2.9 Proteins

Question 1

What are proteins?

Answer 1

polymers made of amino acids (the amino acids are monomers)

Question 2

Proteins examples:

Answer 2

• enzymes
• hormones
• antibodies

Question 3

What is a dipeptide?

Answer 3

formed when two amino acids join together

Question 4

What is a polypeptide?

Answer 4

formed when more than two amino acids join together

Question 5

What is the basic structure of an amino acid?

Answer 5

H
|
.. NH2 - C - COOH
|
R
(ignore the dots - only for formatting)

Question 6

Identify the chemical groups in an amino acid:

Answer 6

NH2 - amine
R - carbon-containing variable side group (different for each amino acid)
COOH - carboxyl group

Question 7

How many amino acids are there?

Answer 7

20

Question 8

What do you call the amino acids that animals have to obtain through their diet?

Answer 8

essential amino acids

Question 9

How are polypeptide chains formed?

Answer 9

• amino acids are joined by condensation reactions
• the OH on the COOH and the H on the NH2 are lost to form water
• the bond forms between C and N

Question 10

What are the bonds between amino acids called?

Answer 10

peptide bonds

Question 11

What is the primary structure of a protein?

Answer 11

the sequence of amino acids in the polypeptide chain

Question 12

What is the secondary structure of a protein? How does it form?

Answer 12

• hydrogen bonds form between amino acids in the polypeptide chain
• this causes the polypeptide chain to either
• coil into an alpha helix (like an extended spring)
• or fold into a beta pleated sheet

Question 13

What is the tertiary structure of a protein?

Answer 13

• the polypeptide chain further bends and folds into a precise 3D structure
• this is due to the bonds that form between amino acids in the polypeptide chain
• the further bending and folding also depends on the polarity of the R groups: polar R groups face inwards (away from water) and non-polar R groups face outwards (towards water)
• the bonds that form include:
  ionic bonds
  chemical bonds e.g. disulfide bonds
  more hydrogen bonds
• for proteins made from a single polypeptide chain, their tertiary structure forms their final 3D structure

Question 14

Describe the ionic and disulfide bonds that form at the tertiary structure of a protein:

Answer 14

• ionic bonds: the electrostatic attraction between oppositely charged ions (some R groups ionise, and so ionic bonds can form)
• disulfide bonds: covalent bonds that form between a sulfur atom on one cysteine (amino acid) and a sulfur atom on another cysteine

Question 15

What is the quaternary structure of a protein?

Answer 15

• some proteins are made of several different polypeptide chains held together by bonds
• the quaternary structure forms when different polypeptide chains assemble together
• e.g. haemoglobin, insulin, collagen

Question 16

What are conjugated proteins?

Answer 16

proteins containing another chemical group called a prosthetic group (a group that is not part of an amino acid)
e.g. iron group in haemoglobin

Question 17

State the bonds that form at the different structural levels of proteins:

Answer 17

• primary structure: peptide bonds
• secondary structure: hydrogen bonds
• tertiary structure:
  ionic bonds
  disulfide bonds (and other chemical bonds)
  hydrophobic and hydrophilic interactions
  hydrogen bonds
• quaternary structure: all of the above

Question 18

What is the significance of a protein’s primary structure in determining its three-dimensional structure and properties?

Answer 18

• the primary structure of a protein is the amino acid sequence in a polypeptide chain
• this sequence determines what types of bonds will form
• e.g. disulfide bonds will form if cysteines are present in the amino acid sequence
• the types of bonds and where these bonds form will determine how the polypeptide chain folds
• and so will determine its 3D structure/shape
• the 3D structure of the protein then determines its properties and its function in the body

Question 19

What are globular proteins? What are their properties?

Answer 19

• spherical, compact proteins made up of multiple polypeptide chains
• they are soluble and so can be easily transported in fluids: this is because the hydrophilic parts of the chains face outwards and the hydrophobic parts of the chain face inwards
• their 3D structures are crucial for binding to other substances

Question 20

What are examples of globular proteins? Explain how their 3D structures are important:

Answer 20

• e.g. haemoglobin: a globular protein made of 4 polypeptide chains. its solubility means it can be easily transported in the blood. this is important because it is responsible for transporting oxygen. the iron-containing haem groups present bind to oxygen
• e.g. enzymes: their 3D structures are crucial for catalysing reactions

Question 21

What are fibrous proteins? What are their properties?

Answer 21

• proteins made of long, insoluble polypeptide chains that coil tightly to each other to form a rope like shape
• the chains are held together by lots of bonds (e.g. disulfide and hydrogen bonds) - this makes them strong
• cross links between chains add additional strength
• often found in supportive tissue due to their strength

Question 22

What are examples of fibrous proteins? Explain how their 3D structures are important:

Answer 22

• e.g. collagen: a strong, fibrous protein that forms connective tissue in animals. its 3D structure is important as it provides strength
• e.g. keratin: found in hair, nails, and skin