Proteins

PROTEINS – Complete Exam Notes

Introduction

Proteins are the most abundant and functionally diverse molecules in living systems. They play crucial roles in virtually all biological processes. The term protein is derived from the Greek word "proteios", meaning "of first importance" or "primary". This highlights their fundamental role in life processes.

Proteins are high molecular weight polymers composed of amino acids linked together by peptide bonds. They exhibit a wide range of structures and functions, making them essential for growth, repair, and regulation within organisms.

Definition and Composition

Proteins are complex molecules characterized by:

• High molecular weight polymers

• Composed of amino acids

• Linked via peptide bonds

General Formula

There is no single universal formula for proteins as their composition varies depending on the amino acid sequence.

Elemental Composition

• Carbon (C): 50–55%

• Hydrogen (H): 6–8%

• Oxygen (O): 20–23%

• Nitrogen (N): 15–18% (a key feature that distinguishes proteins from carbohydrates and lipids)

• Sulfur (S): 0–4% (present in some amino acids like cysteine and methionine)

[Figure: General structure of a protein showing amino acids linked together]

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Biological Importance of Proteins

Function Description Examples

Catalytic Enzymes accelerate biochemical reactions, facilitating vital metabolic processes. Amylase, Pepsin, Lipase

Structural Provide shape, support, and mechanical strength to cells and tissues. Collagen, Keratin, Elastin

Transport Carry substances in blood and across cell membranes. Hemoglobin (O₂), Albumin (Fats), Transferrin (Iron)

Defense Protect against pathogens and foreign invaders. Antibodies (Immunoglobulins)

Regulatory Control cellular activities and physiological processes. Hormones like Insulin, Growth Hormone

Contractile Enable movement within muscles and other tissues. Actin, Myosin

Storage Store amino acids and nutrients for future use. Ferritin (Iron), Casein (Milk)

Receptor Receive signals from outside the cell to trigger responses. Insulin Receptor, G-protein Coupled Receptors (GPCRs)

Signaling Transmit signals between cells to coordinate activities. Cytokines, Growth Factors

Buffering Maintain pH stability of body fluids. Albumin, Hemoglobin

[Figure: Functions of proteins diagram with icons]

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Amino Acids – The Building Blocks

Definition

Amino acids are organic compounds containing two functional groups:

1. Amino group (-NH₂) – Basic

2. Carboxyl group (-COOH) – Acidic

Each amino acid also has a side chain (R group) that determines its unique properties and functions.

General Structure of an Amino Acid

Visualize the structure as:

Where:

• H₂N = Amino group

• COOH = Carboxyl group

• H = Hydrogen atom

• R = Side chain (varies for each amino acid)

All amino acids (except glycine) have a chiral carbon (α-carbon), which exhibits optical activity, important in stereochemistry and biological recognition.

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Classification of Amino Acids

1. Based on Structure (Chemical nature of R group)

A. Aliphatic amino acids

• Glycine (Gly, G) – R=H (no chiral carbon)

• Alanine (Ala, A) – R=CH₃

• Valine (Val, V) – Branched chain

• Leucine (Leu, L) – Branched chain

• Isoleucine (Ile, I) – Branched chain

B. Hydroxyl group containing

• Serine (Ser, S) – R=CH₂OH

• Threonine (Thr, T) – R=CH(OH)CH₃

C. Sulfur containing

• Cysteine (Cys, C) – R=CH₂SH (forms disulfide bonds)

• Methionine (Met, M) – R=CH₂CH₂SCH₃

D. Acidic amino acids and their amides

• Aspartic acid (Asp, D) – R=CH₂COOH

• Glutamic acid (Glu, E) – R=CH₂CH₂COOH

• Asparagine (Asn, N) – Amide of Aspartic acid

• Glutamine (Gln, Q) – Amide of Glutamic acid

E. Basic amino acids

• Lysine (Lys, K) – Contains extra NH₂

• Arginine (Arg, R) – Contains guanidino group

• Histidine (His, H) – Contains imidazole ring

F. Aromatic amino acids

• Phenylalanine (Phe, F) – Benzene ring

• Tyrosine (Tyr, Y) – Phenol ring

• Tryptophan (Trp, W) – Indole ring

G. Imino acid

• Proline (Pro, P) – Contains secondary amino (imino) group

[Figure: All 20 amino acids structures grouped by category]

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2. Based on Polarity (Important for protein folding)

Type Nature Amino acids

Non-polar (hydrophobic) Avoid water, found inside proteins Gly, Ala, Val, Leu, Ile, Met, Pro, Phe, Trp

Polar (hydrophilic) Attract water Ser, Thr, Cys, Tyr, Asn, Gln

Acidic (negatively charged at pH 7) –COO⁻ Asp, Glu

Basic (positively charged at pH 7) –NH₃⁺ Lys, Arg, His

[Figure: Polarity classification of amino acids table]

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3. Based on Nutritional Requirement

Type Meaning Amino acids

Essential Cannot be synthesized by body; must come from diet Val, Leu, Ile, Lys, Met, Phe, Thr, Trp, His, Arg (Arg and His only in children)

Non-essential Can be synthesized by body Ala, Asn, Asp, Cys, Glu, Gln, Gly, Pro, Ser, Tyr

Conditionally essential Essential only in certain conditions (illness, stress) Arg, Cys, Gln, Tyr, His, Pro, Gly

Mnemonic for essential amino acids: PVT TIM HALL (Phe, Val, Thr, Trp, Ile, Met, His, Arg, Leu, Lys). A better mnemonic is MATTVIL PHL (Met, Arg, Thr, Trp, Val, Ile, Leu, Phe, His, Lys).

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Properties of Amino Acids

Physical Properties

Property Description

Color Colorless, crystalline solids

Taste Sweet (Gly, Ala), Tasteless (Leu), Bitter (Arg, Ile)

Solubility Soluble in water, insoluble in organic solvents

Melting point High (>200°C)

Optical activity All except Gly are optically active (L-form in nature)

Chemical Properties

1. Zwitterion (Amphoteric nature)

At neutral pH, amino acids exist as zwitterions, which are dipolar ions:

In zwitterion form:

• -NH₂ accepts H⁺ → -NH₃⁺

• -COOH donates H⁺ → -COO⁻

2. Isoelectric point (pI)

The pH at which an amino acid has zero net charge, meaning the positive and negative charges balance out. This is critical in protein purification and analysis.

• At pH < pI: Net positive charge (moves toward cathode)

• At pH = pI: No net charge (no movement in electric field)

• At pH > pI: Net negative charge (moves toward anode)

Example: Glycine pI = 6.0

3. Reactions of amino group

• Ninhydrin reaction: All amino acids react to produce a purple color; proline yields yellow.

• Sanger's reagent (FDNB): Used for N-terminal amino acid identification.

• Dansyl chloride: Also used for N-terminal analysis.

4. Reactions of carboxyl group

• Can form esters with alcohols.

• Can form amides with ammonia.

5. Reactions of side chains (specific to certain amino acids)

• Cysteine: Forms disulfide bonds (-S-S-)

• Tyrosine: Reacts with Millon's reagent (red color)

• Tryptophan: Reacts with glyoxylic acid (purple ring - Hopkins-Cole test)

• Arginine: Sakaguchi test (red color)

[Figure: Common amino acid reactions – ninhydrin, Sanger, etc.]

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Peptide Bond

Definition

A peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of a water molecule.

Formation of Peptide Bond

Dehydration synthesis occurs when:

Reaction: 

H₃N⁺—CH—COOH + H₃N⁺—CH—COOH → 

H₃N⁺—CH—CO—NH—CH—COOH + H₂O

Characteristics of Peptide Bond

Property Description

Type of bond Covalent (C-N)

Bond length Shorter than typical C-N, with partial double bond character

Rotation Restricted; rigid and planar

Configuration Always trans (except for proline)

Hydrolysis Requires strong acid/base or enzyme (protease)

Naming of Peptides

Number of amino acids Name

2 Dipeptide

3 Tripeptide

4 Tetrapeptide

5-10 Oligopeptide

10-50 Polypeptide

>50 Protein

[Figure: Peptide chain showing multiple peptide bonds]

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Classification of Proteins

1. Based on Shape (Structure)

A. Fibrous Proteins (Scleroproteins)

Property Description

Shape Long, thread-like, filamentous

Solubility Insoluble in water

Strength Strong, tough, resistant to digestion

Function Structural support

Examples Collagen, Keratin, Elastin, Fibroin

Collagen: Most abundant protein in mammals (25-30% of total body protein). Found in skin, bones, tendons, cartilage. Contains hydroxyproline and hydroxylysine, which are modified amino acids essential for stability.

Keratin: Found in hair, nails, horns, feathers. Rich in cysteine, forming disulfide bonds that provide rigidity and resilience.

[Figure: Fibrous protein structure – collagen triple helix]

B. Globular Proteins

Property Description

Shape Spherical, compact

Solubility Soluble in water and salt solutions

Function Dynamic functions such as enzymes, carriers, regulators

Examples Albumin, Globulins, Hemoglobin, Insulin, Enzymes

Hemoglobin: Oxygen carrier in blood. Contains four subunits (2α, 2β) and heme groups that bind oxygen efficiently.

Albumin: Maintains osmotic pressure in blood plasma and transports various substances.

[Figure: Globular protein structure – hemoglobin]

C. Intermediate Proteins

• Properties lie between fibrous and globular proteins.

• Example: Fibrinogen, involved in blood clotting.

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2. Based on Composition

A. Simple Proteins

• Yield only amino acids upon hydrolysis.

• Examples: Albumin, Globulin, Collagen, Elastin, Histone, Prolamin, Glutelin, Protamine.

B. Conjugated Proteins

• Contain a prosthetic group (non-protein component) attached to the protein.

• Yield amino acids plus the non-protein part upon hydrolysis.

Class Prosthetic Group Examples

Nucleoproteins Nucleic acid (DNA/RNA) Chromosomes, Ribosomes

Glycoproteins Carbohydrate Mucin, Antibodies, Hormones (TSH)

Lipoproteins Lipid LDL, HDL, VLDL

Hemoproteins Heme (iron-porphyrin) Hemoglobin, Myoglobin, Cytochromes

Metalloproteins Metal ion Ferritin, Ceruloplasmin

Phosphoproteins Phosphate group Casein, Vitellin

Chromoproteins Colored group Hemoglobin, Rhodopsin

C. Derived Proteins

o Obtained by hydrolysis of simple or conjugated proteins.

o Primary derived: Proteans, Metaproteins, Coagulated proteins.

o Secondary derived: Proteoses, Peptones, Peptides.

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3. Based on Function

Functional Class Function Examples

Catalytic Enzymes catalyze biochemical reactions. Pepsin, Trypsin, DNA Polymerase

Structural Support and shape in tissues. Collagen, Keratin, Elastin

Contractile Facilitate movement in muscles. Actin, Myosin, Tubulin

Transport Carry substances within organisms. Hemoglobin, Albumin, Transferrin

Storage Reserve amino acids and nutrients. Ferritin, Casein, Ovalbumin

Protective Defense against pathogens. Immunoglobulins, Complement Proteins

Regulatory Hormonal regulation. Insulin, Growth Hormone, Glucagon

Receptor Signal reception on cell surfaces. Insulin receptor, Neurotransmitter receptors

Toxic Defense or attack mechanisms. Snake venom, Diphtheria toxin, Ricin

Nutrient Food source of amino acids. Ovalbumin, Casein, Glutelin

[Figure: Functional classification of proteins table with icons]

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Levels of Protein Structure

Understanding the four levels of protein structure is essential for grasping how proteins function and how their structure relates to their activity.

1. Primary Structure (1°)

Definition: The linear sequence of amino acids in a polypeptide chain.

Bonds involved: Peptide bonds and disulfide bonds (if present).

Determination: Through amino acid analysis, N-terminal sequencing (Sanger method, Edman degradation), C-terminal analysis, and mass spectrometry.

Example: Insulin has 51 amino acids in two chains (A and B) linked by disulfide bonds.

[Figure: Primary structure – linear sequence of amino acids]

Clinical Significance: Sickle cell anemia results from a substitution of glutamic acid by valine at position 6 of the β-globin chain, causing abnormal hemoglobin.

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2. Secondary Structure (2°)

Definition: Local folding patterns of the polypeptide backbone stabilized by hydrogen bonds between –NH and –C=O groups.

Types:

A. α-Helix

o Right-handed coil

o 3.6 amino acids per turn

o Hydrogen bonds between C=O of residue n and NH of residue n+4

o R groups point outward

o Found in keratin, myoglobin, troponin

B. β-Pleated Sheet

o Extended zigzag conformation

o Hydrogen bonds between adjacent chains

o Types: Parallel and antiparallel (more stable)

o R groups point above and below the sheet

o Found in silk fibroin

C. β-Turn (Reverse turn)

o Allows chain to reverse direction

o Usually contains glycine or proline

o Stabilized by hydrogen bonds

D. Random Coil (Loop)

o No regular repeating structure

o Flexible region

[Figures: α-helix, β-sheet structures]

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3. Tertiary Structure (3°)

Definition: The three-dimensional folding of a single polypeptide chain, involving interactions of side chains.

Bonds/interactions involved:

Interaction Type Strength Between

Hydrophobic interactions Non-covalent Weak Non-polar R groups (cluster inside)

Hydrogen bonds Non-covalent Weak Polar R groups

Ionic bonds (salt bridges) Non-covalent Weak Charged R groups

Disulfide bonds Covalent Strong –SH of cysteine residues

Van der Waals forces Non-covalent Very weak All atoms

Domains are independently folded regions within a protein, often 100-200 amino acids long. Example: Myoglobin, which contains a heme group in a hydrophobic pocket.

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4. Quaternary Structure (4°)

Definition: Arrangement of two or more polypeptide subunits into a functional protein.

Subunits may be:

o Homodimer: Two identical subunits

o Heterodimer: Two different subunits

o Multimer: Multiple subunits

Bonds involved: Same as tertiary (hydrophobic, hydrogen bonds, ionic, disulfide bonds).

Examples:

Protein Subunits Function

Hemoglobin 4 subunits (2α, 2β) Oxygen transport

Immunoglobulin G 4 subunits (2 heavy, 2 light chains) Antibody

Lactate dehydrogenase 4 subunits (H or M types) Enzyme

Insulin 2 subunits (A and B chains linked by disulfide bonds) Hormone

Cooperativity, as seen in hemoglobin, enhances oxygen binding efficiency, resulting in a sigmoidal oxygen dissociation curve.

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Summary of Protein Structure Levels

Level Definition Bonds Example

Primary Linear sequence of amino acids Peptide, disulfide bonds Sequence: H₂N-Met-Ala-Gly...

Secondary Local folding (α-helix, β-sheet) Hydrogen bonds (backbone) α-helix in keratin

Tertiary 3D folding of single chain Hydrophobic, H-bonds, ionic, disulfide Myoglobin

Quaternary Assembly of multiple subunits Same as tertiary Hemoglobin (4 subunits)

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Denaturation of Proteins

Definition: Loss of secondary, tertiary, and quaternary structure, leading to loss of biological function, without breaking primary peptide bonds.

[Figure: Native vs. denatured protein]

Agents Causing Denaturation

Agent Examples

Heat Boiling (egg white solidifies)

pH extremes Strong acids or bases

Organic solvents Ethanol, acetone

Heavy metal ions Hg²⁺, Pb²⁺, Ag⁺

Detergents SDS (Sodium Dodecyl Sulfate)

Reducing agents β-mercaptoethanol (breaks disulfide bonds)

Mechanical forces Vigorous shaking

Urea or Guanidine HCl Disrupt hydrogen bonds

Characteristics of Denaturation

Property Native Protein Denatured Protein

Biological activity Active Inactive

Solubility Soluble Often insoluble (precipitates)

Shape Compact, folded Random coil, unfolded

Susceptibility to digestion Less susceptible More susceptible

Renaturation: Some proteins can refold to their native structure after denaturation if conditions are restored, exemplified by ribonuclease. This supports primary structure contains all information for folding (Anfinsen's dogma).

[Figure: Renaturation experiment]

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Important Proteins (Exam Focus)

Protein Type Location Function Key Feature

Collagen Fibrous Skin, bones, tendons, cartilage Structural support Triple helix; contains hydroxyproline; most abundant protein

Hemoglobin Globular, conjugated Red blood cells Oxygen transport 4 subunits (2α, 2β); contains heme

Myoglobin Globular, conjugated Muscle tissue Oxygen storage Single subunit; higher oxygen affinity than hemoglobin

Keratin Fibrous Hair, nails, horns, feathers Structural Rich in cysteine (disulfide bonds)

Elastin Fibrous Lungs, blood vessels, skin Elasticity Can stretch and recoil

Albumin Globular Blood plasma Osmotic pressure, transport Most abundant plasma protein

Immunoglobulins Globular Blood, lymph Defense (antibodies) Y-shaped; 4 polypeptide chains

Insulin Globular Pancreas (β-cells) Lowers blood glucose 2 chains (A and B); disulfide bonds

Casein Globular Milk Nutrient (stores amino acids) Phosphoprotein; coagulates to form curd

Fibrinogen Globular (soluble) Blood plasma Blood clotting Converted to fibrin (insoluble clot)

Actin & Myosin Fibrous Muscle Contraction Sliding filament mechanism

Ferritin Globular Liver, spleen Iron storage Hollow sphere stores iron

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Color Reactions of Proteins (Lab Exam Important)

Test Reagent Positive Color Amino Acid Responsible

Ninhydrin test Ninhydrin Purple All amino acids (proline = yellow)

Biuret test CuSO₄ in alkali Violet Peptide bonds (at least 2 peptide bonds)

Xanthoproteic test Conc. HNO₃ → alkali Yellow to orange Aromatic amino acids (Tyr, Trp, Phe)

Millon's test Millon's reagent Red Tyrosine (phenol group)

Hopkins-Cole test Glyoxylic acid + conc. H₂SO₄ Purple ring at junction Tryptophan (indole ring)

Sakaguchi test α-naphthol + sodium hypochlorite Red Arginine (guanidino group)

Lead acetate test Pb acetate + NaOH Black precipitate Cysteine (sulfur as sulfide)

Nitroprusside test Sodium nitroprusside Red Cysteine (free -SH)

[Figure: Color reactions of proteins – test tube colors shown]

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Nutritional Classification of Proteins

Class Definition Contains all essential amino acids? Examples

Complete proteins Support normal growth and maintenance Yes Egg, milk, meat, fish, soy

Partially incomplete proteins Support life but not growth Partially Wheat (lacks lysine), legumes (lacks methionine)

Incomplete proteins Cannot support life or growth Missing one or more essential amino acids Gelatin (lacks tryptophan), zein (corn)

Protein complementation involves combining incomplete proteins to provide all essential amino acids, such as rice and beans.

[Figure: Protein complementation – rice and beans]

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Protein Hydrolysis

Hydrolysis breaks peptide bonds to release individual amino acids, essential in digestion and laboratory analysis.

Method Reagent Conditions Result

Acid hydrolysis 6N HCl 110°C, 24 hours All amino acids; destroys tryptophan

Alkaline hydrolysis NaOH or Ba(OH)₂ Heat Tryptophan preserved; destroys others

Enzymatic hydrolysis Proteases Mild conditions All amino acids preserved

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Common Protein Disorders (Clinical Correlation)

Disorder Protein involved Defect Consequence

Sickle cell anemia Hemoglobin Glutamic acid → Valine (β-chain position 6) RBCs sickle-shaped; anemia

Marfan syndrome Fibrillin Genetic defect Tall, long limbs, heart problems

Osteogenesis imperfecta Collagen Defective collagen synthesis Brittle bones

Ehlers-Danlos syndrome Collagen Defects in collagen processing Hyperelastic skin, loose joints

Alzheimer's disease Amyloid β, Tau protein Misfolding and aggregation Neurodegeneration, dementia

Parkinson's disease α-synuclein Aggregation into Lewy bodies Movement disorders

Prion diseases (CJD, mad cow) Prion protein (PrP) Misfolded PrP induces misfolding of normal PrP Spongiform encephalopathy

Cystic fibrosis CFTR protein Defective chloride channel Thick mucus in lungs and pancreas

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Summary Table for Quick Revision

Topic Key Points

Amino acids 20 standard; essential vs non-essential; zwitterion; pI

Peptide bond C-N bond; partial double bond; trans; rigid

Primary structure Linear sequence; peptide bonds

Secondary α-helix, β-sheet; H-bonds (backbone)

Tertiary 3D folding; hydrophobic, H-bonds, ionic, disulfide

Quaternary Multiple subunits; hemoglobin example

Fibrous proteins Insoluble, structural (collagen, keratin)

Globular proteins Soluble, functional (enzymes, Hb, albumin)

Conjugated proteins Protein + prosthetic group (hemoglobin, glycoproteins)

Denaturation Loss of 2°,3°,4° structure; loss of function

Protein tests Biuret (general), ninhydrin (amino acids), xanthoproteic (aromatic)

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Additional Tips for Exam Preparation

o Familiarize yourself with diagrams of amino acids, protein structures, and reactions. Use Google Images for visual aid.

o Practice naming and classification of amino acids and proteins.

o Understand the biochemical basis of common disorders related to proteins.

o Review laboratory color reactions and their significance.

Memorize the key features and functions of important proteins like collagen, hemoglobin, and enzymes.