Biochemistry Terms Starting With L

Lactate is the ionized form of lactic acid produced when pyruvate undergoes reduction by NADH during anaerobic glycolysis, regenerating the NAD+ needed for glycolysis to continue.

Muscle cells generate lactate during intense exercise when oxygen delivery cannot meet energy demands, a threshold typically reached during sprints lasting more than 10 seconds. Lactate dehydrogenase catalyzes this reversible reaction, and the direction it runs depends on the relative concentrations of pyruvate, lactate, NAD+, and NADH in the cell. Rather than being a metabolic dead end, lactate travels through the blood to the liver, where it is converted back to glucose via the Cori cycle, a pathway first described by Carl and Gerty Cori in the 1920s.

Cardiac muscle and slow-twitch skeletal fibers actively import and oxidize lactate as a preferred fuel during sustained activity.

Lactate dehydrogenase is an enzyme that catalyzes the reversible interconversion of pyruvate and lactate, coupled to the oxidation of NADH to NAD+ or the reduction of NAD+ to NADH depending on metabolic conditions.

This tetrameric enzyme exists in five isoforms in humans, designated LDH-1 through LDH-5, assembled from two subunit types called H and M in different combinations that reflect the metabolic priorities of each tissue. During intense exercise, human skeletal muscle produces up to 25 millimoles of lactate per kilogram of wet muscle per minute through LDH-5 activity, sustaining glycolysis when oxygen supply falls short of demand. Heart muscle expresses predominantly LDH-1, which favors the oxidation of lactate back to pyruvate, allowing cardiac cells to use circulating lactate as fuel.

Red-blooded Antarctic icefish (family Channichthyidae) completely lack functional lactate dehydrogenase in their cardiac tissue, an evolutionary adaptation possible only in the oxygen-rich, near-freezing waters of the Southern Ocean.

Ligand is a molecule or ion that binds reversibly to a specific site on a protein or other macromolecule through noncovalent interactions, altering the macromolecule's conformation or activity.

Ligands range from simple ions like calcium to large molecules such as hormones and synthetic drugs, and their binding specificity arises from complementary shapes and chemical properties between the ligand and its target site. The neurotransmitter acetylcholine binds to nicotinic receptors at the neuromuscular junction, triggering a conformational change that opens an ion channel and initiates muscle contraction in organisms from insects to mammals. Dissociation constants for strong ligand-receptor interactions typically fall between 1 nanomolar and 1 micromolar, meaning the receptor remains occupied even when the ligand is present at very low concentrations.

Competitive inhibitors of enzymes are a class of ligand that occupy the active site without being converted to product, raising the apparent Km without changing maximum velocity.

Lipase is an enzyme that catalyzes the hydrolysis of ester bonds in lipids, releasing glycerol and free fatty acids from triglycerides and other fat molecules.

Human pancreatic lipase can process up to 150 grams of dietary fat per day, secreting roughly 30,000 units of enzymatic activity during a single meal. The enzyme binds to the surface of fat droplets in the small intestine and works most efficiently at pH 8.0, where bile salts have already emulsified the fat into smaller particles that expose more surface area for catalysis. Hormone-sensitive lipase in adipose tissue performs a different role, mobilizing stored triglycerides during fasting or exercise by responding to hormonal signals rather than dietary input.

Bacillus subtilis produces thermostable lipases that remain active above 60°C, properties that make them valuable in industrial detergent formulations and biodiesel production.

Lipid is a hydrophobic or amphipathic biological molecule composed primarily of carbon, hydrogen, and oxygen that is insoluble in water but dissolves readily in nonpolar solvents such as chloroform or ether.

Lipids encompass structurally diverse molecules including triglycerides, phospholipids, steroids, and waxes, each with distinct biological roles ranging from energy storage to membrane architecture to hormonal signaling. The human body stores approximately 135,000 calories of energy as triglycerides in adipose tissue, roughly 80 times more energy than it holds as glycogen. Phospholipids form the bilayer of every cell membrane, with their hydrophilic head groups facing aqueous environments and their hydrophobic fatty acid tails sequestered in the interior.

Antarctic krill (Euphausia superba) synthesize wax esters, a specialized lipid class, to regulate buoyancy in deep ocean water and sustain metabolism during winter months when phytoplankton food sources disappear.

Lipid metabolism is the set of biochemical reactions by which cells break down, synthesize, and modify lipids, converting fatty acids and other lipid molecules into usable energy or incorporating them into structural and signaling compounds.

Beta-oxidation, the primary catabolic pathway, cleaves two-carbon acetyl units from fatty acid chains in the mitochondrial matrix, feeding them into the Krebs cycle. Each 16-carbon palmitic acid molecule yields approximately 106 ATP molecules when fully oxidized, a figure that exceeds the ATP output of an equivalent mass of glucose by roughly 2.4-fold. Adipose tissue in humans stores excess dietary lipids as triglycerides and mobilizes them through hormone-sensitive lipase when blood glucose drops below approximately 70 mg/dL, releasing free fatty acids into circulation.

During upstream migration, Pacific salmon draw on stored lipids so extensively that some populations consume more than 90% of their body fat reserves before reaching spawning grounds.

Lipoprotein is a biochemical assembly of proteins and lipids that transports water-insoluble fats through the bloodstream.

Humans produce five main classes of lipoproteins, ranging from high-density lipoproteins containing about 50% protein to chylomicrons with less than 2% protein content. Each particle consists of a hydrophobic core of triglycerides and cholesterol esters surrounded by a shell of phospholipids, free cholesterol, and apolipoproteins. These protein components determine which cells can recognize and absorb the lipoprotein cargo, giving each class a distinct metabolic destination.

Low-density lipoproteins deliver cholesterol to peripheral tissues, while high-density lipoproteins collect excess cholesterol and return it to the liver.

Luciferin is a light-emitting organic compound that produces bioluminescence when oxidized by the enzyme luciferase in the presence of oxygen.

Luciferase catalyzes the oxidation of luciferin, releasing energy as visible light rather than heat, a process that converts chemical energy to photons with roughly 90% efficiency. Different species have evolved chemically distinct luciferins: fireflies use a benzothiazole-based compound that produces yellow-green light peaking near 560 nanometers, while many deep-sea organisms use coelenterazine, which emits blue light near 480 nanometers. Researchers have exploited firefly (Photinus pyralis) luciferin as a reporter molecule in genetics laboratories since the 1980s, using it to track gene expression in living cells.