Lipid Biochemistry: Fats, Membranes, and Signaling Molecules

Fatty Acids

Fatty acids are long hydrocarbon chains with a carboxyl group at one end. They typically contain an even number of carbon atoms, most commonly 16 (palmitic acid) or 18 (stearic acid, oleic acid), because they are synthesized by adding two-carbon units from acetyl-CoA. The hydrocarbon chain is hydrophobic, while the carboxyl group is hydrophilic, giving fatty acids an amphipathic character.

Saturated fatty acids have no carbon-carbon double bonds in their hydrocarbon chain. The absence of double bonds allows the chains to pack tightly together, producing fats that are solid at room temperature, such as butter and lard. Unsaturated fatty acids contain one (monounsaturated) or more (polyunsaturated) double bonds, each introducing a kink in the chain. These kinks prevent tight packing and result in oils that are liquid at room temperature, such as olive oil and fish oil.

The double bonds in naturally occurring unsaturated fatty acids are almost always in the cis configuration, meaning the hydrogen atoms on each side of the double bond are on the same side. Trans fatty acids, in which the hydrogens are on opposite sides, are rare in nature but are produced industrially by partial hydrogenation of vegetable oils. Trans fats raise LDL cholesterol and lower HDL cholesterol, increasing cardiovascular disease risk, which is why their use in processed foods has been restricted or banned in many countries.

Two polyunsaturated fatty acids are essential in the human diet because they cannot be synthesized by human cells: linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid). These serve as precursors for longer-chain fatty acids and for eicosanoids, a family of signaling molecules that includes prostaglandins, thromboxanes, and leukotrienes.

Triglycerides: Energy Storage

Triglycerides (triacylglycerols) are the primary form of stored fat in animals. Each molecule consists of a glycerol backbone esterified with three fatty acid chains. The ester bonds are formed through condensation reactions between the hydroxyl groups of glycerol and the carboxyl groups of the fatty acids.

Triglycerides are ideal energy storage molecules for several reasons. First, because fatty acid chains are highly reduced (rich in C-H bonds), they yield approximately 9 kilocalories per gram upon complete oxidation, more than twice the energy density of carbohydrates or proteins (about 4 kcal/g each). Second, triglycerides are hydrophobic, so they are stored without associated water, unlike glycogen, which binds roughly two grams of water per gram of glycogen. This means that a gram of stored fat contains roughly six times as much usable energy as a gram of stored glycogen.

Fat is stored in specialized cells called adipocytes, which can expand enormously to accommodate large triglyceride droplets. The average adult human carries enough stored fat to supply energy for several weeks of fasting, while glycogen reserves are depleted within a day. Mobilization of triglycerides begins with lipolysis, catalyzed by hormone-sensitive lipase. This enzyme is activated by epinephrine and glucagon (signaling energy demand) and inhibited by insulin (signaling energy abundance).

Phospholipids and Membranes

Phospholipids are the structural foundation of all biological membranes. Each phospholipid molecule has a hydrophilic head group and two hydrophobic fatty acid tails, making it strongly amphipathic. The most common phospholipids are glycerophospholipids, which have a glycerol backbone with two fatty acid chains esterified at positions 1 and 2, and a phosphate group at position 3 that is linked to a polar head group such as choline (phosphatidylcholine), ethanolamine (phosphatidylethanolamine), serine (phosphatidylserine), or inositol (phosphatidylinositol).

When placed in water, phospholipids spontaneously assemble into a bilayer, with the hydrophobic tails facing inward and the hydrophilic heads facing outward toward the aqueous environment. This bilayer arrangement is the structural basis of all cell membranes. The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the membrane as a two-dimensional fluid in which phospholipids move laterally but rarely flip between layers. Proteins are embedded in or associated with the bilayer, performing functions such as transport, signaling, and enzymatic catalysis.

Membrane fluidity is influenced by fatty acid composition and cholesterol content. Unsaturated fatty acids increase fluidity because their kinked chains prevent tight packing. Cholesterol has a dual effect: at high temperatures it reduces fluidity by restricting phospholipid movement, while at low temperatures it prevents the membrane from becoming too rigid by disrupting the regular packing of fatty acid chains. This buffering effect is important for maintaining membrane function across a range of temperatures.

Steroids and Cholesterol

Steroids are lipids with a characteristic four-ring carbon skeleton: three six-membered rings and one five-membered ring. Cholesterol is the most abundant steroid in animal tissues and serves as the precursor for all other steroid molecules. It is an essential component of cell membranes, where it modulates fluidity, and it is the starting material for the synthesis of steroid hormones, bile acids, and vitamin D.

Steroid hormones include cortisol (a glucocorticoid that regulates metabolism and immune responses), aldosterone (a mineralocorticoid that regulates sodium and potassium balance), testosterone and estradiol (sex hormones that direct reproductive development and function), and progesterone (which maintains pregnancy). Because steroids are hydrophobic, they can cross cell membranes and bind to intracellular receptors that act as transcription factors, directly regulating gene expression.

Cholesterol synthesis occurs primarily in the liver through a complex pathway that begins with acetyl-CoA and proceeds through mevalonate. The rate-limiting enzyme is HMG-CoA reductase, the target of statin drugs. Statins competitively inhibit this enzyme, reducing cholesterol synthesis and lowering blood LDL cholesterol levels. Cholesterol is transported in the blood by lipoproteins: LDL delivers cholesterol to peripheral tissues, while HDL carries excess cholesterol back to the liver for excretion, a process called reverse cholesterol transport.

Lipid Signaling

Beyond their structural and energy storage roles, lipids serve as important signaling molecules. Phosphatidylinositol-4,5-bisphosphate (PIP2), a minor component of the inner leaflet of the plasma membrane, is a key signaling lipid. When a cell-surface receptor is activated, the enzyme phospholipase C cleaves PIP2 into two second messengers: inositol-1,4,5-trisphosphate (IP3), which triggers calcium release from the endoplasmic reticulum, and diacylglycerol (DAG), which activates protein kinase C.

Eicosanoids are signaling lipids derived from arachidonic acid, a 20-carbon polyunsaturated fatty acid released from membrane phospholipids by phospholipase A2. Prostaglandins mediate inflammation, pain, and fever. Thromboxanes promote platelet aggregation and blood clotting. Leukotrienes are involved in allergic and inflammatory responses. Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) work by inhibiting cyclooxygenase (COX), the enzyme that converts arachidonic acid into prostaglandins and thromboxanes.

Sphingolipids, built on a sphingosine backbone rather than glycerol, are another class of signaling lipids. Ceramide and sphingosine-1-phosphate regulate cell growth, differentiation, and apoptosis. These signaling functions highlight that lipids are far more than passive structural materials or inert energy stores.

Fatty Acid Oxidation

When cells need energy, fatty acids are broken down through beta-oxidation, a repetitive four-step cycle that occurs in the mitochondrial matrix. Each cycle removes a two-carbon unit as acetyl-CoA and produces one NADH and one FADH2. The acetyl-CoA enters the citric acid cycle, and the electron carriers donate their electrons to the electron transport chain for ATP production.

Before entering the mitochondria, long-chain fatty acids must be activated by attachment to coenzyme A (forming fatty acyl-CoA) and then transported across the inner mitochondrial membrane by the carnitine shuttle. Carnitine palmitoyltransferase I (CPT-I), located on the outer mitochondrial membrane, is the rate-limiting step and is inhibited by malonyl-CoA, the first committed intermediate of fatty acid synthesis. This reciprocal regulation ensures that fatty acid synthesis and oxidation do not occur simultaneously.

The complete oxidation of a 16-carbon fatty acid (palmitate) yields 106 ATP molecules after accounting for the 2 ATP equivalents used for activation, compared to approximately 30 to 32 ATP from one glucose molecule. This enormous energy yield explains why fats are the preferred fuel for sustained, moderate-intensity exercise and why migratory animals accumulate fat stores before long journeys.