proteins and amino acids

Question 1

what is the general structure of an amino acid? draw this

Answer 1

• (refer to image)
• COOH carboxyl / carboxylic acid group
• R variable side group - consists of carbon chain and may include other functional groups (e.g. benzene ring or -OH (alcohol))
• NH2 amine / amino group
• central carbon
• single hydrogen

Question 2

how do polypeptides form?

Answer 2

• condensation reaction takes place between amino acids, forming peptide bonds (-CONH-)
• there are 4 levels of protein structure

Question 3

describe ‘primary’ structure of a protein

Answer 3

sequence, number and type of amino acids in the polypeptide, determined by the sequence of codons on mRNA

Question 4

describe ‘secondary’ structure of a protein

Answer 4

• hydrogen bonds form between O δ- attached to -C=O and H δ+ attached to -NH
• two types:
  . a-helix = all N-H bonds on same side of protein chain, spiral shape, H-bonds parallel to helical axis
  . B-pleated sheet = N-H and C=O groups alternate from one side to the other

Question 5

describe ‘tertiary’ structure of a protein and the bonds present

Answer 5

• 3D structure formed by further folding
• hydrogen bonds: numerous and easily broken
• ionic bonds: relatively strong bonds between charged R groups
• disulphide bridges: strong covalent S-S bonds between molecules of the amino acid cysteine
• hydrophilic-hydrophobic interactions

Question 6

describe ‘quaternary’ structure of a protein

Answer 6

• functional proteins may consist of more than one polypeptide
• precise 3D structure held together by the same types of bonds as the tertiary structure
• may involve the addition of prosthetic groups (e.g. metal ions or phosphate groups)

Question 7

describe the structure and function of globular proteins

Answer 7

• spherical and compact
• hydrophilic R groups face outwards and hydrophobic R groups face inwards = therefore usually water soluble
• involved in metabolic processes -> e.g. enzymes such as amylase

Question 8

describe the structure of haemoglobin

Answer 8

• globular conjugated protein (has a prosthetic group)
• 2 a-chains and 2 B-chains with 4 prosthetic groups
• water soluble so dissolves in plasma
• Fe^2+ haem group forms co-ordinate bond with O2
• tertiary structure changes so it is easier for subsequent O2 molecules to bind (co-operative binding)

Question 9

describe the structure and function of fibrous proteins

Answer 9

• can form long chains or fibres
• insoluble in water
• useful for structure and support e.g. collagen in skin

Question 10

what is the function of collagen?

Answer 10

component of bones, cartilage, connective tissue, tendons

Question 11

what is the function of elastin?

Answer 11

provides elasticity to connective tissue, arteries, skin, lungs, cartilage, ligaments

Question 12

what is the function of keratin?

Answer 12

structural component of hair, nails, hooves, claws, horns, epithelial cells of outer layer of skin