1.4 Proteins Flashcards
Describe / draw the general
structure of an amino acid
● COOH = carboxyl group
● R = variable side chain / group
● H2N = amine group
How many amino acids are common in all organisms? How do they vary?
The 20 amino acids that are common in all organisms differ only in their side group (R
Describe how amino acids join together
● Condensation reaction
● Removing a water molecule
● Between carboxyl / COOH group of one and amine / NH2 group of another
● Forming a peptide bond
What are dipeptides and polypeptides?
● Dipeptide - 2 amino acids joined together
● Polypeptide - many amino acids joined together
Describe the primary structure of a protein
Sequence of amino acids in a polypeptide chain, joined by peptide bonds
Describe the secondary structure of a protein
● Folding (repeating patterns) of polypeptide chain eg.
alpha helix / beta pleated sheets
● Due to hydrogen bonding between amino acids
● Between NH (group of one amino acid) and C=O (group)
Describe the tertiary structure of a protein
● 3D folding of polypeptide chain
● Due to interactions between amino acid R groups
(dependent on sequence of amino acids)
● Forming hydrogen bonds, ionic bonds and disulfide bridges
Describe the quaternary structure of a protein
● More than one polypeptide chain
● Formed by interactions between polypeptides
(hydrogen bonds, ionic bonds, disulfide bridges)
Describe the test for protein
- Add biuret reagent (sodium hydroxide + copper (II) sulphate)
- Positive result = purple / lilac colour (indicating presence of peptide bonds)
How do enzymes act as
biological catalysts?
● Each enzyme lowers activation energy of reaction it catalyses
● To speed up rate of reaction
Describe the induced-fit model of enzyme action
- Substrate binds to (not completely complementary) active site of enzyme
- Causing active site to change shape (slightly) so it is complementary to its substrate
- So enzyme-substrate complex forms
- Causing bonds in substrate to bend / distort, lowering activation energy
Describe how models of enzyme action have changed over time
● Initially lock and key model (now outdated)
○ Active site a fixed shape, complementary to one substrate
● Now induced-fit model
Explain the specificity of enzymes
● Specific tertiary structure determines shape of active site
○ Dependent on sequence of amino acids (primary structure)
● Active site is complementary to a specific substrate
● Only this substrate can bind to active site, inducing fit and forming an enzyme-substrate complex
Describe and explain the effect of enzyme concentration on the rate of enzyme-controlled reactions
● As enzyme concentration increases, rate of reaction increases
○ Enzyme concentration = limiting factor (excess substrate)
○ More enzymes so more available active sites
○ So more enzyme-substrate complexes form
● At a certain point, rate of reaction stops increasing / levels off
○ Substrate concentration = limiting factor (all substrates in use)
Describe and explain the effect of substrate concentration on the rate of enzyme-controlled reactions
● As substrate concentration increases, rate of reaction increases
○ Substrate concentration = limiting factor (too few substrate molecules to occupy all active sites)
○ More enzyme-substrate complexes form
● At a certain point, rate of reaction stops increasing / levels off
○ Enzyme concentration = limiting factor
○ As all active sites saturated / occupied (at a given time)
Describe and explain the effect of temperature on
the rate of enzyme-controlled reactions
● As temperature increases to optimum, rate of reaction increases
○ More kinetic energy
○ So more enzyme-substrate complexes form
● As temperature exceeds optimum, rate of reaction decreases
○ Enzymes denature - tertiary structure and active site change
shape
○ As hydrogen / ionic bonds break
○ So active site no longer complementary
○ So fewer enzyme-substrate complexes form
Describe and explain the effect of pH on
the rate of enzyme-controlled reactions
● As pH increases / decreases above / below an optimum, rate of
reaction decreases
○ Enzymes denature - tertiary structure and active site
change shape
○ As hydrogen / ionic bonds break
○ So active site no longer complementary
○ So fewer enzyme-substrate complexes form
Describe and explain the effect of concentration of competitive inhibitors on the rate of enzyme-controlled reactions
● As concentration of competitive inhibitor increases, rate of
reaction decreases
○ Similar shape to substrate
○ Competes for / binds to / blocks active site
○ So substrates can’t bind
○ So fewer enzyme-substrate complexes form
● Increasing substrate concentration reduces effect of inhibitors
(dependent on relative concentrations of substrate and inhibitor)
Describe and explain the effect of concentration of
non-competitive inhibitors on the rate of enzyme-controlled reactions
● As concentration of non-competitive inhibitor increases, rate of reaction decreases
○ Binds to site other than the active site (allosteric site)
○ Changes enzyme tertiary structure / active site shape
○ So active site no longer complementary to substrate
○ So substrates can’t bind
○ So fewer enzyme-substrate complexes form
● Increasing substrate concentration has no effect on rate of reaction as change to active site is permanent
