Biochemistry Terms Starting With F

Fatty acid is a carboxylic acid with a long hydrocarbon chain that forms a fundamental structural and energetic component of lipids.

Fatty acids range from 4 to 36 carbon atoms in length, with palmitic acid (16 carbons) and stearic acid (18 carbons) among the most abundant in human tissue. Saturated fatty acids carry only single bonds between carbons and pack tightly into solid structures at room temperature, while unsaturated fatty acids contain one or more double bonds that introduce kinks, keeping them liquid. Complete oxidation of one molecule of palmitic acid through beta-oxidation and the citric acid cycle yields approximately 129 ATP molecules.

Omega-3 and omega-6 fatty acids cannot be synthesized by humans because the enzymes needed to insert double bonds beyond carbon 9 are absent, making dietary sources from fish, flaxseed, and walnuts necessary.

Feedback inhibition is a regulatory mechanism in which the end product of a metabolic pathway binds to and inhibits the first committed enzyme in that pathway, reducing further production of that product.

This mechanism prevents overproduction of metabolites and conserves the raw materials and energy a cell would otherwise waste. In Escherichia coli, the amino acid threonine inhibits aspartokinase, the enzyme catalyzing the first step in threonine biosynthesis, when intracellular threonine concentrations reach approximately 1 mM. Inhibition occurs through allosteric binding, meaning threonine attaches to a regulatory site distinct from the enzyme’s active site and changes the enzyme’s shape to reduce its activity.

Because inhibition targets the first committed step rather than a later one, no unnecessary intermediates accumulate in the pathway.

Fermentation is an anaerobic metabolic process in which cells regenerate NAD+ by transferring electrons from NADH to organic molecules, sustaining glycolysis without oxygen.

During alcoholic fermentation, Saccharomyces cerevisiae converts pyruvate to ethanol and carbon dioxide, regenerating the NAD+ that glycolysis requires to continue oxidizing glucose. Lactic acid bacteria such as Lactobacillus acidophilus reduce pyruvate directly to lactate, a reaction that also regenerates NAD+ and lowers the pH of the surrounding environment. Both pathways yield only 2 ATP per glucose molecule, compared with the 36 to 38 ATP generated by aerobic respiration.

Human skeletal muscle cells temporarily use lactic acid fermentation during intense exercise when oxygen delivery cannot keep pace with energy demand, allowing glycolysis to continue for short bursts of high-power output.

Flavin adenine dinucleotide is a redox cofactor derived from riboflavin that carries electrons between metabolic reactions by cycling between its oxidized form, FAD, and its reduced form, FADH2.

FAD consists of a flavin mononucleotide unit linked to adenosine monophosphate through a phosphoanhydride bond, giving the molecule its characteristic yellow color in solution. During the citric acid cycle, the enzyme succinate dehydrogenase uses FAD as a tightly bound prosthetic group to oxidize succinate to fumarate, accepting two electrons and two protons to become FADH2. FADH2 then donates its electrons directly to Complex II of the mitochondrial electron transport chain, ultimately contributing to the synthesis of approximately 1.5 ATP molecules per FADH2 oxidized, fewer than the 2.5 ATP generated per NADH.

The lower reduction potential of FAD relative to NAD+ makes it better suited for oxidizing substrates that are already partially oxidized, such as succinate.

Flavin mononucleotide is a riboflavin-derived coenzyme that carries electrons in oxidation-reduction reactions and binds as a prosthetic group to the first complex of the mitochondrial electron transport chain.

FMN consists of riboflavin with a phosphate group attached at the 5′ position of its ribitol chain, making it structurally simpler than FAD, which carries an additional adenosine monophosphate unit. Within Complex I of the mitochondrial electron transport chain, FMN accepts two electrons from NADH and passes them sequentially to a series of iron-sulfur clusters, with a standard reduction potential of approximately -0.22 volts. Unlike many coenzymes that associate loosely with enzymes, FMN binds tightly as a prosthetic group and does not dissociate between catalytic cycles.

FMN can also exist in a one-electron-reduced semiquinone radical state, an intermediate that allows it to bridge reactions between two-electron donors and one-electron acceptors.

Free energy is the thermodynamic quantity that measures the maximum useful work a system can perform at constant temperature and pressure, represented by the Gibbs free energy function.

Biochemists express free energy changes as delta G, where a negative value indicates a spontaneous reaction that releases energy and a positive value indicates a reaction that requires an energy input to proceed. ATP hydrolysis releases approximately 30.5 kJ/mol of free energy under standard conditions, but this value rises to about 50 kJ/mol inside living cells because of the low concentrations of ADP and phosphate maintained there. E.

coli and other organisms couple exergonic reactions to endergonic ones so that the combined delta G remains negative, driving biosynthetic reactions that would otherwise stall. Temperature and concentration both shift the actual free energy change away from the standard value, which is why cellular conditions matter as much as the reaction chemistry itself.

Fructose is a six-carbon ketose monosaccharide found naturally in fruits, honey, and root vegetables, and is the sweetest of the common dietary sugars.

Fructose shares the molecular formula C6H12O6 with glucose but differs in structure: its carbonyl group sits at carbon 2, making it a ketose, while glucose carries its carbonyl at carbon 1 as an aldose. At roughly 1.7 times the sweetness of sucrose, fructose requires smaller quantities to achieve the same perceived sweetness in food products. Unlike glucose, which is metabolized in nearly every cell, fructose is processed almost exclusively in the liver, where it enters glycolysis below the main regulatory checkpoint controlled by phosphofructokinase-1.

High fructose intake can therefore drive hepatic lipogenesis at rates that bypass normal feedback controls, contributing to elevated triglycerides and non-alcoholic fatty liver disease in clinical studies.

Functional group is a specific arrangement of atoms within a molecule that determines the chemical properties and reactivity of that compound.

Each functional group confers predictable chemical behavior on any molecule that contains it. The hydroxyl group (-OH) in glucose makes this sugar highly soluble in water, while the carboxyl group (-COOH) in fatty acids gives them their acidic properties and allows them to form ester bonds with glycerol. Many biomolecules carry more than one functional group simultaneously; amino acids, for example, contain both an amino group (-NH2) and a carboxyl group (-COOH), which together allow them to act as buffers and to form peptide bonds during protein synthesis.