Proteins

Question 1

All proteins contain

Answer 1

C H O N

Some very strong proteins contain sulphur

Question 2

Proteins are often

Answer 2

Large molecules and are the principle material in enzymes, hormones, nerves

They are polypeptides made up of chains whose monomer units are amino acids

Question 3

Basic structure of an amino acid

Answer 3

H          R      O
  \         |     //
   N —- C — 
  /         |       \
H         H      OH

Where NH2 is the amine or basic group
Where COOH is the acidic group (carboxylic acid)
Where R is the variable part of the molecule & represents a different carbon-containing side chain

Question 4

Number of commonly occurring amino acids

Answer 4

20, plants are able to synthesise all of these

Animals are only able to synthesise some and must obtain the others in their diet (essential amino acids)

Ex. Glycine, valine, methionine

Question 5

Amino acids are

Answer 5

Small, soluble molecules and a mixture of different amino acids can be separated by electrophoresis or chromatography

Question 6

Dipeptide formation

Answer 6

Two amino acids join in a condensation reaction to form a dipeptide (products: water and the dipeptide)

Peptide bond between 2 amino acids

Hydrolysed by adding water across the bond

Question 7

Dipeptide Formation in words

Answer 7

A hydrogen atom from the amine group of one amino acid joins with OH from the carboxyl group of a second amino acid to produce H2O which is released

This forms a peptide bond

Question 8

Peptide bond

Answer 8

Between carbon & nitrogen

Question 9

N-terminal

Answer 9

Amine group (H2N…. C)

Question 10

C-terminal

Answer 10

Carboxylic group (N||||H… OH)

Question 11

Functional protein may contain

Answer 11

One or more polypeptides

Question 12

Chains of amino acids shape

Answer 12

Chains of amino acids that make up polypeptides often take up 3D shape

Question 13

Disulphide bond

Answer 13

Occur between sulphur containing groups in the amino acids cysteine and methionine, strong covalent bonds

Question 14

Ionic bonds

Answer 14

Free amino and carboxylic acid groups can ionise and then the opposite electrostatic charges can attract

Weaker than disulphide but still strong

Question 15

Hydrogen bonds

Answer 15

Occur between the amine and the carboxyl groups

The hydrogen bond is the electrostatic attraction between these partial charges

Weakest bond

Question 16

additional interactions that will contribute to the structure of a protein

Answer 16

Polar/hydrophilic amino acids will move themselves towards water e.g. histidine and glutamine

Non-polar/hydrophobic amino acids will move themselves away from water e.g. tyrosine, phenylalanine

Question 17

Primary structure

Answer 17

The primary structure of the polypeptide determines its shape and its function,

determined by the sequence of bases in the DNA

Slight change in the primary structure (e.g. substitution of base caused by mutation of DNA) can lead to protein with different properties e.g. sickle cell anaemia

Question 18

SCA

Answer 18

at 6th amino acid, the glutamic acid changes to valine, changing the shape of red blood cells from biconcave disc to sickle shape

Affects: amount of O2 transported, ability to flexibly move around transport system

Question 19

Secondary structure

Answer 19

Describes the coiling of the polypeptide chain with hydrogen bonding alone between the amine group of one amino acid and carboxyl of another

Between NH2 and C=O

Structures formed tend to be a-helix or b-pleated sheet

Question 20

Collagen structure

Answer 20

• 3 helical polypeptide chains which wind around each other to form triple helix
• held together by hydrogen bonds
• at R groups of lysine amino acids, covalent bonds form different triple helices. These cross-links hold many collagen molecules side by side, forming a collagen fibre
• resulting fibres have a large tensile strength
• important structural protein found in skins, tendons and walls of the blood vessels

Question 21

Tertiary structure describes

Answer 21

Describes the shape of the polypeptide chain with hydrogen, ionic and disulphide bonds between the R groups

Question 22

Quaternary structure

Answer 22

Describes the shape of the molecule when several polypeptide chains are wound around each other

Often there may be a non-amino acid part to the molecule, a prosthetic group

e.g. haemoglobin has a quaternary structure and is made of four polypeptide chains wound around each other with 4 haem groups that contain iron

Question 23

Proteins with/without prosthetic groups

Answer 23

Complex

Simple

Question 24

Haemoglobin

Answer 24

Oxygen carrying pigment found in RBC

4 polypeptide chains: 2 a & 2 b chains

Structure is curled up so that hydrophilic chains are on the outside of the molecule, making it soluble in water

Each polypeptide chain has a haem group which is not made of amino acids (prosthetic group)

Each haem contains a Fe2+. One O2 can bind with each Fe2+ so a complete Hb molecule can carry 8 O2 atoms

Question 25

Test for proteins

Answer 25

Biuret test

Add biuret reagent

Positive: purple
Negative: no colour change/blue

Question 26

Main molecular shapes in protein structure

Answer 26

Fibrous such as collagen & keratin that have a structural function
Globular such as enzymes and haemoglobin that carry out metabolic functions

Question 27

Fibrous proteins

Answer 27

Polypeptide chains form long parallel strands, chains wind around each other in the secondary structure and twist into a second helix in the tertiary structure

Repeated, regular sequences of amino acids

Little variability between proteins

Stable and insoluble

Role in support and structure

Question 28

Globular proteins

Answer 28

Polypeptide chains folded into a spherical shape

Irregular sequences of amino acids not repeated

Soluble and less stable

Variability between proteins

Metabolic role

Question 29

Functions of proteins

Answer 29

Enzymes, acting as biological catalysts e.g. amylase, lactase and maltase

Contractile proteins e.g. actin and myosin

Transport proteins, that carry substances around the body e.g. haemoglobin

Structural proteins e.g. collagen, keratin

Hormones, chemical messengers e.g. insulin, glucagon and ADH

Question 30

Properties of proteins

Answer 30

Insoluble in water - some form colloids

Formed by condensation reactions and broken down by hydrolysis. Enzymes involved in hydrolysis - endopeptidases, exopeptidase and membrane-bound dipeptides

denatured by certain factors

Question 31

Denatured

Answer 31

Change in shape due to bonds being broken

Question 32

Factors which denature proteins include

Answer 32

High temperature
Acids
Alkalis
Inorganic solvents
Organic solvents
Mechanical force

Question 33

which types of bonds exist in the tertiary structure of a protein that aren’t present in the secondary structure

Answer 33

Ionic bonds, disulphide