Biological molecules - Proteins Flashcards
giving examples where necessary, what are the roles of proteins in organisms (8 things)
- cell signalling molecules e.g hormones such as insulin
- transport oxygen in red blood cells e.g haemoglobin
- antibodies - fight pathogens
- structural - build cells
- enzymes e.g pepsin, speeding up metabolic reactions
- muscles and contraction e.g actin and myosin
- receptors on surface of membrane
- membrane carriers and channels
what are the two types of proteins
fibrous and globular
what percentage of the organic matter of the cell do proteins make up
50%
what do proteins provide the building materials for
growth, repair of tissues and replacement of cells
are proteins polymers
yes
how many amino acids are there
20
describe how plants get amino acids
plants can make all the amino acids they need - but they need a source of nitrogen. they get this from nitrates in the soil
describe how animals get amino acids
animals can make some amino acids, but must ingest and absorb others (essential amino acids)
what happens to excess amino acids in animals
they cannot be stored so are broken down in the liver to urea and excreted by the kidney
describe the general structure of amino acids
C-C-N backbone with an amino group at one end and a carboxyl group at the other, with an R group and hydrogen attached to the central C
what does the R group represent
any one of 20 possible groups, which determine what particular amino acid it is
what is the bond between two amino acids called
peptide bond
what type of reaction forms and breaks the bond between two amino acids
form - condensation
breaks - hydrolysis
what is the primary structure
the specific sequence of amino acids forming a chain of amino acids joined by peptide bonds
what determines the primary structure
genetic code
where are peptide bonds formed
at the ribosome during translation
what is the secondary structure
how a polypeptide folds. it forms one of two structures - alpha helix or beta pleated sheet
what is the secondary structure stabilised by
hydrogen bonds
what do parts of the polypeptide that have no secondary structure do
join helices and sheets together
what is the tertiary structure
the 3D shape of a protein determined by the way that alpha helices and beta pleated sheets interact with one another
what are four main types of bond/interaction that hold together the tertiary structure
- disulfide bonds
- ionic bonds
- hydrogen bonds
- hydrophilic and hydrophobic interactions
do all proteins have a quaternary structure
no
when does a protein have a quaternary structure
if it is made of two or more polypeptide chains
give examples of proteins with a quaternary structure
haemoglobin, collagen
describe hydrogen bonds in proteins (how does it form, strength and what is it broken by)
- forms between polar groups
- very weak
- broken pH and temperature changes
describe ionic bonds in proteins (how does it form, strength and what is it broken by)
- forms between charged groups
- quite strong
- broken by pH and temperature changes
describe disulfide bonds in proteins (how does it form, strength and what is it broken by)
forms between sulphur atoms in the R-groups (e.g cysteine), very strong, broken by reducing agents e.g strong acids
describe hydrophobic bonds in proteins (how does it form, strength and what is it broken by)
- hydrophobic R-groups (non-polar) point inwards away from water
- weak
- broken by changes to the overall 3-D structure (disruption of other bonds)
describe hydrophilic bonds in proteins (how does it form, strength and what is it broken by)
- hydrophilic R-groups (polar) point outwards towards water
- weak
- broken by changes to the overall 3-D structure (disruption of other bonds)
describe peptide bonds in proteins (how does it form, strength and what is it broken by)
- strong covalent bond between amino acids
- very very strong
- broken by enzymes e.g pepsin
how may a prosthetic group be attached
covalent bonds, ionic interactions or hydrogen bonds
what are conjugated proteins
proteins that contain a non-protein part. this part is known as a prosthetic group
give an example of a conjugated protein
haemoglobin - haem groups are prosthetic group
what are the two types of protein
globular and fibrous
what is the 3D feature of globular proteins
roll up to form balls - spherical
what is the 3D feature of fibrous proteins
form long, narrow fibres
describe the primary structure of globular proteins
very precise, usually a non-repeating amino acid sequence
describe the primary structure of fibrous proteins
often long molecules with simple, repeating amino acid sequence
describe the secondary and tertiary structure of globular proteins
complex
describe the secondary and tertiary structure of fibrous proteins
simple
describe the solubility in water of globular proteins
usually soluble - hydrophilic R groups on outside and in contact with water
describe the solubility in water of fibrous proteins
usually insoluble
describe the role of globular proteins
usually metabolic: enzymes, hormones, transport
describe the role of fibrous proteins
usually structural
what are the three examples of fibrous proteins you need to know
collagen, keratin, elastin
where is collagen found
- artery walls
- tendons
- bones
- cartilage
- skin
where is keratin found
- fingernails
- hair or wool
- claws and hooves
- beaks and horns
- scales and feathers
where is elastin found
- skin
- lungs
- bladder
- blood vessels
what are the properties and functions of collagen
strong and inelastic, providing internal mechanical strength e.g prevents bursting
what are the properties and functions of keratin
- lots of the amino acid cysteine in the polypeptide so lots of disulfide bonds form between chains.
- provides mechanical protection as it is a very strong and hard molecule
- also acts as a waterproof and impermeable barrier
what are the properties and functions of elastin
due to cross-linking and coiling it is strong and extensible, allowing things to stretch or adapt their shape
what are the three globular proteins you need to know
haemoglobin, insulin and pepsin
describe the structure of haemoglobin
a conjugated protein as it contains non-protein prosthetic haem groups which contain Fe2+ which bonds to oxygen. it has four polypeptides in its quaternary structure (2 alpha and 2 beta)
describe the structure of insulin
2 polypeptide chains
A begins with an alpha helix
B ends with a beta pleated sheet
each forms its own tertiary structure - joined by disulfide bonds
describe the structure of pepsin
no quaternary structure
1 polypeptide chain that folds into a symmetrical tertiary structure
of 327 amino acids, only 4 have basic R groups - gives stability in low pH environment
what is the function of haemoglobin
carry oxygen from the lungs to tissues
what is the function of insulin
hormone - binds to glycoprotein receptors on muscle and fat cells to:
increase rate of glucose uptake from blood, increase rate of glucose consumption
what is the function of pepsin
enzyme - digests protein in the stomach (breaks peptide bonds)
what are computer models used for
to predict the structure of proteins
what are the advantages of computer modelling
fast, smaller research teams, cheaper, gives an indication of whether an interaction would work
describe the biuret test for proteins, including a positive result
mix 1cm cubed of suspension or solution with an equal volume of biuret reagent. swirl the tube and look for a colour change (blue to lilac)
how does the biuret test work
the chemicals in the biuret solution react with the peptide bonds in a protein
when may the biuret test not work and why
it will not go lilac with just a mixture of amino acids, as it only detects the presence of proteins because of peptide bonds
can insulin change shape
no
can haemoglobin change shape
yes
state three properties of a fibrous protein that are different to those of a globular protein
- insoluble
- strong
- unreactive
two proteins have the same number and type of amino acids but different tertiary structures. why
different sequence of amino acids therefore R-groups bond differently