3.6 Proteins

Question 1

What are peptides?

Answer 1

Peptides are polymers made up of amino acid molecules.

Question 2

What do proteins consist of?

Answer 2

Proteins consist of one or more polypeptides arranged as complex macromolecules.

Question 3

What is the general structure of amino acids?

Answer 3

A carboxyl group (-COOH) and an amino group (-NH2) attached to a carbon atom. The difference between different amino acids is the variable group they contain.

Question 4

What is the smallest amino acid?

Answer 4

Glycine. The R group is a hydrogen atom.

Question 5

What chemical elements do all amino acids contain?

Answer 5

Carbon, oxygen, hydrogen and nitrogen.

Question 6

What are amino acids linked together by?

Answer 6

Amino acids are linked together by peptide bonds to form dipeptides and polypeptides.

Question 7

What is released during the synthesis of peptides?

Answer 7

A molecule of water. It is a condensation reaction.

Question 8

What breaks the peptide bond?

Answer 8

A molecule of water. It is a hydrolysis reaction.

Question 9

How is a polypeptide formed?

Answer 9

When many amino acids join together by peptide bonds.

Question 10

What is the primary structure?

Answer 10

1) The sequence of amino acids in the polypeptide chain.

2) Different proteins have different sequences of amino acids in their primary structure.

Question 11

What is the secondary structure?

Answer 11

1) The polypeptide chain doesn’t remain flat and straight.
2) Hydrogen bonds form between between nearby amino acids in the chain.
3) This makes it automatically coil into an alpha helix or fold into a beta pleated sheet.

Question 12

What is the tertiary structure?

Answer 12

1) The coiled or folded chain of amino acids is often coiled and folded further.
2) More bonds form between different parts of the polypeptide chains. (h bonds, sulphur brigdes, ionic,)
3) For proteins made from a single polypeptide chain, the tertiary structure forms their final 3D structure.

Question 13

What is the quaternary structure?

Answer 13

association of subunits

Question 14

What bonds are there in the primary structure?

Answer 14

The primary structure is held together by peptide bonds between amino acids.

Question 15

What bonds are there in the secondary structure?

Answer 15

The secondary structure is held together by hydrogen bonds.

Question 16

What bonds are there in the tertiary structure?

Answer 16

1) Ionic bonds: the attractions between negatively charged R-groups and positively charged R groups on different parts of the molecule.
2) Disulfide bonds: Whenever two molecules of the amino acid cytesine come close together, the sulfur atom in one cysteine bonds to the sulfur in the other cysteine, forming a disulfide bond.
3) Hydrophobic & hydrophilic interactions: When hydrophobic R groups are close together in the protein, they tend to clump together. This means that hydrophilic R groups are more likely to be pushed to the outside, which affects how the protein folds up into its final structure.
4) Hydrogen bonds- These weak bonds form between slightly positively charged atoms in some R groups & slightly negatively charged hydrogen atoms in other R groups on the polypeptide chain.

Question 17

What bonds are there in the quaternary structure?

Answer 17

This tends to be determined by the tertiary structure of the individual polypeptide chains being bonded together. Because of this, it can be influenced by all the bonds mentioned above.

Question 18

What are globular proteins?

Answer 18

Globular proteins are spherical, water soluble and compact proteins.

Question 19

What makes globular proteins soluble?

Answer 19

In a globular protein, the hydrophilic R groups on the amino acids tend to be pushed to the outside of the molecule. This is caused by the hydrophobic and hydrophilic interactions in the protein’s tertiary structure. This makes globular proteins soluble, so they’re easily transported in fluids.

Question 20

Examples of globular proteins:

Answer 20

Haemoglobin, insulin, amylase

Question 21

What is haemoglobin?

Answer 21

• Haemoglobin is a globular protein that carries oxygen around the body in red blood cells.
• It’s known as a conjugated protein: this means it’s a protein with a non-protein group attached. This non-protein part is called a prosthetic group.
• Each of the four polypeptide chains in haemoglobin has a prosthetic group called haem.
• A haem group contains iron which oxygen binds to.

Question 22

What is insulin?

Answer 22

• Insulin is a hormone secreted by the pancreas.
• It helps to regulate the blood glucose level.
• Its solubility is important- it means it can be transported in the blood to the tissues where it acts.
• An insulin molecule consists of two polypeptide chains, which are held together by disulfide bonds.

Question 23

What is amylase?

Answer 23

• Amylase is an enzyme that catalyses the breakdown of starch in the digestive system.
• It is made of a single chain of amino acids.
• Its secondary structure contains both alpha-helix and beta-pleated sheet sections. Most enzymes are globular proteins.

Question 24

What are fibrous proteins?

Answer 24

• Fibrous proteins are insoluble and strong.
• They’re structural proteins and fairly unreactive.
• They’re tough and rope-shaped.

Question 25

Examples of fibrous proteins:

Answer 25

Collagen, keratin and elastin.

Question 26

What is collagen?

Answer 26

• Collagen is a fibrous protein found in animal connective tissues, such as bone, skin and muscle.
• It is a very strong molecule.
• Minerals can bind to the protein to increase its rigidity e.g in bone.

Question 27

What is keratin?

Answer 27

• Keratin is a fibrous protein found in many of the external structures of animals, such as skin, hair, nails, feathers and horns.
• It can either be flexible (in skin) or hard and tough (in nails).

Question 28

What is elastin?

Answer 28

• Elastin is a fibrous protein found in elastic connective tissue, such as skin, large blood vessels, and some ligaments.
• It is elastic, so it allows tissues to return to their original shape after they have been stretched.