Biochemistry Terms Starting With E

Electron carrier is a molecule that accepts electrons from one reaction partner and donates them to another, linking oxidation and reduction steps in metabolic pathways.

These molecules form the backbone of energy production in living cells by shuttling electrons between different protein complexes. NAD+ accepts two electrons and one proton to become NADH, which carries this reducing power to the electron transport chain where it releases electrons and drives the synthesis of approximately 2.5 ATP molecules per NADH. Cytochrome c, a small heme-containing protein in the mitochondrial intermembrane space, transfers single electrons between Complex III and Complex IV with a standard reduction potential of +0.25 volts.

Ubiquinone, also called coenzyme Q, is a lipid-soluble carrier that diffuses laterally through the inner mitochondrial membrane to shuttle electrons between Complexes I and III.

Electron transport chain is a series of protein complexes embedded in the inner mitochondrial membrane that transfers electrons from NADH and FADH2 to molecular oxygen while pumping protons across the membrane to drive ATP synthesis.

Located within the cristae of mitochondria, this pathway consists of four main protein complexes, labeled I through IV, that pass electrons through sequential redox reactions of decreasing free energy. Each NADH molecule entering at Complex I ultimately drives the pumping of roughly 10 protons into the intermembrane space, while each FADH2 entering at Complex II contributes electrons that pump approximately 6 protons. During aerobic respiration in humans, the chain produces around 32 to 34 ATP molecules per glucose molecule, with Complex IV, cytochrome c oxidase, catalyzing the final step in which electrons combine with oxygen and protons to form water.

Cyanide poisoning kills by binding irreversibly to the iron center of Complex IV, halting electron flow and collapsing the proton gradient within seconds.

Endergonic reaction is a chemical process that absorbs free energy from its surroundings and has a positive Gibbs free energy change, meaning it cannot proceed spontaneously without an energy input.

These reactions are thermodynamically unfavorable under standard conditions and must be driven by coupling to energy-releasing processes. Protein synthesis in a bacterial cell like Escherichia coli requires endergonic peptide bond formation, with each bond costing approximately 4 to 5 kcal/mol and a single ribosome adding around 20 amino acids per second. Photosynthesis in plants couples endergonic glucose synthesis to the exergonic absorption of light energy, storing roughly 686 kcal/mol in each glucose molecule produced.

Cells also drive endergonic reactions by coupling them to ATP hydrolysis, which releases about 7.3 kcal/mol under standard conditions and shifts the free energy change of the coupled reaction to a negative value.

Enthalpy is a thermodynamic quantity representing the total heat content of a system at constant pressure, equal to the system's internal energy plus the product of its pressure and volume.

During cellular respiration, the complete oxidation of one glucose molecule releases approximately 686 kcal/mol of enthalpy, though cells capture only about 38% of this energy in ATP bonds. Enthalpy changes drive many biochemical processes, including protein folding, where hydrophobic collapse releases enthalpy as van der Waals contacts form between nonpolar side chains. The sign of the enthalpy change indicates direction: a negative value means heat is released to the surroundings, as in the exothermic hydrolysis of ATP, while a positive value means heat is absorbed, as in the melting of a DNA duplex.

Calorimetry experiments by Privalov and colleagues in the 1970s established precise enthalpy values for protein unfolding transitions, revealing that cold denaturation and heat denaturation both occur but through different thermodynamic driving forces.

Entropy is a thermodynamic measure of the number of possible microscopic arrangements of a system, with higher entropy corresponding to greater disorder and a greater number of accessible states.

Biochemical processes demonstrate entropy changes through protein folding and metabolic reactions. During protein denaturation, entropy increases as structured polypeptides unfold into disordered conformations, with configurational entropy gains on the order of 50 to 100 J/mol·K depending on chain length. Hydrophobic interactions, which drive the burial of nonpolar amino acid side chains in protein cores, are themselves entropy-driven: releasing ordered water molecules from around hydrophobic surfaces increases the entropy of the solvent more than the protein loses by folding.

Cellular respiration also increases entropy as one highly ordered glucose molecule breaks down into six carbon dioxide and six water molecules, dispersing chemical energy across many more molecular degrees of freedom.

Enzyme is a biological catalyst, almost always a protein, that accelerates a biochemical reaction by lowering the activation energy without being consumed in the process.

Enzymes bind specific molecules called substrates at a region called the active site, forming a transient enzyme-substrate complex that stabilizes the transition state and accelerates the reaction. Catalase, found in nearly all aerobic organisms, decomposes hydrogen peroxide into water and oxygen at a rate of approximately 40 million molecules per second per enzyme molecule, one of the fastest turnover numbers measured for any enzyme. Some enzymes require non-protein cofactors: carbonic anhydrase depends on a single zinc ion at its active site, and without it the enzyme loses all catalytic activity.

Ribozymes, RNA molecules with catalytic activity discovered by Thomas Cech and Sidney Altman in the early 1980s, demonstrate that proteins do not hold a monopoly on biological catalysis.

Enzyme kinetics is the study of the rates of enzyme-catalyzed reactions and how those rates respond to changes in substrate concentration, temperature, pH, and the presence of inhibitors or activators.

Scientists measure enzyme kinetics by tracking how quickly substrates convert to products under controlled conditions, then fitting the data to mathematical models. The Michaelis-Menten equation describes the hyperbolic relationship between reaction rate and substrate concentration, yielding two key parameters: Km, the substrate concentration at which the reaction proceeds at half its maximum rate, and Vmax, the theoretical maximum rate when all enzyme active sites are saturated. Leonor Michaelis and Maud Menten derived this equation in 1913 using invertase, an enzyme that cleaves sucrose, and their framework remains the foundation of quantitative enzymology.

Competitive inhibitors raise the apparent Km without changing Vmax, while noncompetitive inhibitors lower Vmax without affecting Km, and distinguishing these patterns on a Lineweaver-Burk double-reciprocal plot is a standard diagnostic tool in pharmacology.

Essential amino acids are amino acids that the body cannot synthesize in sufficient quantities and must obtain from food.

Humans require nine essential amino acids for protein synthesis and metabolic function: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The designation “essential” is species-specific and can shift with physiological state; for example, histidine is conditionally essential in infants because their biosynthetic capacity is too low to meet demand. Complete proteins, found in foods such as eggs, meat, and quinoa (Chenopodium quinoa), supply all nine in proportions that match human metabolic needs.

Plant-based diets can meet all nine requirements when varied sources are combined across meals.

Ester bond is a covalent chemical linkage formed between a carboxyl group and a hydroxyl group through a dehydration reaction that releases one water molecule.

In triglycerides, three ester bonds connect individual fatty acid chains to a glycerol backbone, creating the primary form of stored fat in animals and plants. Lipases secreted by the pancreas hydrolyze these bonds in the small intestine, cleaving each ester linkage by adding water across the bond to release free fatty acids for absorption. Ester bonds also appear in phospholipids, where they anchor fatty acid tails to the glycerol portion of the head group, and in wax esters such as those secreted by human sebaceous glands to waterproof skin.

The bond energy of a typical ester linkage is approximately 360 kJ/mol, making it stable under normal physiological conditions but susceptible to enzymatic hydrolysis.

Exergonic reaction is a chemical reaction that releases free energy to its surroundings and proceeds with a negative change in Gibbs free energy.

During cellular respiration, the complete oxidation of one mole of glucose releases approximately 686 kcal of free energy, making glucose catabolism one of the most energetically favorable reactions in biology. Exergonic reactions proceed spontaneously because the products occupy a lower free-energy state than the reactants, but spontaneity does not guarantee speed. Enzymes lower the activation energy barrier so that exergonic reactions proceed at rates compatible with cellular needs.

Cells couple highly exergonic reactions, such as ATP hydrolysis at roughly 30.5 kJ/mol under physiological conditions, to endergonic reactions that would otherwise not proceed.