Simple Lipids: Fats, Oils, and Waxes

• Lipids are a class of heterogeneous, non-polar compounds
• Lipids are largely insoluble in water, but soluble in non-polar solvents like chloroform and benzene

Simple Lipids

• Simple lipids are fatty acid esters with various alcohols
• Fats are esters of fatty acids with glycerol and are also called oils
• Fats are also known as triglycerides because glycerol's three hydroxyl groups are esterified
• Waxes are esters of long-chain fatty acids with aliphatic or alicyclic high molecular weight monohydric alcohols
  • Monohydric alcohol refers to an alcohol with only one hydroxyl group
• Waxes are water-insoluble due to the weakly polar nature of the ester group

Lipid Classification

• Lipids can be divided into four major groups: simple lipids, compound lipids, derived lipids, and miscellaneous lipids

Simple Lipids Details

• Simple lipids include fats, oils, and waxes
• Triglycerides are uncharged due to the lack of ionizable groups
• Triglycerides constitute approximately 98% of the lipids in adipose tissue, 30% of plasma or liver lipids, and less than 10% of erythrocyte lipids
• Triglycerides are esters of glycerol with various fatty acids
• The neutral fats are commonly are called triglycerides due to the esterification of glycerol's three hydroxyl groups
• Esterification of glycerol with one molecule of fatty acid yields monoglyceride, and with two molecules, diglyceride
• Simple triglycerides are those where the three fatty acids connected to glycerol are the same, such as tripalmitin
• Mixed triglycerides are those where the fatty acids are different types, like stearo-diolein and palmito-oleo-stearin
• A triglyceride (TG), also known as triacylglycerol (TAG) or triacylglyceride, is an ester derived from glycerol and three fatty acids
• Triglycerides are blood lipids that enable bidirectional transfer of adipose fat and blood glucose from the liver
• The degree of saturation varies among them depending on the oil source
• Triacylglycerols are esters composed of three fatty acids joined to glycerol (trihydroxy alcohol)
• The difference between fats and oils is their physical state at room temperature
• Fats are solid at 25°C, while oils are liquid at the same temperature
• Melting point differences reflect the degree of unsaturation of the constituent fatty acids
• Wax is an ester of a long-chain alcohol, usually mono-hydroxy which is bound to a fatty acid
• Wax acids and alcohols typically have chains of 12-34 carbon atoms

Compound Lipids

• Compound lipids include phospholipids, glycolipids, proteolipids, and sulfolipids
• Phospholipids yield fatty acids, glycerol, amino alcohol sphingosine, phosphoric acid, and nitrogen-containing alcohol upon hydrolysis
• They can be glycerophospholipids or sphingophospholipids, based on the presence of glycerol or sphingosine

Phospholipids

• One of glycerol's -OH groups is esterified to a phosphate (PO4) group in phospholipids
• Phospholipids form bilayers when suspended in water
• Cellular plasma membranes, mitochondrial membranes, and membranes of other organelles are formed of these lipid bilayers

Complex Lipids

• Complex lipids differ due to variations in glycerol and fatty acids
• Derivatization differs with different groups
• Their reactivity and functional requirements depend on their reactivity and functional requirements (task performing)
• Components may include protein, glycoprotein, phosphorus compounds, additional complex lipids, nitrogen compounds, and nitrogen-containing alcohols

Lipid Functions

• Lipids function both in food and inside the body as an energy storehouse
• Most of the energy needed by the human body is fulfilled by carbohydrates and lipids, with fat substances satisfying 30-70% of energy requirement
• They are a cell membrane structural component that helps to maintain cell liquidity and elasticity
• There are two types of fats in the human body: visceral and subcutaneous
• Visceral fats protect important organs such as the kidney, liver, and heart
• They act as energy storehouses through burning lipid
• Lipids, along with carbohydrates, fulfill energy production through acetyl CoA entering the citric acid cycle, and other metabolic reactions (30-70%)
• They are an integral part of every membrane and maintain cell liquidity elasticity cushioning and anchoring platform

Lipids in Metabolic Pathways

• Lipids act as precursors in many metabolic pathways
• This occurs both in the cytoplasm and through assembling transient functional structures
• They serve as membrane ingredients and anchoring platforms for various protein units serve as metabolic intermediates in fatty acid metabolism
• Lipids serve as precursors for newer metabolites like vitamins, LDL, HDL, and body pigments

Fatty Acid Breakdown

• Long-chain fatty acids become acyl-CoA and undergo β-oxidation
• Acyl-CoA is a group of coenzymes that metabolize fatty acids
• Acyl-CoA's are susceptible to β-oxidation, forming ultimately acetyl-CoA
• Acetyl-CoA enters the citric acid cycle, eventually forming several equivalents of ATP
• Fats are converted to ATP, the universal biochemical energy carrier.
• Dietary and storage lipids from adipose tissue are also a main source of energy

Lipid Biosynthesis

• Complex lipids are synthesized through complex pathways
  • This includes triglyceride, fatty acid, and cholesterol biosynthesis

Krebs Cycle

• Pyruvate transitions into modified forms through carboxyl group removal and oxidation before it is bound to Coenzyme A.
• Acetyl CoA facilitates the transfer of its acetyl group to oxaloacetate, which results in the formation of citrate, and the beginning of the citric acid cycle.

Triglycerides

• Triglycerides are broken down into glycerol and fatty acids
• Glycerol is converted to glycerol-3-phosphate via glycerol kinase
• Glycerol-1-phosphatase catalyzes the hydrolysis of glycerol 3-phosphate to regenerate glycerol, allowing glycerol fermentation

Glycolipids

• Glycolipids are esters of fatty acids with alcohol and contain carbohydrate groups
• The typical glycolipid has a mono- or oligosaccharide group attached to an alcohol sphingolipid or glycerol group

Diglycerides

• A diglyceride, or diacylglycerol (DAG), is a glyceride with two fatty acid chains bonded to a single glycerol molecule through ester linkages
• When one OH group of DAG is linked to carbohydrates, it becomes a glycolipid

Glycolipid Classification

• Glycolipids are classified based on their alcohol component:
• Glycoglycerolipids
• found more in plants, incorporate glycerol
• Glycosphingolipids
• found predominantly in animals and humans, incorporate sphingosine
• Sphingolipids are also called Ceramides
• This is especially true with sphingolipid molecules that are linked to a Carbohydrate

Cerebrosides

• Cerebrosides contain a mono-glycosyl-ceramide and can be further classified:
  • Kerasin has Lignoceric acid (C24) SFA
  • Cerebron has Cerebronic acid (C24) Hydroxy SFA
  • Nervon has Nervonic acid (C24) MUFA
  • Oxynervon has Oxynervonic acid (C24) MUFA

Gangliosides

• Gangliosides are a class of molecules that contain structure carbohydrate moieties that includes
• Glucose
• Galactose
• N-Acetyl Galactosamine
• N-Acetyl Neuraminic Acid (NANA)/Sialic acid
• Depending on chemical structure, 30+ types have been isolated, varying in number and positions of NANAs.
• Types based on number of NANA residues

Glycosphingolipids

• The most complex glycolipids present in animals are gangliosides
• Glycosphingolipids that contain phosphatidylinositol as the lipid component are connected to the carbohydrate and glycophosphatidylinositols

Globosides

• Globosides are structurally Ceramide, a structure with an association with Oligosaccharide components and complexes