Amino Acids and Proteins: Primary Structure and Peptide Formation

Amino Acids and Proteins

Primary Structure of Proteins

The primary structure of a protein refers to the specific sequence of amino acids in a polypeptide chain, held together by peptide bonds. This sequence determines the protein's unique properties and biological activity.

• Protein: A polypeptide of 50 or more amino acids with biological activity.

• Peptide bond: An amide bond formed between the carboxyl group of one amino acid and the amino group of another.

• Polypeptide: A chain of many amino acids linked by peptide bonds.

• The sequence of amino acids is critical; even small changes can alter function.

Example: The hormone thyrotropin-releasing hormone is a tripeptide with a specific sequence required for activity.

Formation of Peptides

Peptides are formed by linking amino acids through peptide bonds. The process involves the reaction of the carboxyl group of one amino acid with the amino group of another, releasing water.

• Dipeptide: Two amino acids joined by a peptide bond.

• Tripeptide: Three amino acids joined by peptide bonds.

• Tetrapeptide: Four amino acids joined by peptide bonds.

• Pentapeptide: Five amino acids joined by peptide bonds.

• Polypeptide: Longer chains of amino acids.

Peptide bond formation equation:

$\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Peptide} + \text{H}_2\text{O}$

Example: Glycine and alanine react to form glycylalanine and water.

Naming Peptides

The naming of peptides follows a systematic approach:

• The name begins with the N-terminal amino acid residue.

• All following amino acid residues up to the C-terminal have their "ine" or "ate" endings replaced with "yl".

• The complete name of the C-terminal amino acid is retained.

Example: A tripeptide consisting of alanine, glycine, and serine is named alanylglycylserine.

Three-letter abbreviations: Used for concise representation, e.g., Gly-Ala-Ser.

Peptide Structure: N-Terminus and C-Terminus

Peptides have two distinct ends:

• N-terminus: The end with a free amino group (NH2).

• C-terminus: The end with a free carboxyl group (COOH).

Example: In the peptide Gly-Ala, glycine is at the N-terminus and alanine at the C-terminus.

Primary Structure of Insulin

Insulin was the first protein to have its primary structure determined. It consists of two polypeptide chains (A and B) linked by disulfide bonds.

• Chain A: 21 amino acids

• Chain B: 30 amino acids

• Disulfide bonds connect specific cysteine residues between and within chains.

Example: The sequence and disulfide linkages are essential for insulin's biological activity.

Essential Amino Acids

Of the 20 amino acids used to build proteins in the body, only 11 can be synthesized by humans. The other nine are essential amino acids and must be obtained from dietary proteins.

• Complete proteins: Contain all essential amino acids (e.g., eggs, milk, meat, fish).

• Incomplete proteins: Deficient in one or more essential amino acids (e.g., grains, beans, nuts).

Table: Essential Amino Acids for Adults

Essential Amino Acid	Source
Histidine	Meat, fish, dairy
Isoleucine	Eggs, soy, fish
Leucine	Meat, legumes
Lysine	Meat, dairy
Methionine	Eggs, nuts
Phenylalanine	Milk, nuts
Threonine	Eggs, cheese
Tryptophan	Turkey, milk
Valine	Fish, soy

Additional info: The table above is inferred from standard sources, as the original table was not fully visible.

Peptide Drawing and Naming Practice

To draw and name a peptide:

1. Draw the structure for each amino acid, starting with the N-terminus.

2. Remove the O atom from the carboxylate group of the N-terminal amino acid and two H atoms from the ammonium group in the adjacent amino acid. Repeat until the C-terminus is reached.

3. Connect amino acid residues using peptide bonds.

Example: Glycylserylmethionine (Gly-Ser-Met) is a tripeptide formed from glycine, serine, and methionine.

Tripeptide Combinations

Tripeptides can be formed from different combinations of amino acids. For example, with two glycines and one alanine:

• Glycylglycylalanine (Gly-Gly-Ala)

• Glycylalanylglycine (Gly-Ala-Gly)

• Alanylglycylglycine (Ala-Gly-Gly)