Unit 2: Lipids and Amino acids - Subjective Questions

Fatty acids are carboxylic acids with long aliphatic hydrocarbon chains (typically 4–36 carbons) ending in a carboxyl group ().

General structure:

Properties:

• Saturation: Saturated fatty acids have no double bonds (e.g., palmitic acid, ); unsaturated ones have one or more double bonds (e.g., oleic acid).
• Melting point: Increases with chain length and decreases with degree of unsaturation. Saturated fatty acids are solid, unsaturated are liquid at room temperature.
• Solubility: Poorly soluble in water; solubility decreases with chain length. Short-chain acids are somewhat water soluble.
• Cis/trans isomerism: Natural unsaturated fatty acids are mostly in cis configuration, producing a kink that lowers packing and melting point.
• Amphipathic nature: The polar carboxyl head is hydrophilic while the hydrocarbon tail is hydrophobic.
• Chemical reactivity: Undergo esterification, saponification (with alkali to form soaps), hydrogenation, and oxidation (rancidity).

Lipids are broadly classified as follows:

1. Simple lipids — esters of fatty acids with alcohols:

• Fats and oils (triacylglycerols): esters of glycerol with fatty acids.
• Waxes: esters of long-chain fatty acids with long-chain monohydric alcohols.

2. Compound (complex) lipids — contain additional groups besides fatty acid and alcohol:

• Phospholipids: contain phosphoric acid (e.g., lecithin, cephalin).
• Glycolipids: contain carbohydrate (e.g., cerebrosides, gangliosides).
• Lipoproteins: lipids combined with proteins (e.g., HDL, LDL).

3. Derived lipids — substances obtained on hydrolysis of simple and compound lipids:

• Fatty acids, glycerol, steroids (e.g., cholesterol), fat-soluble vitamins (A, D, E, K), and ketone bodies.

Functional classification:

• Storage lipids (triacylglycerols)
• Structural/membrane lipids (phospholipids, glycolipids, cholesterol)
• Signaling and precursor lipids (steroids, eicosanoids).

Structure:

Triacylglycerol (triglyceride) is an ester formed from one glycerol molecule and three fatty acid molecules.

• The three hydroxyl groups of glycerol are esterified with three fatty acid carboxyl groups.
• If all three fatty acids are identical → simple triacylglycerol (e.g., tripalmitin).
• If different → mixed triacylglycerol.

Biological significance:

• Major energy store: yields ~9 kcal/g, more than double that of carbohydrates or proteins.
• Efficient storage: stored anhydrous in adipose tissue, occupying less space.
• Thermal insulation: subcutaneous fat helps maintain body temperature.
• Mechanical protection: cushions vital organs (kidney, heart).
• Metabolic fuel: hydrolyzed by lipases into glycerol and fatty acids for β-oxidation.

Waxes are esters of long-chain saturated or unsaturated fatty acids with long-chain monohydric alcohols.

Chemical nature:

where both and are long hydrocarbon chains (typically –).

Properties:

• Completely water-insoluble and highly hydrophobic.
• Solid at room temperature with high melting points.
• Chemically inert and resistant to hydrolysis.
• Not readily digested by most organisms.

Biological roles:

• Waterproofing: coat skin, fur, feathers, and leaves to prevent water loss.
• Protection: form protective coatings on fruits and insect exoskeletons.
• Energy storage: in marine organisms such as plankton and whales.
• Examples: beeswax, carnauba wax, lanolin, spermaceti.

Phospholipids are complex lipids containing phosphoric acid, along with fatty acids and an alcohol (glycerol or sphingosine).

Classification:

1. Glycerophospholipids (based on glycerol backbone):

• Phosphatidylcholine (lecithin)
• Phosphatidylethanolamine (cephalin)
• Phosphatidylserine
• Phosphatidylinositol
• Cardiolipin (diphosphatidylglycerol)

2. Sphingophospholipids (based on sphingosine backbone):

• Sphingomyelin — found in nerve and brain tissue.

General structure of glycerophospholipid:

• Glycerol backbone with two fatty acids at C-1 and C-2 (hydrophobic tails).
• A phosphate group esterified at C-3, linked to a polar head group (choline, ethanolamine, etc.).

Importance in membranes:

• Amphipathic nature: hydrophilic phosphate head and hydrophobic fatty acid tails allow formation of the lipid bilayer.
• Provide fluidity and flexibility to membranes.
• Act as emulsifying agents and in signal transduction (e.g., phosphatidylinositol).
• Serve as a selective permeability barrier.

Glycolipids are lipids containing carbohydrate residues attached to a lipid, usually built on a sphingosine (ceramide) backbone. They lack phosphate groups.

Structure:

• Ceramide (sphingosine + fatty acid) linked to one or more sugar units via a glycosidic bond.

Types:

• Cerebrosides: contain a single sugar (glucose or galactose). Example: galactocerebroside in nerve tissue.
• Globosides: contain two or more sugar residues but no sialic acid.
• Gangliosides: contain oligosaccharides with one or more sialic acid (N-acetylneuraminic acid) residues; abundant in nerve tissue.
• Sulfatides: cerebrosides esterified with sulfate.

Functions:

• Major components of the outer leaflet of the plasma membrane.
• Involved in cell–cell recognition and communication.
• Serve as antigenic determinants (e.g., ABO blood group antigens).
• Act as receptors for hormones, toxins, and viruses.
• Important for nerve tissue structure and function.

Structure of cholesterol:

• A steroid consisting of a cyclopentanoperhydrophenanthrene ring system (three fused six-membered rings and one five-membered ring).
• Contains a hydroxyl group () at C-3 (polar head), a double bond between C-5 and C-6, and an 8-carbon branched hydrocarbon side chain at C-17.
• Molecular formula: .
• Amphipathic: the group is hydrophilic; the ring system and tail are hydrophobic.

Biological significance:

• Membrane component: regulates membrane fluidity and permeability in animal cells.
• Precursor of steroid hormones: glucocorticoids, mineralocorticoids, sex hormones (estrogen, testosterone, progesterone).
• Precursor of bile acids: aids in fat digestion and absorption.
• Precursor of vitamin D.
• Clinical relevance: high LDL cholesterol contributes to atherosclerosis and cardiovascular disease.

Stereochemistry of amino acids:

• All amino acids (except glycine) contain a chiral (asymmetric) α-carbon bonded to four different groups: an amino group (), a carboxyl group (), a hydrogen atom, and a variable R group.
• This chirality gives rise to optical isomers (enantiomers) designated D and L using the Fischer projection and comparison with glyceraldehyde.

D and L designation:

• In the Fischer projection, with the carboxyl group at top and R group at bottom:
  • L-amino acid: on the left.
  • D-amino acid: on the right.

Why L-configuration predominates:

• Proteins are synthesized almost exclusively from L-amino acids.
• Enzymes and ribosomes are stereospecific and recognize only L-forms.
• This uniformity ensures correct protein folding and biological function.

Note: D-amino acids do occur but rarely — e.g., in bacterial cell walls and certain peptide antibiotics (gramicidin).

Threonine and isoleucine have two chiral centers, giving four stereoisomers each.

Amino acids are classified based on the properties of their side chains (R groups):

1. Nonpolar, aliphatic R groups:

• Glycine, Alanine, Valine, Leucine, Isoleucine, Proline, Methionine.
• Hydrophobic; cluster in protein interior.

2. Aromatic R groups:

• Phenylalanine, Tyrosine, Tryptophan.
• Largely nonpolar; absorb UV light at 280 nm.

3. Polar, uncharged R groups:

• Serine, Threonine, Cysteine, Asparagine, Glutamine.
• Contain groups (–OH, –SH, amide) that form hydrogen bonds.

4. Positively charged (basic) R groups:

• Lysine, Arginine, Histidine.
• Carry a net positive charge at physiological pH.

5. Negatively charged (acidic) R groups:

• Aspartate, Glutamate.
• Carry a net negative charge at physiological pH due to carboxyl side chains.

Other classifications:

• Essential vs. non-essential amino acids.
• Glucogenic, ketogenic, or both based on metabolic fate.

Titration of glycine:

Glycine has two ionizable groups: the α-carboxyl and the α-amino group. When a fully protonated glycine solution is titrated with a strong base (), the following transitions occur:

Regions of the curve:

• Stage 1 (low pH): Fully protonated form (net charge ). As base is added, the carboxyl group loses a proton.
• pK₁ ≈ 2.34: midpoint where ; acts as a buffer zone.
• Isoelectric point (pI): net charge is zero; the zwitterion predominates.
• pK₂ ≈ 9.60: midpoint where ; second buffer zone.
• Stage 2 (high pH): fully deprotonated form (net charge ).

Definitions:

• : the pH at which a given ionizable group is half-dissociated; equals the pH at the midpoint of a buffering region.
• Isoelectric point (pI): the pH at which the molecule carries no net electrical charge. For glycine:

Uncommon amino acids are amino acids not among the 20 standard protein-forming amino acids. They arise either by post-translational modification of standard residues or occur as free/non-protein amino acids.

Examples formed by post-translational modification:

• 4-Hydroxyproline and 5-hydroxylysine: found in collagen; stabilize its triple-helix structure.
• γ-Carboxyglutamate: in prothrombin and other clotting factors; binds .
• Desmosine: cross-links elastin fibers.
• N-Methyllysine: found in myosin.
• Phosphoserine, phosphothreonine, phosphotyrosine: important in cell signaling.

Non-protein (free) amino acids:

• Ornithine and citrulline: intermediates of the urea cycle.
• Homocysteine: intermediate in methionine metabolism.
• β-Alanine: component of coenzyme A and pantothenic acid.
• GABA (γ-aminobutyric acid): inhibitory neurotransmitter.
• Selenocysteine: the '21st amino acid,' incorporated during translation.

Significance: They perform specialized structural, regulatory, metabolic, and signaling functions.

Additional notes:

• Essential fatty acids (linoleic, α-linolenic) are polyunsaturated and cannot be synthesized by the body.
• Unsaturated fatty acids increase membrane fluidity.
• Notation such as indicates 18 carbons, one double bond at position 9.

Zwitterion:

A zwitterion is a dipolar ion carrying both a positive and a negative charge simultaneously, with a net charge of zero. At physiological pH, amino acids exist predominantly as zwitterions:

• The carboxyl group loses a proton to become .
• The amino group gains a proton to become .

Amphoteric nature:

Amino acids can act as both acids and bases (amphoteric/ampholytes):

• As an acid: the group can donate a proton.
• As a base: the group can accept a proton.

Effect of pH on net charge:

• At low pH (acidic): amino acid is fully protonated → net positive charge ().
• At the isoelectric point (pI): exists as zwitterion → net zero charge.
• At high pH (basic): fully deprotonated → net negative charge ().

Below the pI the molecule migrates toward the cathode (positive), and above the pI toward the anode (negative) during electrophoresis.

Common features:

• Both are amphipathic complex lipids with hydrophilic heads and hydrophobic tails.
• Both are integral components of biological membranes.
• Both contribute to membrane structure and cellular communication.

(a) Saponification value:

• The number of milligrams of required to saponify 1 gram of fat or oil.
• Inversely proportional to the average molecular weight of the fatty acids: higher value → shorter chain fatty acids.

(b) Iodine number (Iodine value):

• The number of grams of iodine absorbed by 100 g of fat.
• A measure of the degree of unsaturation: higher iodine number → more double bonds. Useful to distinguish oils from fats.

(c) Acid value (Acid number):

• The number of milligrams of required to neutralize the free fatty acids in 1 gram of fat.
• Indicates the extent of rancidity/hydrolysis; higher value means more free fatty acids (spoilage).

(d) Reichert-Meissl (RM) number:

• The number of milliliters of required to neutralize the volatile, water-soluble fatty acids distilled from 5 g of fat.
• Used to detect adulteration of butter, which is rich in short-chain volatile fatty acids like butyric acid.

General structure:

An α-amino acid consists of a central α-carbon bonded to:

• An amino group ()
• A carboxyl group ()
• A hydrogen atom ()
• A variable side chain ()

The R group determines the identity and chemical properties of the amino acid.

Buffering action:

• Amino acids resist changes in pH near their values because of the equilibrium between protonated and deprotonated forms.
• According to the Henderson–Hasselbalch equation:

• Maximum buffering capacity occurs when , i.e., when .
• Glycine buffers effectively around pH 2.34 (carboxyl group) and pH 9.60 (amino group).
• Amino acids with ionizable side chains (e.g., histidine, ) can buffer near physiological pH, making histidine important in proteins like hemoglobin.

Essential amino acids:

• Cannot be synthesized by the human body and must be obtained from the diet.
• There are 9 essential amino acids:
  • Phenylalanine, Valine, Threonine, Tryptophan, Methionine, Histidine, Isoleucine, Leucine, Lysine.
  • (Mnemonic: PVT TIM HaLL)

Non-essential amino acids:

• Can be synthesized by the body from other compounds/metabolic intermediates.
• Examples: Alanine, Asparagine, Aspartate, Glutamate, Serine, Glycine, Proline, etc.

Conditionally essential amino acids:

• Normally non-essential but become essential during illness, stress, or in infants.
• Examples: Arginine, Cysteine, Glutamine, Tyrosine.

Nutritional importance:

• Complete proteins (e.g., animal proteins, eggs) contain all essential amino acids.
• Incomplete proteins (many plant proteins) lack one or more; combining foods (e.g., rice + beans) provides all essential amino acids.
• Deficiency of essential amino acids leads to negative nitrogen balance, impaired growth, and protein-energy malnutrition (e.g., kwashiorkor).

Cholesterol and membrane fluidity:

• Cholesterol is inserted into the phospholipid bilayer with its hydroxyl group oriented toward the polar heads and its rigid ring system interacting with fatty acyl chains.
• Its effect on fluidity is temperature-dependent:
  • At high temperatures: cholesterol restrains the movement of phospholipid tails, decreasing fluidity and increasing order.
  • At low temperatures: it prevents tight packing of fatty acyl chains, increasing fluidity and preventing the membrane from crystallizing.
• Thus cholesterol acts as a fluidity buffer, maintaining optimal membrane fluidity and reducing permeability to small molecules.

Transport in blood via lipoproteins:

Because cholesterol is hydrophobic, it is carried in the plasma packaged into lipoprotein particles:

• Chylomicrons: transport dietary lipids from intestine.
• VLDL (Very Low Density Lipoprotein): carries endogenous triacylglycerols from liver.
• LDL (Low Density Lipoprotein): carries cholesterol to tissues; high levels are atherogenic ('bad cholesterol').
• HDL (High Density Lipoprotein): carries cholesterol from tissues back to liver (reverse cholesterol transport); protective ('good cholesterol').

An imbalance (high LDL, low HDL) increases risk of atherosclerosis and coronary heart disease.

Derivation:

Consider the dissociation of a weak acid :

The acid dissociation constant is:

Rearranging for :

Taking the negative logarithm of both sides:

Since and :

This is the Henderson–Hasselbalch equation.

Application to amino acids:

• Allows calculation of the ratio of protonated to deprotonated forms of each ionizable group at a given pH.
• When : ; the group is half-ionized (buffering maximum).
• When : the protonated (acidic) form predominates.
• When : the deprotonated (basic) form predominates.
• Used to predict the net charge of an amino acid or protein at any pH and to determine the isoelectric point (pI), which is critical in techniques like electrophoresis and ion-exchange chromatography.

Isoelectric point (pI):

The pI is the pH at which an amino acid carries no net charge. For amino acids with an ionizable side chain, the pI is calculated by averaging the two values that flank the zwitterionic (neutral) form.

Rule:

• Acidic amino acids (extra ): average the two lowest values.
• Basic amino acids (extra /basic group): average the two highest values.

Example 1 — Aspartate (acidic):

• (-COOH)
• (side-chain COOH)
• (-NH₃⁺)

Since aspartate is acidic, average the two lowest:

Example 2 — Lysine (basic):

• (-COOH)
• (-NH₃⁺)
• (side-chain -NH₃⁺)

Since lysine is basic, average the two highest:

Significance: At its pI an amino acid is least soluble and does not migrate in an electric field, which is exploited in electrophoretic and chromatographic separations.