2.3.6 & 2.3.7 Proteins

OCR A-Level Biology Module 2 Chapter 3 - Proteins

What are the monomer units of proteins called?

amino acids

What is the name given to a molecule made of 2 amino acids?

dipeptide

What type of reaction form dipeptides and polypeptides?

condensation

How many amino acids is a polypeptide made from?

3 or more

What elements do proteins contain?

carbon, hydrogen, oxygen and nitrogen

Are proteins macromolecules?

yes

What is the R group of an amino acid?

a variable group

How many different types of amino acids are commonly found in cells?

20

What are non-essential amino acids?

amino acids that our body can make from other amino acids if they can’t be obtained from food in our diets

What are essential amino acids?

amino acids that cannot be made by the body so have to come from food in our diets

How many non-essential amino acids are there?

5

How many essential amino acids are there?

15

What are the 2 main groups of molecules in the structure of an amino acid?

amine group (NH2) and a carboxyl group (COOH)

What is an amphoteric molecule?

a molecule with 2 different (contrasting) properties

Why are amino acids amphoteric molecules?

the amine group is basic (alkaline) and the carboxyl group is acidic

What is the name of the simplest amino acid and what is its R group made of?

glycine - single H atom

What happens to the properties of the amino acid if you add more carboxyl groups to the R group?

it becomes more acidic

True or false? The R group of an amino acid can only contain the elements H, O, C, and N

false - they can contain any element

Which amino acid contains sulphur?

cysteine

How do plants manufacture amino acids?

with nitrates from the soil and organic molecules produced by photosynthesis

True or false? Animals cannot store excess amino acids because the amine group makes them toxic

true

How are excess amino acids removed from the body?

they are transported to the liver where the amine group is removed in deamination, and then converted to urea which is excreted via urine

What are the bonds between amino acids in polypeptides called?

peptide bonds

Are peptide bond ionic or covalent?

covalent

Are the R groups involved in the condensation reactions that form polypeptides?

no

What type of enzymes can hydrolyse polypeptides?

proteases

What is a reagent?

a mixture of 2 or more chemicals

Is Biuret a reagent? Why/why not?

yes - it is contains both copper sulphate and an alkali such as potassium hydroxide and sodium hydroxide

What reaction takes place in the Biuret test?

The Cu2+ ions are reduced to Cu+ ions which forms

biuret test

What is the primary structure of a protein?

the sequence of the amino acids in the polypeptide chain bonded by strong covalent peptide bonds

True or false? the primary structure is specific for each protein

true

Why could a mutation in DNA affect the primary structure of a protein?

the amino acids are coded for by DNA, so a mutation in the base sequence could cause a different amino acid to be added to the polypeptide chain affect the structure and the function of the protein by causing it to fold differently

What two structures can be formed in the secondary structure of a protein?

alpha helix or a beta pleated sheet

What causes the protein to fold into its secondary structure?

hydrogen bonds

What shape is an alpha helix and how is it held together?

a coil held in the centre by hydrogen bonds

What shape is an beta pleated sheet and how is it held together?

a folded sheet where 2 parts of the polypeptide chain are antiparallel to each other, held by hydrogen bonds

True or false? A polypeptide chain either form an alpha helix or a beta pleated sheet in it secondary structure 

False - different regions of the same polypeptide chain coil or fold

Fill in the blank: In an alpha helix, hydrogen bonds form after every ___ peptide

4th

What can cause the hydrogen bonds in proteins to break?

high temperatures and extreme pH changes

What is the tertiary structure of a protein?

further folding and coiling of the secondary structure to determine its 3D shape

What are the bonds in the tertiary structure found between?

additional bonds between R groups

What happens to the tertiary structure when a enzyme denatures?

the bonds in the tertiary structure break so the 3D shape of the enzymes active site changes meaning it is no longer complementary to the substrate

What are the 4 interactions that could occur between R groups in the tertiary structure?

disulphide bonds/bridges, ionic bonds, hydrogen bonds, hydrophilic/hydrophobic interactions

Is a disulphide bond weak or strong?

strong

When a protein denatures, are any disulphide bonds broken? Why/why not?

no - they are too strong

How do disulphide bridges form? Give an example of an amino acid that forms these

form in proteins that contain the sulphur in their amino acids, such as between 2 cysteine amino acids

What type of bond are disulphide bonds?

covalent

What do ionic bonds in the tertiary structure form between?

oppositely charged R-groups

How can ionic bonds be affected by a decrease in pH?

there is an increase in acidity and the number of H+ ions in the solution, which may bond to the proteins, breaking other bonds

What is the mostly commonly found bond in tertiary structures?

hydrogen bond

What do hydrogen bonds in the tertiary structure form between?

slightly polar R-groups

How do hydrophilic and hydrophobic interactions affect the structure of a protein?

Hydrophobic forces cause non-polar amino acids to be repelled by water and pushed into the middle of the protein, with hydrophilic and polar amino acids moving to the outside of the molecule

Fill in the blanks: the quaternary structure of some proteins is formed through the interaction of multiple ____ ___ held together by bonds

polypeptide chains

Fill in the blanks: a protein with a quaternary structure can be called a (f)____ (m)____

functional macromolecule

True of false? all proteins have a quaternary structure

false

What is each polypeptide chain called in a quaternary protein?

subunit

What is a term describing a protein with 2 polypeptide chains?

dimer

What is a term describing a protein with 3 polypeptide chains?

trimer

What is a term describing a protein with 4 polypeptide chains?

tetramer

What is a homomer protein ?

a protein with a quaternary structure where all polypeptide chains are identical

What is the term used to describe a protein with a quaternary structure consisting of non-identical polypeptide chains?

heteromer

Choose the correct word: globular proteins are loose/compact, water soluble/insoluble and usually spherical/strands

compact, soluble, spherical

What type of reactions are globular proteins usually involved in?

metabolic

True or false? globular proteins do not unravel and denature easily when temperature of pH deviates from the optimum level

false

Why do globular proteins have a spherical shape and what causes them to be water soluble?

the non-polar hydrophilic R-groups are arranged on the outside, surrounding the hydrophilic amino acids on the inside

What is a conjugated protein?

a protein that contains a prosthetic group

What is the term that describes a permanent, non-protein part of a protein molecule?

prosthetic group

Fill in the blanks: a fibrous protein contain many polypeptide chains in long, narrow ___ that are cross-linked by ___ and often ___ bonds

stands, hydrogen, disulphide

Are fibrous proteins soluble or insoluble? Why/why not?

insoluble - they contain a large number of small, hydrophobic R-groups

What is the function of most fibrous proteins?

structural roles

Why are fibrous proteins good for structural roles?

they have a limited number of amino acids, creating a repetitive primary structure, which , along with small R-groups makes them well organised and compact

True of false? fibrous proteins have a complex teritiary structure

false - it is simple are they aren’t folded

Choose the correct words: Haemoglobin is a fibrous/globular protein with a tertiary/quaternary structure made of 2/3/4/6 polypeptides

globular, quaternary, 4

What are the polypeptide chains that make up haemoglobin called?

2 a-globin chains and 2 b-globin chains

True or false? all the polypeptide chains in haemoglobin have their own tertiary structure

true

Explain why haemoglobin is a conjugated protein

It contains a haem group which is a prosthetic group

What ions are found in haem groups?

iron (II) ions

How many haem groups does haemoglobin have?

4

Choose the correct words: Insulin is a globular/fibrous protein made form 2/3/4 polypeptide chains held together by 2/3/4 ionic bonds/covalent bonds/disulphide bridges

globular, 4, 3, disulphide bridges

How does the structure of the hormone insulin allow it to be transported?

it has amino acids with hydrophilic R-groups on the outside of the molecule so it is soluble in water (and blood)

Fill in the blanks: insulin binds to (g)____ ____ on the outside of ___ and ___ cells to increase/decrease their uptake of ____ from the blood, reducing blood ____ levels

glycoprotein receptors, muscle and fat, increase, glucose, glucose

Fill in the blanks: the enzyme catalase is a ____ protein with a ____ structure containing ___(number) ____ prosthetic groups. The presence of the ____ ions in the prosthetic group allows it to catalyse the breakdown of ____ ____ (a harmful byproduct of many metabolic reactions) by converting it into ___ and _____

globular, quaternary, 4, haem, iron II, hydrogen peroxide, water and oxygen

Fill in the blanks: Keratin is the group of _____ proteins found in hair, skin and nails. It contains lots of ____ amino acids, with many ___ ___ making it strong. It provides mechanical protection, an impermeable barrier and a relatively inflexible shape.

fibrous, cysteine, disulphide bridges

Fill in the blanks: Elastin is a ____ protein found in elastic fibres (such as in the skin, lungs and bloods vessels).It is made from many stretchy molecules called ____ which are cross-linked and branched/coiled/in chains

fibrous, tropoelastin, coiled

What is the function of collagen?

it is a connective tissue found in skin, tendons, ligaments, artery walls and bones

Fill in the blanks: collagen is a ___ protein with a quaternary structure of ___(number) polypeptide chains twisted around each other in a ____ ____. The chains are cross-linked to form a collagen ____ giving it a high ___ strength (also due to repetitive primary structure of only 3 amino acids) but also flexibility. They are soluble/insoluble.

fibrous, 3, triple helix, fibril, tensile