Lipolysis

Triacylglycerol (TG)

Also known as Triglyceride, it comprises 95% of lipid consumed in the diet and functions as adipose storage. Its structure consists of a glycerol "backbone" attached to three fatty acids.

Lipids in Food

Common oils include Sunflower, Safflower, Corn, Oleic, Coconut, and Palm oil. Common fats include Butter, Lard, and Tallow. Cholesterol is primarily found in animal foods.

Simple TG

A type of triglyceride where all three fatty acids (FAs) are identical.

Mixed TG

A type of triglyceride where the fatty acids are different.

Saturated Fatty Acids (SFA)

Fatty acids that have no double bonds between carbons. Examples include Palmitic acid (16:0) and Stearic acid (18:0).

Unsaturated Fatty Acids (UFA)

Fatty acids that contain one or more double bonds. Examples include Oleic acid (18:1) and Linoleic acid (18:2).

Fatty Acid Structure

Consists of carbon atoms bonded to hydrogens. It has a polar carboxyl end (COOH), which is water soluble, and a nonpolar methyl end (CH3), which is water insoluble.

Fatty Acid Chain Lengths

Classified as short chains (2-4 carbons), medium chains (6-12 carbons), and long chains (14-26 carbons).

Monounsaturated Fatty Acid

A fatty acid containing one double bond between carbons.

Polyunsaturated Fatty Acid

A fatty acid containing multiple double bonds between carbons.

Lipogenesis

The metabolic process of synthesizing fatty acids from acetyl CoA, primarily in the liver, but also in adipose tissue, mammary glands, lungs, brain, and kidneys.

Acetyl CoA in Lipogenesis

Derived from carbohydrates, fats, ketogenic amino acids, and alcohol, serving as a precursor for fatty acid synthesis.

Fatty Acid Synthesis Location

Occurs in the cytosol, starting from the methyl end towards the carboxylic acid end, with palmitate (16C) being the primary synthesized fatty acid.

Key Enzyme in Fatty Acid Synthesis

Acetyl CoA carboxylase, which forms a complex called fatty acid synthase (FAS).

FAS Components

Include the Condensing enzyme (CE) and Acyl carrier protein (ACP), both having free –SH groups for attachment.

Regulation of Fatty Acid Synthesis

Enhanced by insulin, cytosolic citrate, and high carbohydrate diets; inhibited by long-chain acyl-CoA, cAMP, glucagon, and elevated plasma free fatty acids (FFA).

Malonyl CoA Formation

Synthesized when acetyl CoA is carboxylated by acetyl CoA carboxylase; this is an irreversible and rate-limiting step in fatty acid biosynthesis.

Fatty Acid Elongation

Occurs in the endoplasmic reticulum or mitochondria, where malonyl CoA contributes two-carbon units and reducing power comes from NADPH, catalyzed by fatty acid elongase enzymes.

Fatty Acid Desaturation

Involves enzymes like Delta-5, -6, and -9 desaturases. Humans cannot desaturate beyond the 9th carbon.

Essential Fatty Acids (EFA)

Linoleic (18:2 w-6) and linolenic acid (18:3 w-3) are crucial for growth, reproduction, cell membrane formation, brain development, and eicosanoid production.

Cis vs. Trans Fatty Acids

Cis FAs have hydrogens on the same side of the double bond, causing kinks. Trans FAs have hydrogens on opposite sides, leading to straighter chains and less membrane fluidity, which can impair metabolic functions.

Phospholipids Structure

Comprise a glycerol backbone, two fatty acids, and a phosphatidic acid unit. They have distinct hydrophilic and hydrophobic sides, facilitating their function in cell membranes.

Sterols

Includes steroid hormones (e.g., estrogen, androgens), cholesterol, and vitamin D.

Cholesterol Synthesis

Approximately 700 mg are synthesized daily in the liver, originating from the condensation of acetyl CoA, involving the formation of HMG-CoA catalyzed by HMG-CoA reductase.

Digestion of Lipids

Involves enzymes like lingual lipase, gastric lipase, and intestinal mucosal-secreted lipase, occurring in intraluminal, mucosal, and secretory phases.

Intraluminal Phase

Occurs in the upper jejunum, relying on emulsification by bile salts to form micelles for fat absorption.

Mucosal Phase

Absorption of free fatty acids and monoglycerides occurs; shorter-chain fatty acids enter the portal vein, while longer-chain fatty acids are re-synthesized into triglycerides and absorbed as chylomicrons.

Secretory Phase

Chylomicrons are released from mucosal cells into lymphatic circulation, while short-chain fatty acids exit to the portal vein.

Lipoproteins

Integral components for lipid transport, including triglycerides, cholesterol esters, phospholipids, and proteins. Types include Chylomicrons, VLDL, LDL, and HDL.

Ketogenesis

The production of ketone bodies when lipolysis exceeds triglyceride formation, typically seen with low insulin levels or insufficient carbohydrate intake.

Ketone Bodies

Main sources of ketone production from fatty acid oxidation, amino acids, and alcohol oxidation. Key ketones are acetoacetate, acetone, and eta-hydroxybutyrate, providing energy for cells except red blood cells.

Triacylglycerol (TG)

Also known as Triglyceride. Comprises 95% of lipid consumed in the diet and functions as adipose storage, consisting of a glycerol backbone attached to three fatty acids.

Phospholipids

Lipids that contain a glycerol backbone, two fatty acids, and a phosphatidic acid unit substituting for the third fatty acid.

Sterols

A variety of steroid hormones, cholesterol, and vitamin D.

Simple TG

A type of triglyceride where all three fatty acids (FAs) are identical.

Mixed TG

A type of triglyceride where the fatty acids are different.

Saturated Fatty Acids (SFA)

Fatty acids with no double bonds between carbons, such as Palmitic acid (16:0) and Stearic acid (18:0).

Unsaturated Fatty Acids (UFA)

Fatty acids containing one or multiple double bonds, such as Oleic acid (18:1) and Linoleic acid (18:2).

Polar Carboxyl End

The water-soluble COOH end of a fatty acid.

Nonpolar Methyl End

The water-insoluble CH_3 end of a fatty acid, also identified as the omega end (ω).

Short-chain Fatty Acids

Fatty acids with 2-4 carbons.

Medium-chain Fatty Acids

Fatty acids with 6-12 carbons.

Long-chain Fatty Acids

Fatty acids with 14-26 carbons.

Monounsaturated Fatty Acids

Fatty acids that contain one double bond.

Polyunsaturated Fatty Acids

Fatty acids that contain multiple double bonds.

Lipogenesis

The metabolic process of synthesizing fatty acids from acetyl CoA, primarily in the liver.

Acetyl CoA

A precursor for fatty acid synthesis, derived from carbohydrates, fats, ketogenic amino acids, and alcohol.

Citrate Lyase

An enzyme that converts citrate to oxaloacetate (OAA) and acetyl CoA in the cytoplasm, making acetyl CoA available for fatty acid synthesis.

Palmitate (16C)

The primary fatty acid synthesized first in the cytosol, which can then undergo elongation or desaturation.

Acetyl CoA carboxylase

A key enzyme in fatty acid synthesis that catalyzes the irreversible, rate-limiting step of malonyl CoA formation from acetyl CoA.

Fatty Acid Synthase (FAS)

A complex of enzymes formed during fatty acid synthesis, including condensing enzyme (CE) and acyl carrier protein (ACP).

Malonyl CoA

An intermediate in fatty acid synthesis, formed when acetyl CoA is carboxylated by acetyl CoA carboxylase.

Fatty Acid Elongation

The process where malonyl CoA contributes two-carbon units to a fatty acid chain in the endoplasmic reticulum or mitochondria, catalyzed by fatty acid elongase enzymes.

Fatty Acid Desaturation

The process of introducing double bonds into fatty acids, primarily by Delta-5, -6, and -9 desaturases, though humans cannot desaturate beyond the 9th carbon.

Cis FAs

Fatty acids where hydrogens are on the same side of the double bond, causing kinks that maintain fluidity.

Trans FAs

Fatty acids where hydrogens are on opposite sides of the double bond, leading to straighter chains and less membrane fluidity.

Essential Fatty Acids (EFA)

Fatty acids like Linoleic (18:2 w-6) and Linolenic acid (18:3 w-3) that humans cannot synthesize and must obtain from the diet, crucial for growth, reproduction, and cell membrane formation.

Esterification

The process by which fatty acids are joined via ester linkages to form triglycerides, monoglycerides, and diglycerides.

Lecithin

A phospholipid made in the liver.

Steroid Hormones

Examples include estrogen, androgens, DHEA, and adrenocorticoid hormones, which are types of sterols.

Cholesterol Synthesis

The process, primarily in the liver, where approximately 700 mg of cholesterol are synthesized daily from acetyl CoA, involving the formation of HMG-CoA.

PPARs (Peroxisome proliferator-activated receptors)

Nuclear hormone receptors (PPARα, PPARδ, PPARλ) that govern molecular control mechanisms of fat metabolism.

Lingual Lipase

An enzyme involved in the digestion of lipids.

Gastric Lipase

An enzyme involved in the digestion of lipids, secreted in the stomach.

Intraluminal Phase

The initial phase of lipid digestion occurring in the upper jejunum, relying on emulsification by bile salts to form micelles.

Micelles

Structures formed from mixed pancreatic secretions and chyme, essential for fat absorption during the intraluminal phase.

Mucosal Phase

The phase of lipid digestion where free fatty acids and monoglycerides are absorbed; shorter-chain fatty acids enter the portal vein, while longer-chain fatty acids are re-synthesized into triglycerides and absorbed as chylomicrons.

Secretory Phase

The final phase of lipid digestion where chylomicrons are released from mucosal cells into lymphatic circulation, and short-chain fatty acids exit to the portal vein.

Lipoproteins

Integral components for lipid transport, including triglycerides, cholesterol esters, phospholipids, and proteins; types include Chylomicrons, VLDL, LDL, and HDL.

Chylomicrons

A type of lipoprotein involved in transporting re-synthesized triglycerides from mucosal cells into lymphatic circulation.

Ketogenesis

The production of ketone bodies when lipolysis exceeds triglyceride formation, typically seen in conditions with low insulin levels or insufficient carbohydrate intake.

Ketone Bodies

Main sources of energy for cells (excluding red blood cells) derived from fatty acid oxidation, amino acids, and alcohol oxidation, including acetoacetate, acetone, and β-hydroxybutyrate.