1.3-Proteins

amino acid

monomer from which proteins are made

protein

polymer made of amino acid monomers linked together by peptide bonds via condensation reactions

polypeptide

long chain polymer made up of several amino acids linked together by condensation

primary sequence

base sequence of amino acids in a polypeptide

sequence of amino acids determined by DNA

therefore primary structure determines final tertiary structure

secondary structure

hydrogen bonding causes a helix or B pleated sheets

tertiary structure

further folding of whole chain fold into specific 3D shape

held in place by ionic, hydrogen bonds + disulfide bridges

creates active site in enzymes

what happens when a protein is subjected to high temperatures or extreme pH?

denatures as the hydrogen bonds break first

quaternary structure

further folding of more than one polypeptide chain

what are globular proteins?

e.g enzymes and haemoglobin- carry out metabolic functions

what are fibrous proteins?

long chains running parallel to each other with cross bridges between the chains- produces very stable molecules e.g collagen

biuret test for proteins?

blue to purple/lilac if protein is present

what is the lock and key model?

active site is always exact complementary shape (at optimum temp.), so reactions are fastest in these conditions

substrate is complementary in shape to active site, therefore binds and forms ES complex

what is the induced fit model?

substrate and active site not exactly complementary to begin with

when substrate binds to active site, induces changes in enzyme structure

active site changes shape to become complementary

what does the ‘strained fit’ between the active site and substrate do?

distorts particular bond in the substrate and so reduces activation energy needed to break the bond

how to calculate rate?

amount of product/time taken

why is the rate of reaction fastest at the start of the reaction, and slower as the reaction progresses?

less substrates for the enzyme to bind to at the end of the reaction compared to at the start of the reaction, so the curve becomes a line that levels off

why do both curves level (i.e slow and fast reaction) off at the same amount of product?

same substrate concentration from the start

what happens to enzymes at low temperatures?

less kinetic energy- fewer collisions between enzyme and substrate

what happens to enzymes at higher temperatures?

more kinetic energy

collide more frequently

more ES complexes form

increases rate of reaction

what happens to enzymes at temperatures above 40c (optimum temp)?

causes enzyme to denature

breaks bonds holding the tertiary structure

active site changes shape

substrate no longer complementary to enzyme’s active site

ES complexes cant form

slows down rate of reaction

what happens to an enzyme when the pH is too high or too low?

alters charges on amino acids that make up active site

breaks bonds holding the tertiary structure

alters shape of active site

ES complexes no longer form as enzyme is denatured

why enzymes work at the optimum pH?

shape of active site isn’t altered so it is complementary to the substrate

therefore max. no. of ES complexes form

explain the effect of increasing the substrate conc. on rate of reaction

more substrates collide + bind to active site of enzyme- more ES complexes form

active sites start becoming saturated

enzyme conc. becomes the limiting factor

curve levels off

when does an increase in enzyme conc. lead to a proportionate increase in the rate of reaction?

when there is an excess of substrate

what happens to the rate of reaction if the substrate is limiting?

not enough substrates to occupy active sites at one time

enzyme comc. will have no effect on the rate of reaction

rate of reaction will level off

what happens during competitive inhibition?

competitive inhibitors have a similar shape to that of a substrate

binds to active site of enzyme

fewer ES complexes form + rate of reaction reduced

explain the effect of increasing substrate conc. on competitive inhibition?

at first- rate is reduced as there is low substrate conc

effect of competitive inhibitor is reduced as there are more substrates that are likely to bind to the active sites

max rate of reaction is reached as there is no inhibition

what happens during non competitive inhibition?

bind to allosteric site of enzyme

changes shape of active site- no longer complementary- substrate can’t bind

fewer ES complexes form + rate of reaction of decreases