Structure of Proteins

Question 1

Define a protein.

Answer 1

• Highly complex substance that is present isn all living organism
• Required for the structure, function and regulation of the body’s tissues and organs

Question 2

State the functions of proteins.

Answer 2

• Enzymes: catalyse chemical reactions in cells - speed up reactions that would be too slow to sustain life
• Structural support: collagen provides structural support for tissue, such as skin and bones
• Hormones: insulin regulate various physiological processes in the body such as blood sugar levels
• Cell signalling: involved in transmitting signals within and between cells, regulating processes like cell growth and differentiation

Question 3

What are proteins made up of?

Answer 3

They are polymers of amino acids

Question 4

Describe the structure of amino acids.

Answer 4

Contain both an amine (alkali group) and a carboxylic acids (acidic group)

Question 5

Describe the reaction of amino acids.

Answer 5

• Acids react with amines to produce salts
• In neutral pH amino acids react with themselves producing a zwitterion

Question 6

What is a zwitterion?

Answer 6

• A double ion or two charges on the same molecule

Question 7

When does a cation (+) form?

Answer 7

At low pH

Question 8

When does an anion (-) form?

Answer 8

At high pH

Question 9

How do dimers, trimers form?

Answer 9

• When amino acids react with other amino acids
• Form by producing a new chemical bond called an amide bond or a peptide bond

Question 10

What enzyme is involved in the formation of a peptide bond between amino acids?

Answer 10

Peptidyl transferases

Question 11

What is a peptide?

Answer 11

Two or more amino acids joined together by peptide bonds

Question 12

What is a polypeptide?

Answer 12

A chain of many amino acids

Question 13

Describe protein structure.

Answer 13

Primary structure - a linear chain of amino acids in a polypeptide chain

Secondary structure: folded structures that from within a polypeptide due to interaction between atoms of the backbone

Tertiary structure: 3D structure of a polypeptide - formed due to interactions between the R groups of the amino acids in the protein

Quaternary structure: multiple polypeptide chains

Question 14

Which bond is involved in the secondary structure?

Answer 14

Hydrogen bonds

Question 15

What are the two main types of arrangement of the secondary structure?

Answer 15

• Alpha helix
• Beta pleated sheets

Question 16

Describe the alpha helix structure.

Answer 16

• A narrow tube, coiled in a right hand spiral
• Hydrogen bonds between the C=O of one peptide and the N-H of another peptide group 4 units ahead in the primary structure
• Hydrogen bonds run in parallel to the helix, within one polypeptide chain
  to stabilised the protein structure

Question 17

Describe beta pleated sheets.

Answer 17

• Two or more lengths of the polypeptide chains lie in parallel to each other
• R groups of the amino acids above and below the sheets, while the NH and C=O parts of the peptide bond point towards each other
• Hydrogen bonds between these sections
• The backbone of the polypeptide chain is extended into a zigzag rather than
  helical structure

Question 18

Which bonds are involved in the tertiary structure?

Answer 18

Disulfide bonds, peptide bonds, covalent bonds, hydrogen bonds and van der Waals forces

Question 19

Where are disulphide bonds primarily found?

Answer 19

In secreted, extracellular proteins, e.g. hormone, insulin

Question 20

State the role of metal ions in the formation of the tertiary structure.

Answer 20

• Assists in the folding and stabilising of protein structure i.e. calcium in calmodulin protein

Question 21

Describe the role of calmodulin.

Answer 21

• A multifunctional intermediate calcium-binding messenger protein
• The structure changes with the addition of calcium
• This change occurs with rotation and bonding with the calcium

Question 22

What bonds are involved in the quaternary structure?

Answer 22

same as tertiary structure

Question 23

State the two categories of proteins in the quaternary structure.

Answer 23

Fibrous and globular

Question 24

Define proteolysis.

Answer 24

Polypeptide chain is cleaved into fragments which take on different shapes

Question 25

Define glycosylation.

Answer 25

The addition of sugars to polypeptide chains - is important in targeting and recognition

Question 26

Define phosphorylation.

Answer 26

The addition of phosphate groups
- often changes the shape of the protein, exposing an active site of an enzyme or binding site for another molecule

Question 27

What is insulin and where is it produced?

Answer 27

• A peptide hormone
• Produced in the beta cells of the pancreas

Question 28

What does insulin regulate?

Answer 28

• The metabolism of carbohydrates, fats and proteins

Question 29

What does insulin promote?

Answer 29

• Absorption of glucose from the blood into liver, fat and skeletal muscle cells

Question 30

What does a decrease of insulin activity result in?

Answer 30

• Diabetes mellitus - a condition of high blood sugar level (hyperglycaemia)

Question 31

Why is insulin administered via injection?

Answer 31

• It is too big to pass through the phospholipid bilayer of the gastrointestinal cell
• Would be digested by enzymes in the gastrointestinal tract

Question 32

How does folding affect insulin activity?

Answer 32

• Folding pattern allows the protein to bind to the receptor adequately to initiate the reaction (makes insulin more stable)

Question 33

How does hydration affect insulin activity.

Answer 33

• Affects solubility and folding
• If water molecules are attached certain fat soluble hydrophobic amino acids will move away from these water molecules; this affects folding and the 3D structure
• Hydrophobic amino acid side chains tend to cluster in a protein’s interior, away from water
• When the protein is folded, these clusters form a hydrophobic protein core

Question 34

Why can’t we dry protein such as insulin?

Answer 34

All the associated water molecules will be removed which will alter the folding pattern of the protein which cannot be rectified by rehydration - it is impossible to weigh insulin and other therapeutic proteins

Question 35

How are therapeutic proteins quantified?

Answer 35

• Pharmacological testing
• We measure how much the glucose level in the rabbit has reduced with our injection

Question 36

How is insulin harvested?

Answer 36

human DNA is injected into bacteria or other living organisms - recombinant DNA technology