Proteins

Question 1

What are proteins?

Answer 1

Polymers made of monomers called amino acids

The sequence, type & number of amino acids within a protein determines its shape & function

Question 2

What are the structure of Amino acid?

Answer 2

General structure: a central carbon atom, an amine group (-NH2), a carboxylic acid group (-COOH), a hydrogen atom & an R group (differs depending on the amino acid)

Question 3

What is a peptide bond?

Answer 3

Peptide bonds form between an amine group of one amino acid and a carboxylic acid group of another → water is released (a condensation reaction)

Question 4

What is dipeptides?

Answer 4

Are formed by the condensation of two amino acids

Question 5

What is oligopeptides?

Answer 5

Are formed by the condensation of ~3 amino acids

Question 6

What is polypeptides

Answer 6

Are formed by the condensation of 3+ amino acids

Question 7

Protein structure

Answer 7

A functional protein may contain one or more polypeptides.

There are four levels of structure in proteins

Different chemical bonds in each level allow proteins to have different structure & functions:

Question 8

What bonds increase in strength?

Answer 8

Hydrogen bonds - between polar R groups

Weak hydrophobic interactions - between nonpolar R groups

Ionic bonds - between +ve & -ve charged R groups (can be broken by pH changes)

Disulphide bonds / bridges - a covalent bond between cysteine R groups

Question 9

Primary structure of a protein

Answer 9

Primary structure: the sequence of amino acids bonded by covalent peptide bonds

Specific for each protein

DNA determines the primary structure through protein synthesis → affects the shape → affects the function

Question 10

Secondary structure of a protein

Answer 10

Secondary structure: hydrogen* bonding between amino acids enable two shapes to form:
Α-helix - H bonds between every 4th peptide bond
β-pleated sheet - H bonds between parallel parts of a polypeptide chain

Most fibrous proteins have a secondary structure

Question 11

Tertiary structure of a protein

Answer 11

Tertiary structure: more conformational changes & additional bonds form between R-groups

-Hydrogen bonds
-Disulphide
-Ionic
-Weak hydrophobic interactions

This structure is common in globular proteins

Question 12

Quaternary structure of a protein

Answer 12

Quaternary structure: Occurs in proteins that have more than one polypeptide chain working together as a functional macromolecule

Each polypeptide chain is called a subunit of the protein

A good example is haemoglobin

Question 13

Biochemical test for proteins

Answer 13

Biuret solution
Positive= purple
Negative = Blue

Question 14

Example of a globular protein: Haemoglobin

Answer 14

Quaternary structure (4 polypeptide chains)

4 subunits (two α–globins and two β–globins) held together by disulphide bonds

Each subunit contains a haem prosthetic group: these contains Fe2+ which reversibly combine with oxygen molecules → forms oxyhaemoglobin (+ what makes blood red!)

Question 15

Example of a fibrous protein: Collagen

Answer 15

Three polypeptide chains held together by H bonds → form a triple helix - tropocollagen

Each polypeptide chain is a helix shape & contains lots of glycine, proline & hydroxyproline amino acid monomers

Covalent cross links also form between multiple triple helices → form collagen fibrils

Many fibrils are then arranged together to form collagen fibres