2.4 Proteins

Question 1

Polypeptides are

Answer 1

Polypeptides are, as the name implies, a number of linked peptides. They are also called proteins – the names are often used interchangeably. The basic unit of a polypeptide is an amino acid: a carbon-based compound with a carboxyl group (-COOH) and an amino group (-NH 2 ).

Question 2

Peptides are

Answer 2

Peptides are defined as molecules that consist of between 2 and 50 amino acids. Proteins are polymers that are made up of 51 or more amino acids.

Question 3

The bond formed between the carboxyl group of one amino acid and the amine group of another amino acid is called the peptide bond.

Answer 3

Peptide bond

Question 4

primary structure

Answer 4

sequence of amino acids in a protein is called the primary structure. It defines all aspects of the structure and function of a protein.

Question 5

tertiary structure

Answer 5

When the polypeptide folds and coils to form a complex three-dimensional shape, it gives rise to the tertiary structure.

Question 6

A quaternary structure

Answer 6

A quaternary structure only occurs in proteins that are made up of two or more polypeptide chains and refers to the way the multiple subunits are held together in a multi-subunit complex

Question 7

Globular proteins

Answer 7

as the name implies, are globe-like or spherical. They tend to play active roles in the cell’s metabolism. They consist of complex polypeptide chains that can be linked to other chains to form large complex proteins. An example of this is hemoglobin, which has two alpha and two beta chains. Globular proteins are usually soluble in water because their hydrophobic R groups are folded into the core of the molecule, away from the surrounding water molecules.

Question 8

Fibrous proteins

Answer 8

Fibrous proteins are more like a fibre; long and thread-like. Usually, fibrous proteins are made of long polypeptide chains where the hydrophobic R groups are exposed, making the molecule insoluble. They are often found in structural parts of organisms, such as tendons and skin (e.g. collagen and keratin).

Question 9

Rubisco

shape and function

Answer 9

Globular An enzyme involved in the fixation of CO 2 in chloroplasts.

Question 10

Insulin

shape and function

Answer 10

Globular A hormone produced by the beta cells of the pancreas, which is involved in glucose uptake from the blood.

Question 11

Immunoglobulin

shape and function

Answer 11

Globular These are large Y-shaped proteins, also called antibodies, involved in fighting infections by specifically recognising and binding to antigen molecules.

Question 12

Rhodopsin

shape and function

Answer 12

Globular A protein linked to pigment, found on the membrane of rod (photoreceptor) cells of the retina, where it allows very low light intensities to be detected.

Question 13

Collagen

shape and function

Answer 13

Fibrous A structural protein, found in muscles, tendons and ligaments, where it gives tensile strength. It also occurs in skin and bones, where it prevents tearing and fractures, respectively.

Question 14

Spider silk

shape and function

Answer 14

Fibrous A fibrous protein produced by spiders for their webs. It can be extended and is very resistant to breaking.