Biochemistry Terms Starting With K
Biochemistry Glossary: K
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Keto Acid
/ KEE-toe AS-id / · From Greek keto (ketone) + Latin acidus (sour)
Keto Acid keto acid is an organic compound that contains both a carboxylic acid group and a ketone carbonyl group, produced during amino acid metabolism through deamination or transamination reactions.
Keto acids are the carbon skeletons that remain after amino groups are removed from amino acids during metabolic processing. In humans, alpha-ketoglutarate, derived from glutamate, feeds directly into the citric acid cycle as a key intermediate, linking amino acid catabolism to cellular energy production. Roughly 20 distinct keto acids correspond to the 20 standard amino acids, and each can be converted into glucose through gluconeogenesis, oxidized for ATP production, or used to resynthesize amino acids when nitrogen donors are available.
Maple syrup urine disease, a genetic disorder affecting about 1 in 185,000 newborns, results from the inability to degrade branched-chain keto acids derived from leucine, isoleucine, and valine, causing toxic accumulation in blood and urine.
Alpha-ketoglutarate, a keto acid derived from glutamate, extends lifespan in Caenorhabditis elegans worms by up to 50% when supplemented in their diet, an effect linked to inhibition of ATP synthase and activation of longevity-associated signaling pathways.
Mitochondria Functions →Keto acids and ketone bodies are the same molecules. Keto acids contain both a carboxylic acid group and a ketone group and arise from amino acid metabolism, while ketone bodies such as acetoacetate and beta-hydroxybutyrate are distinct compounds produced from acetyl-CoA during fatty acid catabolism.
Pyruvate, a three-carbon keto acid, accumulates in yeast (Saccharomyces cerevisiae) cells at the end of glycolysis and is decarboxylated to acetaldehyde during alcoholic fermentation, ultimately yielding ethanol and carbon dioxide.
Fermentation Biology →Ketogenesis
/kee-toh-JEN-eh-sis/ · Greek keton (acetone) + genesis (origin, creation)
Ketogenesis is the metabolic process by which the liver converts acetyl-CoA molecules derived from fatty acid oxidation into ketone bodies that fuel tissues when glucose availability is low.
During prolonged fasting or carbohydrate restriction, the human liver produces approximately 150 grams of ketone bodies daily to supply the brain and heart with oxidizable fuel. Three enzymatic steps drive the process in liver mitochondria: two acetyl-CoA molecules condense to form acetoacetyl-CoA, which then converts to HMG-CoA before being cleaved into acetoacetate; acetoacetate is subsequently reduced to beta-hydroxybutyrate or spontaneously decarboxylated to acetone. Arctic ground squirrels (Urocitellus parryii) dramatically upregulate ketogenesis during their 7-month hibernation period, producing ketone bodies at rates roughly 5 times higher than during active months to preserve muscle protein while relying almost entirely on fat stores.
Insulin suppresses ketogenesis by promoting malonyl-CoA accumulation, which blocks the carnitine shuttle and prevents fatty acids from entering mitochondria for oxidation.
The human brain can derive up to 70% of its energy from ketone bodies during prolonged fasting, shifting away from its usual dependence on glucose within approximately 3 days of severe carbohydrate restriction.
Ketogenesis only occurs on a ketogenic diet. Ketogenesis begins naturally after roughly 12 to 16 hours without food, once liver glycogen is depleted and rising glucagon levels redirect fatty acid oxidation toward ketone body production.
Dairy cows experience heightened ketogenesis in the weeks immediately after calving, when the energy demand of milk production outpaces dietary intake; affected cows exhale detectable acetone, and blood beta-hydroxybutyrate concentrations can exceed 3 millimolar in clinical cases.
Ketone Body
/ KEE-tone BAH-dee / · Ketone from German 'aketon' (acetone), from Latin 'acetum' (vinegar); Body from Old English 'bodig' (trunk, chest)
Ketone bodies are water-soluble molecules produced primarily in the liver mitochondria from fatty acids during periods of low glucose availability, consisting of acetoacetate, beta-hydroxybutyrate, and acetone.
During prolonged fasting lasting more than 12 hours, the human liver can produce up to 150 grams of ketone bodies per day to fuel the brain and other tissues. Heart muscle preferentially uses beta-hydroxybutyrate over glucose when both are available, obtaining approximately 30% more ATP per oxygen molecule consumed. Arctic ground squirrels (Urocitellus parryii) rely heavily on ketone body metabolism during their 7 to 8 month hibernation period, when they neither eat nor drink.
Acetoacetate and beta-hydroxybutyrate are interconverted by the enzyme beta-hydroxybutyrate dehydrogenase, with the ratio between them reflecting the mitochondrial redox state of the liver.
Newborn babies naturally exist in a state of mild ketosis for the first few days of life, and their brains can derive up to 25% of their energy from ketone bodies present in breast milk.
Ketone bodies are toxic waste products of metabolism. At physiological concentrations reached during fasting, they are regulated fuels that spare muscle protein from breakdown and supply up to two-thirds of the brain's energy needs.
Migrating monarch butterflies (Danaus plexippus) produce ketone bodies from stored fat reserves during their roughly 3,000-mile journey from Canada to Mexico when nectar sources become scarce, sustaining flight over stretches where no food is available.
Most Colorful Butterflies →Kinase
/ KY-nase / · Greek kinein meaning to move or cause motion
Kinase is an enzyme that catalyzes the transfer of a phosphate group from a high-energy donor molecule such as ATP to a specific substrate molecule, altering that substrate's activity or function.
The human genome encodes over 500 different kinases that regulate nearly every cellular process from metabolism to cell division. Protein kinase A in muscle cells responds to adrenaline signals by phosphorylating enzymes that break down glycogen into glucose, providing rapid energy during exercise. Each kinase positions ATP and its target substrate in precise orientations within the active site, lowering the activation energy needed for phosphate transfer.
This positional specificity explains why a single amino acid difference in a substrate’s recognition sequence can determine whether a kinase phosphorylates it at all.
The bacterial kinase PknB in Mycobacterium tuberculosis is required for the pathogen's survival inside human macrophages and has no close human equivalent, making it a promising drug target for tuberculosis therapy.
Are Enzymes Proteins? →Kinases only add phosphate groups to proteins. These enzymes phosphorylate many substrate classes including sugars, lipids, and nucleotides, and phosphorylation of each class produces distinct downstream consequences in the cell.
Hexokinase in yeast (Saccharomyces cerevisiae) phosphorylates glucose molecules immediately upon entry into the cell, trapping the sugar inside and committing it to glycolysis at a rate that keeps pace with the cell's energy demand.
Building Blocks of Carbohydrates →Km
/ kay-EM / · Abbreviation derived from Michaelis constant, named after German biochemist Leonor Michaelis and Canadian physician Maud Menten who developed enzyme kinetics theory in 1913
Km is the substrate concentration at which an enzyme reaches half of its maximum reaction velocity, and it reflects the affinity of the enzyme for its substrate under steady-state conditions.
Hexokinase exhibits a Km of approximately 0.1 millimolar for glucose, indicating high substrate affinity that supports efficient glucose phosphorylation even at low blood sugar concentrations. Glucokinase, a related enzyme found in liver and pancreatic beta cells, has a Km of roughly 10 millimolar, meaning it only becomes active when blood glucose rises substantially after a meal. This difference in Km between the two enzymes allows the liver to act as a glucose buffer, capturing excess sugar after eating without competing with brain and muscle tissue during fasting.
Enzymes with Km values in the nanomolar range bind their substrates so tightly that they remain nearly saturated under most physiological conditions.
Carbonic anhydrase, which maintains blood pH by converting carbon dioxide and water into bicarbonate, has a Km for carbon dioxide of around 8 millimolar and a turnover rate exceeding 1 million reactions per second, making it one of the fastest enzymes known.
Km always measures how tightly a substrate binds to an enzyme. Km also depends on the rate at which the enzyme-substrate complex breaks down to form product, so it equals the true binding constant only when product formation is much slower than substrate release.
Lactase in humans shows a Km of about 18 millimolar for lactose, which means the enzyme operates well below saturation at typical intestinal lactose concentrations, explaining why individuals with reduced lactase expression struggle to digest large dairy servings efficiently.
Krebs Cycle
/ KREBS SY-kul / · Named after German-British biochemist Hans Krebs who discovered the pathway in 1937; cycle from Greek kyklos meaning circle
Krebs cycle is a series of eight enzymatic reactions occurring in the mitochondrial matrix that oxidizes acetyl-CoA to carbon dioxide while generating electron carriers and one molecule of GTP per turn.
Each turn of the cycle generates three NADH molecules, one FADH2, one GTP, and two carbon dioxide molecules from a single acetyl-CoA. In human heart cells, which contract approximately 100,000 times daily, the cycle runs continuously to meet the enormous energy demands of cardiac muscle. Oxaloacetate, the four-carbon molecule that initially accepts the acetyl group from acetyl-CoA, is regenerated at the end of each turn, allowing the pathway to function as a true metabolic loop rather than a linear sequence.
Beyond energy production, the cycle supplies biosynthetic precursors for amino acids, nucleotides, and heme, making it central to anabolism as well as catabolism.
Hans Krebs worked out the complete cycle in 1937 using pigeon breast muscle, which has exceptionally high metabolic rates to support sustained flight, and he published his findings after his initial manuscript was rejected by Nature.
Biochemistry Discoveries of 2019 →The Krebs cycle is where cells make most of their ATP. Each turn of the cycle directly produces only one GTP; the cycle's primary output is NADH and FADH2, electron carriers that transfer energy to the electron transport chain, which then generates approximately 34 ATP molecules per glucose.
Do Prokaryotes Have Mitochondria? →In yeast cells (Saccharomyces cerevisiae) during aerobic growth on grape sugars, the Krebs cycle runs at full capacity, with each cell turning over its entire pool of oxaloacetate many times per minute to sustain the ATP demands of active fermentation and biosynthesis.
Top Biochemistry News of 2021 →