: Oxidative phosphorylation, oxidant production, and methods of measurement The Krebs cycle breaks acetyl-CoA down and loads its energy onto carrier molecules called NADH and FADH2.11PraxiLabs. The Krebs Cycle A Step-by-Step Explanation
Those carriers feed the electron transport chain, which makes most of the cell’s ATP using oxygen. Oxygen is the final acceptor of electrons at the end of the chain. Without it the chain backs up and the carriers can’t unload.
During intense effort, the body uses ATP faster than the other pathways can replace it.12PubMed Central. Role of the phosphocreatine system on energetic homeostasis in skeletal and cardiac muscles Muscles keep a fast backup. They store phosphocreatine, which rebuilds ATP almost instantly.13PubMed Central. Role of the phosphocreatine system on energetic homeostasis in skeletal and cardiac muscles
The Chemistry of Digestion
Proteins, carbohydrates, and fats are too large to absorb, so they have to be broken into small pieces such as simple sugars.14Concepts of Biology. 15.3 Digestive System Processes The body splits them using water.15U.S. National Institutes of Health, National Cancer Institute. Introduction to the Digestive System On its own, this reaction is very slow, so digestive enzymes speed it up.16U.S. National Institutes of Health, National Cancer Institute. Introduction to the Digestive System These enzymes work in the mouth, stomach, pancreas, and first part of the small intestine.17Wikipedia. Digestive enzyme
Follow a sandwich through the process. Enzymes in saliva start breaking down the starch in the bread while you chew. Stomach acid and stomach enzymes begin breaking down the protein. In the small intestine, pancreatic enzymes finish the job on fats, proteins, and carbohydrates.
Fiber is the exception. It passes through undigested until bacteria in the colon ferment it.18PubMed Central. Short chain fatty acids and its producing organisms: An overlooked therapy for IBD? That fermentation makes short-chain fatty acids, mainly acetate, propionate, and butyrate.19PubMed Central. Health Benefits and Side Effects of Short-Chain Fatty Acids These fatty acids are the main fuel for the cells lining the colon.20Medicine in Microecology. Short-chain fatty acids (SCFAs) in gut health: Implications for drug metabolism and therapeutics
Getting Rid of Carbon Dioxide and Keeping pH Steady
Red blood cells contain an enzyme called carbonic anhydrase that turns carbon dioxide and water into bicarbonate and acid.21RCSB: PDB-101. PDB101: Molecule of the Month: Carbonic Anhydrase When the blood reaches the lungs, the same enzyme turns bicarbonate back into carbon dioxide, which you breathe out.22RCSB: PDB-101. PDB101: Molecule of the Month: Carbonic Anhydrase The reaction can happen without the enzyme, but the enzyme makes it up to a million times faster.23RCSB: PDB-101. PDB101: Molecule of the Month: Carbonic Anhydrase
The same reaction mops up excess acid quickly, which keeps the body’s pH-sensitive processes stable.24PubMed Central. Role of Carbonic Anhydrases and Inhibitors in Acid–Base Physiology: Insights from Mathematical Modeling Enzymes work only within a narrow band of acidity, and small shifts can change their shape, so blood pH has to stay in a tight range. The lungs and kidneys share the job of correcting the balance.
How the Body Breaks Down Drugs and Toxins
Most foreign chemicals are processed in two phases.25Deranged Physiology. Phase I and Phase II biotransformation reactions In phase one, liver enzymes called cytochrome P450 mainly oxidize the drug.26IntechOpen. Drug Metabolism: Phase I and Phase II Metabolic Pathways In phase two, the body attaches one of its own molecules, such as glucuronic acid or sulfate, to the drug.27IntechOpen. Drug Metabolism: Phase I and Phase II Metabolic Pathways This makes the drug dissolve more easily in water, usually makes it less active, and lets it leave the body, mostly in urine or bile.28IntechOpen. Drug Metabolism: Phase I and Phase II Metabolic Pathways
The liver is the main site because blood from the gut travels first to the liver through a dedicated vein. Most swallowed substances pass through it before reaching the rest of the body, and it is rich in the enzymes for both phases. This early pass can remove a large share of some oral drugs before they circulate, which is one reason doses differ between pills and injections.
Some drugs and foods can speed up or slow down the cytochrome P450 enzymes. That is why labels warn about mixing certain medicines, or taking some with grapefruit juice. Tell your doctor or pharmacist about everything you take, including supplements.
Building Proteins, DNA, and Hormones
The ribosome links amino acids together into proteins and speeds that bonding reaction about 10 million times.29PubMed. Mechanism of peptide bond formation on the ribosome Unusually, the working part of the ribosome is made of RNA rather than protein.30PubMed. Mechanism of peptide bond formation on the ribosome The recipe comes from DNA, which holds the instructions for each protein. Those instructions are copied into messenger RNA, which carries them to the ribosome. The ribosome reads them and joins amino acids in the right order, and that order decides how the protein folds and what it does.
DNA itself is copied by enzymes called DNA polymerases, which add one building block at a time to a growing strand.31biomath.nyu.edu. What Chemical Reactions Happen in the Human Body? Copying DNA is what lets cells divide, so the body can grow and repair itself.32Nature Reviews Chemistry. Diverse and contrasting functions of DNA polymerases The polymerases are fast but not perfect, and can insert a wrong building block now and then. Many can back up and remove a mismatched one, and separate repair systems check the copy afterward. Errors that slip through are mutations, which are usually harmless but can sometimes contribute to disease.
Body proteins are always being broken down and rebuilt.33PubMed Central. Historical and contemporary stable isotope tracer approaches to studying mammalian protein metabolism Gut organs replace about 50% of their protein a day, while muscle replaces only about 1 to 1.5%.34PubMed Central. Historical and contemporary stable isotope tracer approaches to studying mammalian protein metabolism Between meals, muscle breaks down more protein than it builds, and eating protein reverses this.35PubMed Central. Historical and contemporary stable isotope tracer approaches to studying mammalian protein metabolism The amino acids from food are the raw material for that rebuilding, so spreading protein across meals keeps it going through the day.
Enzymes make signaling molecules too. Every steroid hormone, including cortisol, estrogen, and testosterone, starts when an enzyme in the mitochondria turns cholesterol into pregnenolone.36PubMed Central. Cellular cholesterol delivery, intracellular processing and utilization for biosynthesis of steroid hormones Nerve chemicals such as acetylcholine and GABA are each made by a single enzyme step.37Foundations of Neuroscience. Small-Molecule Neurotransmitters
Reactions That Damage and Defend
Making energy produces reactive forms of oxygen, often called free radicals, as by-products. In the electron transport chain, electrons sometimes escape and react directly with oxygen, forming the superoxide that enzymes later handle. Free radicals are also used in small amounts as signals inside cells, so the goal is control rather than total elimination.
In larger amounts, these molecules can damage DNA, proteins, and fats, and can kill cells.38PubMed Central. Reactive oxygen species and antioxidant defense in human gastrointestinal diseases One enzyme turns the most common one, superoxide, into hydrogen peroxide.39PubMed Central. Reactive oxygen species and antioxidant defense in human gastrointestinal diseases Other enzymes turn hydrogen peroxide into water and oxygen.40PubMed Central. The Chemistry of Reactive Oxygen Species (ROS) Revisited: Outlining Their Role in Biological Macromolecules (DNA, Lipids and Proteins) and Induced Pathologies
The body’s main antioxidant defense is these enzymes, not food. Antioxidants from food, like those in fruit and vegetables, add a second, non-enzyme layer. Taking more antioxidant pills doesn’t automatically mean more protection.
Immune cells turn the same chemistry into a weapon. Some white blood cells use an enzyme to turn hydrogen peroxide and chloride into hypochlorous acid, the active ingredient in bleach.41PubMed. Measuring chlorine bleach in biology and medicine They use it to kill bacteria they have swallowed.42PubMed. Chlorination of bacterial and neutrophil proteins during phagocytosis and killing of Staphylococcus aureus But the same bleach can harm the body’s own tissue during inflammation.43PubMed. Measuring chlorine bleach in biology and medicine
Reactions That Happen Without Enzymes
Vision starts with light rather than an enzyme. When light hits the eye, a molecule called retinal changes shape.44LibreTexts Chemistry. What Chemical Reactions Happen in the Human Body That change in shape starts the signal that lets you see.
Sugar, meanwhile, slowly sticks to proteins. Glucose can react with proteins without any enzyme, the same browning reaction that happens when food is cooked.45Eurasian Journal of Medicine and Oncology. Non-enzymatic glycation of proteins: Mechanisms and roles in biological aging and the pathogenesis of metabolic and neurodegenerative diseases In the body this happens far more slowly. This forms permanent cross-links between proteins, called advanced glycation end products, that are linked to aging and disease.46Eurasian Journal of Medicine and Oncology. Non-enzymatic glycation of proteins: Mechanisms and roles in biological aging and the pathogenesis of metabolic and neurodegenerative diseases Long-term high blood sugar speeds this up. Collagen in skin and blood vessels lasts a long time, so it gradually builds up these cross-links, which contributes to stiffer tissues with age.
The A1C blood test relies on this. It measures how much sugar has stuck to hemoglobin, the oxygen-carrying protein in red blood cells, and so reflects average blood sugar over the past 2 to 3 months.
Bone, too, builds its own mineral crystals. It hardens as calcium and phosphate clump into tiny clusters that grow into crystals within a framework of collagen.47PubMed Central. Biomineralization of bone tissue: calcium phosphate-based inorganics in collagen fibrillar organic matrices