Free Fatty Acids: Structure, Metabolism, Functions, and Measurement

What are Free Fatty Acids?

Free fatty acids (FFAs) are non-esterified fatty acids released from triglycerides or phospholipids through the action of lipases. FFAs are organic molecules consisting of a long hydrocarbon chain, typically composed of 12 to 24 carbon atoms, with a carboxylic acid group (-COOH) at one end. These fatty acids can be categorized into saturated and unsaturated forms, depending on the presence of double bonds in the carbon chain. Examples of common FFAs include palmitic acid, oleic acid, and linoleic acid.

Structure of Free Fatty Acids

General Structure

Free fatty acids are characterized by their distinct molecular structure, consisting of a carboxylic acid group (-COOH) attached to a hydrocarbon chain. The variability in the length of the hydrocarbon chain and the presence or absence of double bonds significantly influence the chemical properties and biological functions of FFAs.

Hydrocarbon Chain Length

The hydrocarbon chain of FFAs can vary widely in length, typically ranging from 4 to 28 carbon atoms. Based on chain length, FFAs can be classified into:

• Short-Chain Fatty Acids (SCFAs): These contain fewer than six carbon atoms. Examples include butyric acid (C4) and propionic acid (C3). SCFAs are primarily produced through the fermentation of dietary fibers by gut microbiota and play critical roles in gut health and metabolic regulation.
• Medium-Chain Fatty Acids (MCFAs): These contain 6-12 carbon atoms. Examples include caprylic acid (C8) and lauric acid (C12). MCFAs are absorbed more rapidly by the body and are readily oxidized for energy.
• Long-Chain Fatty Acids (LCFAs): These contain 13-21 carbon atoms. Examples include palmitic acid (C16) and stearic acid (C18). LCFAs are the most common FFAs found in the diet and body tissues.
• Very Long-Chain Fatty Acids (VLCFAs): These contain more than 22 carbon atoms. Examples include lignoceric acid (C24) and cerotic acid (C26). VLCFAs are integral to certain biological membranes and specialized lipid functions.

Degree of Saturation

The degree of saturation refers to the number of double bonds present in the hydrocarbon chain, and FFAs can be categorized as:

• Saturated Fatty Acids (SFAs): These have no double bonds between the carbon atoms in the hydrocarbon chain. SFAs are typically solid at room temperature. Common examples include palmitic acid (C16:0) and stearic acid (C18:0). SFAs are found in animal fats and some plant oils.
• Monounsaturated Fatty Acids (MUFAs): These contain one double bond in the hydrocarbon chain. MUFAs are usually liquid at room temperature. Oleic acid (C18:1) is a prominent example, abundant in olive oil and other plant oils.
• Polyunsaturated Fatty Acids (PUFAs): These contain two or more double bonds in the hydrocarbon chain. PUFAs are essential fatty acids, meaning they must be obtained through the diet. They are subdivided into omega-3 and omega-6 fatty acids based on the position of the first double bond from the methyl end of the molecule. Examples include alpha-linolenic acid (ALA, C18:3) and linoleic acid (LA, C18:2).

Configuration of Double Bonds

The configuration of double bonds in unsaturated fatty acids can be in either cis or trans forms:

• Cis Configuration: In the cis configuration, the hydrogen atoms attached to the carbon atoms of the double bond are on the same side. This configuration introduces a kink in the hydrocarbon chain, preventing tight packing and thereby increasing fluidity. Most naturally occurring unsaturated fatty acids have cis double bonds.
• Trans Configuration: In the trans configuration, the hydrogen atoms are on opposite sides of the double bond, resulting in a straighter hydrocarbon chain. Trans fatty acids can occur naturally in small amounts in some animal products but are more commonly found in partially hydrogenated vegetable oils. Trans fats are associated with adverse health effects, including an increased risk of cardiovascular disease.

Functional Groups and Derivatives

The carboxylic acid group of FFAs can participate in various chemical reactions, forming derivatives such as:

• Esters: FFAs can form esters with alcohols, creating compounds such as triglycerides and phospholipids.
• Amides: Reaction with amines can form fatty acid amides, which play roles in signaling and regulatory functions.
• Hydroxy Fatty Acids: The addition of hydroxyl groups can create hydroxy fatty acids, which have unique properties and functions, such as in the synthesis of ceramides and other complex lipids.

Fig. 1 Free fatty acid receptors in health and disease. (Kimura, Ikuo, et al, 2019)

Free Fatty Acid Metabolism

FFA metabolism is a complex and vital component of overall lipid metabolism, involving several intricate biochemical pathways that are crucial for energy production, cellular signaling, and maintaining homeostasis.

Synthesis of Free Fatty Acids

De Novo Lipogenesis

De novo lipogenesis is the metabolic process through which FFAs are synthesized from acetyl-CoA and malonyl-CoA in the cytoplasm of cells. This process is particularly active in the liver and adipose tissue and involves a multi-step enzymatic pathway:

• Formation of Malonyl-CoA: Acetyl-CoA is carboxylated to malonyl-CoA by the enzyme acetyl-CoA carboxylase (ACC), a key regulatory step.
• Fatty Acid Synthase (FAS) Complex: The FAS enzyme complex sequentially adds two-carbon units from malonyl-CoA to a growing acyl chain, elongating it step by step. NADPH provides the necessary reducing power.
• Termination and Release: Once the desired chain length is achieved, the fatty acid is released from the FAS complex, typically as palmitic acid (C16:0), the primary product of de novo lipogenesis.

Dietary Intake

FFAs can also be directly obtained from dietary sources. Triglycerides in the diet are broken down by pancreatic lipases into FFAs and monoglycerides, which are absorbed in the small intestine. These absorbed FFAs can be utilized immediately for energy, re-esterified into triglycerides for storage, or incorporated into other lipid molecules.

Oxidation of Free Fatty Acids

The oxidation of FFAs is a critical process for energy production, particularly during periods of fasting, prolonged exercise, or carbohydrate restriction.

Beta-Oxidation

Beta-oxidation is the primary pathway for FFA catabolism, occurring in the mitochondria of cells:

• Activation: FFAs are activated to acyl-CoA derivatives by the enzyme acyl-CoA synthetase.
• Transport: Acyl-CoA derivatives are transported into the mitochondrial matrix by the carnitine shuttle, involving carnitine palmitoyltransferase I (CPT I), carnitine translocase, and carnitine palmitoyltransferase II (CPT II).
• Beta-Oxidation Cycle: Within the mitochondria, the acyl-CoA undergoes a series of four reactions—dehydrogenation, hydration, another dehydrogenation, and thiolysis—catalyzed by specific enzymes. Each cycle shortens the acyl chain by two carbons, producing acetyl-CoA, NADH, and FADH2.
• Energy Production: Acetyl-CoA enters the citric acid cycle, while NADH and FADH2 donate electrons to the electron transport chain, leading to the production of ATP.

Peroxisomal Oxidation

Peroxisomal beta-oxidation is responsible for the breakdown of very long-chain fatty acids (VLCFAs) that are too long to be efficiently oxidized in mitochondria. This pathway generates acetyl-CoA and medium-chain acyl-CoAs, which are then transferred to mitochondria for further oxidation.

Storage and Mobilization of Free Fatty Acids

FFAs are stored as triglycerides in adipose tissue, which acts as an energy reservoir. The mobilization of these stored fats is tightly regulated by hormonal signals:

Triglyceride Storage

• Synthesis: In adipocytes, FFAs are re-esterified to glycerol to form triglycerides. This process is catalyzed by enzymes such as glycerol-3-phosphate acyltransferase (GPAT) and diacylglycerol acyltransferase (DGAT).
• Storage: Triglycerides are stored in lipid droplets within adipocytes, providing a concentrated energy source.

Lipolysis

Lipolysis is the process of breaking down stored triglycerides into FFAs and glycerol, mediated by specific lipases:

• Hormonal Regulation: Hormones such as epinephrine, norepinephrine, glucagon, and insulin regulate lipolysis. Epinephrine and glucagon stimulate lipolysis, while insulin inhibits it.
• Enzymatic Hydrolysis: Hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) catalyze the hydrolysis of triglycerides into diacylglycerol and monoacylglycerol, eventually releasing FFAs and glycerol.
• FFA Release and Transport: The liberated FFAs are released into the bloodstream, where they bind to albumin for transport to tissues requiring energy.

Regulation of Free Fatty Acid Metabolism

FFA metabolism is subject to complex regulation to maintain energy homeostasis and respond to physiological needs:

Allosteric Regulation: Key enzymes involved in FFA synthesis and oxidation, such as ACC and CPT I, are regulated by allosteric effectors. For example, malonyl-CoA inhibits CPT I, preventing the simultaneous synthesis and oxidation of FFAs.

Hormonal Regulation: Insulin promotes FFA synthesis and storage, while glucagon and catecholamines stimulate FFA mobilization and oxidation.

Nutritional Status: The balance between fed and fasting states influences FFA metabolism. During feeding, insulin levels rise, promoting storage. During fasting, glucagon levels rise, promoting mobilization and oxidation.

Fig. 2 A schematic view of obesity-associated, fatty acid-induced insulin resistance and metabolic dysfunction in skeletal muscle. (Tumova, Jana, Michal Andel, and Jan Trnka, 2016)

Functions of Free Fatty Acid

Energy Production

FFAs are a primary energy source, especially during periods of fasting, prolonged exercise, or carbohydrate restriction. The oxidation of FFAs through beta-oxidation in the mitochondria produces acetyl-CoA, which enters the citric acid cycle to generate ATP, the cell's energy currency. This process is crucial for tissues with high energy demands, such as:

• Skeletal Muscle: During prolonged exercise, skeletal muscles rely heavily on FFA oxidation to meet their energy needs, as glycogen stores become depleted.
• Cardiac Muscle: The heart preferentially oxidizes FFAs for energy, as they provide a dense source of ATP necessary for continuous cardiac function.
• Liver: The liver oxidizes FFAs to produce ATP and ketone bodies, which are alternative energy sources for the brain and other tissues during fasting or low-carbohydrate diets.

Signaling Molecules

FFAs act as signaling molecules, influencing various metabolic and cellular processes through interactions with specific receptors and transcription factors:

Peroxisome Proliferator-Activated Receptors (PPARs)

FFAs serve as ligands for PPARs, a family of nuclear receptors that regulate gene expression involved in lipid metabolism, glucose homeostasis, and inflammation. The three main isoforms of PPARs are:

• PPAR-alpha: Primarily expressed in the liver, heart, and muscle, it promotes fatty acid oxidation and ketogenesis.
• PPAR-gamma: Predominantly found in adipose tissue, it regulates adipogenesis, lipid storage, and insulin sensitivity.
• PPAR-delta (or PPAR-beta): Ubiquitously expressed, it enhances fatty acid oxidation and energy expenditure in various tissues.

G-Protein-Coupled Receptors (GPCRs)

FFAs also interact with specific GPCRs, such as GPR40 (FFAR1) and GPR120 (FFAR4), modulating insulin secretion, inflammation, and energy metabolism. These interactions are crucial for maintaining glucose homeostasis and metabolic health.

Membrane Structure

FFAs are essential components of phospholipids in cell membranes, influencing their fluidity, permeability, and functionality:

Membrane Fluidity

The type and proportion of FFAs in phospholipids affect membrane fluidity. Unsaturated FFAs, with their kinked structures due to cis double bonds, prevent tight packing of lipid molecules, enhancing membrane fluidity. This fluidity is critical for:

• Membrane Protein Function: Proper functioning of membrane-bound proteins, such as receptors, ion channels, and transporters, requires an optimal membrane environment.
• Cell Signaling: Fluid membranes facilitate the movement and interaction of signaling molecules and receptors, essential for effective signal transduction.

Lipid Rafts

FFAs are involved in the formation of lipid rafts, specialized microdomains in cell membranes enriched with cholesterol, sphingolipids, and certain proteins. Lipid rafts play key roles in:

• Signal Transduction: They serve as platforms for the assembly of signaling complexes, enhancing the efficiency and specificity of signal transduction pathways.
• Membrane Trafficking: Lipid rafts are involved in the sorting and trafficking of membrane proteins and lipids, influencing cellular processes such as endocytosis and exocytosis.

Cellular Differentiation and Growth

FFAs influence cellular differentiation and growth by modulating the activity of transcription factors and signaling pathways:

Adipogenesis

PPAR-gamma, activated by FFAs, is a master regulator of adipogenesis, the process by which preadipocytes differentiate into mature adipocytes. This differentiation is essential for:

• Energy Storage: Mature adipocytes store excess energy in the form of triglycerides, which can be mobilized during energy demand.
• Metabolic Regulation: Adipose tissue acts as an endocrine organ, secreting adipokines that regulate appetite, insulin sensitivity, and inflammation.

Cell Proliferation and Apoptosis

FFAs can influence cell proliferation and apoptosis through various signaling pathways, including:

• MAPK Pathway: FFAs activate the mitogen-activated protein kinase (MAPK) pathway, which regulates cell growth, differentiation, and survival.
• NF-kB Pathway: FFAs modulate the nuclear factor-kappa B (NF-kB) pathway, influencing inflammatory responses and cell survival mechanisms.

Inflammatory Response

FFAs play dual roles in inflammation, acting as both pro-inflammatory and anti-inflammatory agents depending on their types and concentrations:

Pro-Inflammatory Actions

• Saturated Fatty Acids (SFAs): High levels of SFAs can activate toll-like receptors (TLRs) on immune cells, triggering inflammatory responses. This activation is linked to metabolic disorders such as insulin resistance and atherosclerosis.
• Omega-6 PUFAs: Arachidonic acid (an omega-6 PUFA) is a precursor for pro-inflammatory eicosanoids, which mediate various inflammatory and immune responses.

Anti-Inflammatory Actions

• Monounsaturated Fatty Acids (MUFAs): MUFAs, such as oleic acid, have been shown to exert anti-inflammatory effects by inhibiting the expression of pro-inflammatory cytokines and adhesion molecules.
• Omega-3 PUFAs: Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (omega-3 PUFAs) are precursors for anti-inflammatory eicosanoids and resolvins, which help resolve inflammation and promote tissue repair.

Difference Between Fatty Acids and Free Fatty Acids

Free Fatty Acid Measurement

Accurate measurement of FFAs is crucial for understanding their physiological and pathological roles. Several methods are employed for the quantification of FFAs, including colorimetric assays, high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), and nuclear magnetic resonance (NMR) spectroscopy. These techniques allow researchers and healthcare professionals to monitor changes in free fatty acid profiles and assess their association with various diseases. Here are the main steps.

• Sample Preparation: Determine the type of sample (e.g., plasma, serum, food product) and select an appropriate extraction method to release FFAs. Common techniques include solvent extraction, solid-phase extraction, or protein precipitation.
• Method Optimization: Optimize the chosen method to enhance sensitivity, selectivity, and accuracy. This may involve adjusting parameters such as sample volume, extraction solvent, column type, mobile phase composition, or detection wavelength.
• Data Analysis: Perform data analysis using appropriate software or calculations specific to the chosen analytical technique. Quantify the concentration of FFAs in the sample based on the calibration curve.
• Interpretation: Interpret the results in the context of your research or application. Compare the measured concentrations to relevant reference values or establish correlations with other parameters of interest.