Amino Acids and Peptide Bonds — Revision Notes

⚡ 30-Second Revision

Amino Acid Structure: — $alpha$-carbon, $-\text{NH}_2$, $-\text{COOH}$, $-\text{H}$, R-group.
Chirality: — Most are chiral (L-form in proteins); Glycine is achiral.
Zwitterion: — Dipolar ion at physiological pH (net charge = 0).
Isoelectric Point (pI): — pH at which net charge is zero.
Peptide Bond: — Amide linkage ($-\text{CO-NH}-$) formed via dehydration.
Peptide Bond Properties: — Partial double-bond character, rigid, planar, restricted rotation around C-N bond.
Directionality: — N-terminus (free $-\text{NH}_3^+$) to C-terminus (free $-\text{COO}^-$).
Essential AAs: — Must be obtained from diet (e.g., Leucine, Lysine, Valine).

2-Minute Revision

Amino acids are the fundamental building blocks of proteins, each featuring a central alpha-carbon connected to an amino group, a carboxyl group, a hydrogen atom, and a unique R-group. This R-group determines the amino acid's specific chemical properties, allowing classification into nonpolar, polar, acidic, or basic types.

All amino acids, except glycine, are chiral, with L-forms being biologically prevalent. At physiological pH, amino acids exist as zwitterions, carrying both positive and negative charges, resulting in a net neutral charge at their isoelectric point (pI).

Proteins are formed by linking amino acids via peptide bonds. A peptide bond is an amide linkage ($-\text{CO-NH}-$) formed through a dehydration reaction between the carboxyl group of one amino acid and the amino group of another.

Crucially, this bond exhibits partial double-bond character, making it rigid and planar, which restricts rotation around the C-N bond and is vital for protein folding. Polypeptide chains have a distinct N-terminus (free amino group) and C-terminus (free carboxyl group).

Remember the list of essential amino acids, which humans cannot synthesize and must obtain from their diet.

5-Minute Revision

Amino acids are the monomers of proteins, characterized by a central $alpha$-carbon bonded to an amino group ($-\text{NH}_2$), a carboxyl group ($-\text{COOH}$), a hydrogen atom, and a variable R-group.

The R-group dictates the amino acid's properties, leading to classifications like nonpolar (e.g., Alanine, Valine), polar uncharged (e.g., Serine, Threonine), acidic (e.g., Aspartate, Glutamate), and basic (e.

g., Lysine, Arginine, Histidine). All amino acids, except glycine (whose R-group is also H), are chiral, existing as L- and D-enantiomers, with L-forms being dominant in proteins.

At physiological pH, amino acids exist as zwitterions, where the amino group is protonated ($-\text{NH}_3^+$) and the carboxyl group is deprotonated ($-\text{COO}^-$), resulting in a net zero charge at their isoelectric point (pI). If the pH is below pI, the amino acid is positively charged; if above pI, it's negatively charged.

Amino acids link to form polypeptides via peptide bonds. This is a dehydration synthesis (condensation) reaction where the carboxyl group of one amino acid reacts with the amino group of another, releasing a water molecule.

The resulting amide linkage ($-\text{CO-NH}-$) is highly stable. A critical feature of the peptide bond is its partial double-bond character due to resonance. This makes the bond rigid and planar, restricting rotation around the C-N bond and influencing protein secondary structure.

Polypeptide chains have a distinct N-terminus (free amino group) and C-terminus (free carboxyl group), defining their directionality.

Example: Consider the dipeptide Ala-Gly. Alanine's carboxyl group reacts with Glycine's amino group. The N-terminus is Alanine, and the C-terminus is Glycine. The peptide bond between them is planar and rigid.

Remember essential amino acids (e.g., Leucine, Lysine, Valine, Isoleucine, Methionine, Phenylalanine, Threonine, Tryptophan, Histidine) must be obtained from the diet, unlike non-essential ones which the body can synthesize. Understanding these fundamental concepts is crucial for comprehending protein structure, function, and related biological processes.

Prelims Revision Notes

Amino Acids: The Building Blocks

General Structure: — Central $alpha$-carbon, amino group ($-\text{NH}_2$), carboxyl group ($-\text{COOH}$), hydrogen atom, and a unique R-group (side chain).
R-group: — Determines amino acid properties (polarity, charge, size). Key for classification.

* Nonpolar Aliphatic: Gly, Ala, Val, Leu, Ile, Met, Pro. * Aromatic: Phe, Tyr, Trp (relatively nonpolar). * Polar Uncharged: Ser, Thr, Cys, Asn, Gln (can form H-bonds). * Acidic: Asp, Glu (negatively charged at physiological pH). * Basic: Lys, Arg, His (positively charged at physiological pH).

Chirality: — All except Glycine are chiral ( $alpha$-carbon bonded to 4 different groups). L-amino acids are found in proteins.
Essential Amino Acids: — Cannot be synthesized by the body; must be obtained from diet. (e.g., Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Tryptophan, Threonine, Lysine, Histidine).
Zwitterion: — At physiological pH, amino acids exist as dipolar ions with both $-\text{NH}_3^+$ and $-\text{COO}^-$ groups, resulting in a net zero charge.
Isoelectric Point (pI): — The pH at which an amino acid has a net electrical charge of zero. Below pI, net positive charge; above pI, net negative charge.

Peptide Bonds: The Linkage

Formation: — A covalent amide linkage ($-\text{CO-NH}-$) formed by a dehydration (condensation) reaction between the $alpha$-carboxyl group of one amino acid and the $alpha$-amino group of another. A molecule of water is released.
Characteristics:

* Partial Double-Bond Character: Due to resonance, the C-N bond has characteristics of both single and double bonds. * Rigid and Planar: The partial double-bond character restricts rotation around the C-N bond, making the peptide bond and its adjacent atoms lie in a single plane. * Trans Configuration: Favored over cis due to steric hindrance. * Polarity: The C=O and N-H groups are polar, capable of hydrogen bonding, crucial for secondary structures.

Polypeptide Directionality: — Has an N-terminus (free amino group) and a C-terminus (free carboxyl group). Sequence is read N to C.
Hydrolysis: — Peptide bonds can be broken by proteases (enzymes) or strong acid/base treatment.

Vyyuha Quick Recall

To remember the essential amino acids, think: PVT TIM HALL

Phenylalanine
Valine
Threonine
Tryptophan
Isoleucine
Methionine
Histidine
Arginine (conditionally essential)
Leucine
Lysine