Biochemistry Terms Starting With C
Biochemistry Glossary: C
Carbohydrate
/KAR-bo-HY-drate/ · French carbohydrate (carbon + water)
Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms, typically in a 1:2:1 ratio, that supply the primary energy source for most living organisms.
These biomolecules exist in three main structural forms: monosaccharides, disaccharides, and polysaccharides. Plants such as corn (Zea mays) produce starch, a polysaccharide that can contain over 1,000 glucose units linked together by alpha-1,4 and alpha-1,6 glycosidic bonds. Humans obtain approximately 45 to 65% of their daily calories from carbohydrates, which are broken down during cellular respiration to produce ATP.
Beyond energy, carbohydrates form structural materials: cellulose, the most abundant organic polymer on Earth, makes up the rigid cell walls of plants and contains the same glucose monomer as starch but linked by beta-1,4 bonds that human digestive enzymes cannot cleave.
Rice contains approximately 80% carbohydrates by dry weight, making it one of the most carbohydrate-dense staple foods and the primary caloric source for more than half the world's population.
Building Blocks of Carbohydrates →Not all carbohydrates are sugars. Complex carbohydrates such as starch and cellulose are long chains of sugar molecules bonded together, with very different physical and digestive properties from their simple sugar building blocks.
Potatoes (Solanum tuberosum) store energy as starch granules in their tubers, accumulating up to 18% of fresh weight as starch. When the plant mobilizes this reserve for sprouting, amylase enzymes hydrolyze the granules into glucose units that fuel new shoot and root growth.
Catabolism
/ka-TAB-oh-lizm/ · Greek kata (down) + ballein (to throw)
Catabolism is the set of metabolic pathways that break down complex molecules into simpler ones while releasing energy that cells capture for biological work.
These breakdown reactions occur continuously in all living cells to power cellular processes. During cellular respiration in humans, glucose molecules are catabolized through glycolysis and the citric acid cycle, ultimately yielding 30 to 38 ATP molecules per glucose depending on cellular conditions. Protein catabolism also runs constantly: the average human turns over roughly 250 to 300 grams of protein per day through proteasomal and lysosomal degradation, recycling amino acids for new synthesis.
Bacteria such as Escherichia coli deploy catabolic pathways to extract energy and carbon skeletons from diverse environmental nutrients, switching between pathways within minutes when substrate availability changes.
The human body catabolizes approximately 160 billion red blood cells every single day, recycling their components for new cellular construction.
Catabolism only activates during exercise or starvation. Catabolic reactions run continuously in every living cell to recycle damaged molecules, regulate protein turnover, and supply precursors for biosynthesis even during periods of rest and adequate nutrition.
Mitochondria Functions →Yeast cells (Saccharomyces cerevisiae) catabolize glucose into ethanol and carbon dioxide during alcoholic fermentation. Under anaerobic conditions, a single yeast cell can process roughly 1 to 2 femtomoles of glucose per second, producing ethanol concentrations that can reach 15% before becoming toxic to the cells themselves.
Fermentation Biology →Catalysis
/ka-TAL-uh-sis/ · Greek katalysis (dissolution)
Catalysis is the acceleration of a chemical reaction by a substance that remains unchanged at the end of the process.
Biological catalysis occurs primarily through enzymes, which can increase reaction rates by factors of millions to billions. Carbonic anhydrase, found in red blood cells, catalyzes the conversion of carbon dioxide and water to bicarbonate and protons at a rate of approximately 1 million reactions per second. Most catalysts work by lowering the activation energy required for reactions to proceed, creating alternative reaction pathways that require less energy input.
This reduction in activation energy does not change the thermodynamics of the reaction, meaning the equilibrium position remains the same whether or not a catalyst is present.
The enzyme catalase can decompose 40 million molecules of hydrogen peroxide per second, making it one of the most efficient catalysts known in nature.
Are Enzymes Proteins? →Catalysts speed up reactions without being consumed. Each catalyst molecule emerges unchanged after each reaction cycle and can be reused repeatedly, which is why even tiny amounts of an enzyme can process enormous quantities of substrate.
Pepsin catalyzes protein digestion in the acidic environment of the human stomach, where its active site is shaped to bind peptide bonds between specific amino acid residues. At a pH of around 2.0, pepsin cleaves those bonds at rates far exceeding what would occur without enzymatic assistance. This specificity means pepsin targets dietary proteins rather than the stomach's own mucus lining, which is protected by a different chemical composition.
Building Blocks of Proteins →Citric Acid Cycle
/ SIT-rik AS-id SY-kul / · Latin citrus (citrus fruit) + acidus (sour) + Greek kyklos (circle)
Citric Acid Cycle is a series of eight enzyme-catalyzed reactions in the mitochondrial matrix that oxidizes acetyl-CoA to produce ATP, NADH, FADH2, and carbon dioxide.
During this metabolic pathway, each glucose molecule generates two acetyl-CoA molecules that enter the cycle, producing a total of 2 ATP, 6 NADH, 2 FADH2, and 4 CO2 per glucose. The cycle operates in all aerobic organisms, from bacteria like Escherichia coli to complex multicellular animals. Each complete turn regenerates oxaloacetate, which combines with another acetyl-CoA to continue the process.
Intermediates such as alpha-ketoglutarate and oxaloacetate also feed into amino acid biosynthesis, linking the cycle to nitrogen metabolism.
The citric acid cycle produces only 2 ATP molecules directly, but the 24 electrons captured in NADH and FADH2 generate approximately 30 additional ATP through the electron transport chain.
The citric acid cycle directly produces most of the ATP from glucose breakdown. It generates only 2 ATP molecules per glucose; its primary role is producing electron carriers that drive the bulk of ATP synthesis in the electron transport chain.
Heart muscle cells in mammals contain thousands of mitochondria that run the citric acid cycle continuously to meet the high energy demands of cardiac contractions. A single human cardiomyocyte can consume and regenerate its entire ATP pool roughly every 10 seconds during vigorous exercise. This turnover rate reflects how heavily the heart depends on aerobic metabolism rather than glycolysis alone.
Coenzyme
/KO-en-zyme/ · Latin co (together) + Greek en (in) + zyme (leaven)
Coenzyme is a non-protein organic molecule that binds to an enzyme and assists in catalyzing biochemical reactions.
Coenzymes temporarily carry atoms or functional groups between enzymatic reactions, acting as mobile shuttles within metabolic pathways. NAD+ (nicotinamide adenine dinucleotide) transfers hydrogen atoms and electrons during cellular respiration, accepting them from glucose-derived intermediates in glycolysis and the citric acid cycle. Many coenzymes are derived from vitamins; for example, NAD+ is synthesized from niacin (vitamin B3), which is why niacin deficiency causes the disease pellagra, characterized by disrupted energy metabolism.
Unlike the enzyme itself, a coenzyme changes its chemical form during the reaction but is regenerated elsewhere in the cell.
The human body recycles coenzyme NAD+ approximately 1,000 times per day, making it one of the most actively recycled molecules in cellular metabolism.
Coenzymes and cofactors mean the same thing. Coenzymes are a specific subset of cofactors that must be organic molecules, whereas cofactors also include inorganic ions such as magnesium or zinc.
Coenzyme A carries acetyl groups during fatty acid synthesis in the liver cells of mammals. Each acetyl group added to a growing fatty acid chain requires one molecule of coenzyme A to deliver it to the fatty acid synthase complex. A single round of palmitate synthesis, producing a 16-carbon fatty acid, consumes 8 acetyl-CoA molecules in this way.
Cofactor
/CO-fac-tor/ · Latin cofactor (joint factor)
Cofactor is a non-protein chemical compound that binds to an enzyme and is required for catalytic activity.
Cofactors fall into two broad categories: inorganic ions such as zinc, magnesium, or iron, and organic molecules called coenzymes. Carbonic anhydrase requires a zinc ion to catalyze the conversion of carbon dioxide to bicarbonate at a rate of approximately one million reactions per second. Without the correct cofactor, many enzymes adopt an inactive form called an apoenzyme; binding the cofactor converts it to the active holoenzyme.
Iron-sulfur clusters in mitochondrial electron transport proteins illustrate how tightly bound inorganic cofactors can transfer electrons with remarkable precision.
The human body contains over 300 different enzymes that depend on zinc cofactors, meaning zinc deficiency can disrupt numerous metabolic pathways simultaneously.
Most cofactors stay permanently attached to their enzymes. Many cofactors bind transiently and dissociate after each reaction cycle, functioning more like co-substrates that are regenerated elsewhere in the cell.
DNA polymerase in human cells requires magnesium ions as cofactors to coordinate the nucleotide triphosphate substrate during DNA replication. Without magnesium, the enzyme cannot properly position incoming nucleotides, and replication stalls. Each magnesium ion stabilizes the transition state of the phosphoryl transfer reaction, lowering the activation energy needed to form each new phosphodiester bond.
Competitive Inhibition
/k?m-PET-i-tiv in-hi-BISH-?n/ · Latin competere (to strive together) + inhibere (to hold back)
Competitive inhibition is a form of enzyme regulation in which an inhibitor molecule occupies the active site of an enzyme, blocking substrate binding.
During competitive inhibition, the inhibitor molecule resembles the natural substrate closely enough to bind reversibly to the enzyme’s active site. Statins such as lovastatin competitively inhibit HMG-CoA reductase by mimicking the natural substrate’s structure, reducing cholesterol synthesis in liver cells. The degree of inhibition depends on the relative concentrations of substrate and inhibitor; raising substrate concentration can displace the inhibitor and restore enzyme activity.
This concentration-dependence is reflected kinetically by an increase in apparent Km without any change in Vmax.
Aspirin works as a competitive inhibitor of cyclooxygenase enzymes, but unlike typical competitive inhibitors, it binds irreversibly by acetylating a serine residue in the active site.
Competitive inhibitors permanently disable enzymes. They bind reversibly to the active site, and increasing substrate concentration can outcompete them, restoring normal enzyme activity.
Malonate competitively inhibits succinate dehydrogenase in the citric acid cycle by binding to the enzyme's active site in place of succinate. The two molecules share a similar dicarboxylate structure, which allows malonate to fit the binding pocket without being oxidized. At high enough succinate concentrations, the inhibition is fully reversed, confirming its competitive nature.
Conjugated Protein
/CON-ju-gay-ted PRO-teen/ · Latin conjugatus (joined together) + Greek proteios (primary)
Conjugated Protein is a protein that contains one or more non-protein components, called prosthetic groups, covalently or tightly bound to its amino acid chain.
These prosthetic groups include metal ions, carbohydrates, lipids, or nucleic acids, and they are required for the protein’s biological function. Hemoglobin is a well-known example, containing four heme groups, each with a central iron atom that reversibly binds one oxygen molecule for transport through the bloodstream. Unlike simple proteins composed only of amino acids, conjugated proteins depend on their non-protein components to maintain proper structure and activity.
Glycoproteins, another class, carry carbohydrate chains that determine blood type antigens on the surface of red blood cells.
Hemoglobin contains approximately 0.34% iron by weight, with each molecule carrying exactly four iron atoms that can bind up to four oxygen molecules simultaneously.
Conjugated proteins can function normally without their attached non-protein groups. Removing the prosthetic group typically destroys the protein's activity because the group directly participates in binding, catalysis, or structural stability.
Catalase, found in nearly all aerobic organisms including the common baker's yeast (Saccharomyces cerevisiae), contains four heme groups with iron that enable it to decompose hydrogen peroxide into water and oxygen. Each heme iron cycles between oxidation states during the reaction, processing up to 40 million hydrogen peroxide molecules per second. This rate makes catalase one of the fastest-acting enzymes known.
Creatine Phosphate
/KREE-ah-teen FOSS-fate/ · Greek kreatin (meaning flesh) + phosphate (phosphorous-bearing salt)
Creatine Phosphate is a high-energy phosphate compound stored in muscle and nerve cells that rapidly regenerates ATP by donating its phosphate group to ADP.
During intense physical activity, creatine phosphate transfers its phosphate group to ADP through the enzyme creatine kinase, regenerating ATP within milliseconds. Human skeletal muscle contains approximately 3 to 5 times more creatine phosphate than ATP, providing an energy buffer during the first 10 to 15 seconds of high-intensity exercise. This system operates most efficiently in fast-twitch muscle fibers, where immediate energy demands exceed what glycolysis or oxidative phosphorylation can supply in time.
Once creatine phosphate stores are depleted, the muscle must rely on slower pathways, which is why maximal sprint efforts cannot be sustained beyond roughly 10 seconds.
The creatine phosphate system can regenerate ATP at rates up to 10 times faster than glycolysis, making it the body's most rapid energy system.
Creatine phosphate directly powers muscle contraction. Muscle fibers use ATP, not creatine phosphate, to drive the myosin cross-bridge cycle; creatine phosphate's role is to replenish ATP stores quickly when demand spikes.
Sprint runners rely heavily on creatine phosphate stores during the explosive start of a 100-meter dash. Studies measuring phosphocreatine levels in sprinters show that stores can drop by more than 50% within the first 5 seconds of maximal effort. After the race, creatine kinase rebuilds creatine phosphate over the following minutes as aerobic metabolism resumes.