Peptide bond; primary, secondary, tertiary, quaternary protein structure

4.5.3 · Chemistry › Biomolecules

1. The Peptide Bond

YEH KYON banta hai? Amino acids bifunctional hote hain — har ek mein ek acid ( $- C O O H$ ) aur ek base ( $- N H_{2}$ ) dono hote hain. Toh ek molecule ka acid agle molecule ke amine se react kar sakta hai, bilkul jaise acid + amine → amide.

KYA hota hai (condensation):

$aa_{1} H_{2} N - C H R_{1} - C O O H + aa_{2} H_{2} N - C H R_{2} - C O O H ⟶ H_{2} N - C H R_{1} - peptide bond C O - N H - C H R_{2} - C O O H + H_{2} O$

KAISE hota hai (mechanism, scratch se):

$- N H_{2}$ ke $N$ par lone pair, $- C O O H$ ke carbonyl carbon par attack karta hai (electrophilic C).
Ek tetrahedral intermediate banta hai; $- O H$ , $H_{2} O$ ke roop mein nikal jaata hai.
Result: $C$ aur $N$ ab ek $C - N$ single bond se jude hain jisme partial double-bond character hai.

2. Primary Structure

Kis cheez se tika hai: covalent peptide bonds (aur disulphide $- S - S -$ links).
Bas yeh hai ki kaunsa amino acid kahaan aata hai — protein ki "spelling."
KYON matter karta hai: ek amino acid badlo → poori 3D shape aur function badal sakta hai. (Sickle-cell anaemia = ek Glu→Val swap.)

3. Secondary Structure

Do main types:

Type	Shape	H-bonds	Example
α-helix	right-handed coil	usi chain ke andar	keratin (hair, wool)
β-pleated sheet	zig-zag flat sheets	adjacent chains ke beech	silk fibroin

KYON H-bonds, covalent nahi? Flat, rigid peptide units sirf $C_{α}$ par tilt kar sakte hain. Har $N - H$ aur $C = O$ ko H-bond se satisfy karte hue unhe pack karne ka sabse stable tarika ek coil ya sheet hai.

Figure — Peptide bond; primary, secondary, tertiary, quaternary protein structure

4. Tertiary Structure

R-side-chains ke beech interactions se stabilise hoti hai:

hydrophobic interactions (non-polar groups andar chhup jaate hain)
hydrogen bonds (side-chain)
ionic / electrostatic (salt bridges)
disulphide bonds ( $- S - S -$ , yahan sirf covalent wala)

Do broad shapes:

Fibrous proteins — lambe thread-like, water-insoluble, structural (keratin, collagen, myosin).
Globular proteins — ball shape mein fold, aksar water-soluble, functional (enzymes, insulin, haemoglobin).

5. Quaternary Structure

Sirf >1 chain wale proteins mein hoti hai.
Example: haemoglobin = 4 subunits (2 α + 2 β).
Isi forces se tiki hoti hai jaise tertiary (H-bonds, ionic, hydrophobic, kabhi kabhi $- S - S -$ ), lekin chains ke beech.

Worked Examples

Common mistake Common errors ko steel-man karna

"Denaturation primary structure bhi tod deta hai." Kyon sahi lagta hai: protein clearly change/"break" hoti hai, toh lagta hai sab kuch tootta hai. Fix: Denaturation sirf weak forces (H-bonds, ionic, hydrophobic) ko disrupt karta hai. Covalent peptide bonds (primary structure) intact rehte hain. Sequence preserve hoti hai; sirf folding khatam hoti hai.

"Peptide bond = koi bhi C–N bond / yeh ester hai." Kyon sahi lagta hai: yeh acid + kuch aisa jodta hai jisme $O$ involved ho. Fix: Yeh ek amide ( $- C O - N H -$ ) hai, jo acid + amine se bana hai, acid + alcohol se nahi (woh ester hota hai). Link mein nitrogen hai, oxygen nahi.

"Peptide bond normal single bond ki tarah freely rotate kar sakta hai." Kyon sahi lagta hai: paper par yeh ek single $C - N$ bond ki tarah draw hota hai. Fix: Resonance ise partial double-bond character → planar aur rigid deti hai, aur yahi regular helices/sheets ko possible banata hai.

Recall Feynman: 12-saal ke bacche ko samjhao

Socho LEGO ke log (amino acids) haath pakad rahe hain. Haath pakadna = peptide bond — ek mazboot grip jo nahi chhootti. Line mein logon ka order primary structure hai (kaun kahaan khada hai). Agar line ka koi hissa spring mein curl ho jaaye ya zig-zag mat mein fold ho, woh secondary hai. Jab poori line ek ball mein simatt jaaye, woh tertiary hai. Aur jab kai aise balls ek kaam karne ke liye clump ho jaayein (jaise 4 dost ek bada box uthaa rahe hain = haemoglobin oxygen uthaa raha hai), woh quaternary hai. Heat unhe hilati hai toh folds khul jaate hain (denaturation) — lekin unke haath locked rehte hain, toh order kabhi nahi badalta.

Flashcards

Peptide bond kaunsa functional group hai?

Ek amide group,

- C O - N H -

Peptide bond banane ke liye kaunse do groups react karte hain, aur kya nikalta hai?

Ek amino acid ka

- C O O H

+ agle ka

- N H_{2}

; ek

H_{2} O

nikalta hai (condensation)

Peptide bond planar aur rigid kyon hota hai?

Resonance N ka lone pair C=O mein delocalise karta hai, partial double-bond character deta hai → free rotation nahi

Polypeptide mein n amino acids hain; kitne peptide bonds?

n - 1

peptide bonds (aur

n - 1

water molecules released)

Primary structure define karo aur ise hold karne wala bond batao.

Amino acids ki exact sequence (N→C terminus); covalent peptide bonds (aur

- S - S -

) se tiki hai

Secondary structure ko kya stabilise karta hai?

Backbone

> C = O

aur

> N - H

groups ke beech hydrogen bonds

Do types ki secondary structure ke naam aur examples batao.

α-helix (keratin); β-pleated sheet (silk fibroin)

Tertiary structure ko kaunsi forces stabilise karti hain?

Hydrophobic interactions, H-bonds, ionic (salt bridges), disulphide

- S - S -

bonds

Fibrous aur globular proteins mein kya fark hai?

Fibrous: thread-like, insoluble, structural (keratin); Globular: ball-shaped, soluble, functional (enzymes, insulin)

Quaternary structure kya hai? Ek example do.

≥2 polypeptide subunits ka ek functional protein mein assembly; e.g. haemoglobin (2α + 2β)

Denaturation ke baad kaunsa structural level bachta hai aur kyon?

Primary structure — iske covalent peptide bonds mild heat/pH se nahi tootte; sirf weak forces (sec/ter/quat) khatam hoti hain

Tertiary structure ko stabilise karne wala sirf covalent bond kaunsa hai?

Disulphide bond,

- S - S -

N-terminus aur C-terminus kis cheez ko refer karte hain?

N-terminus = free

- N H_{2}

wala end; C-terminus = free

- C O O H

wala end

Connections

Amino acids — structure, classification, zwitterion
Enzymes — globular proteins as biocatalysts
Denaturation of proteins
Haemoglobin & oxygen transport
Carbohydrates — glycosidic bond (compare: ek aur condensation linkage)
Amides — resonance & planarity