Proteins And Enzymes

Amine group/-NH₂; carboxyl group/-COOH; hydrogen atom; R group/variable side chain (1 mark for any two; 2 marks for all four)

Condensation reaction occurs; the amine group of one amino acid reacts with the carboxyl group of the other; a peptide bond forms; one water molecule is released (any 3)

They are small molecules that join together by condensation reactions/peptide bonds; to form polymers/polypeptides/proteins.

The specific sequence/order of amino acids in a polypeptide chain.

Hydrogen bonds; ionic bonds; disulfide bridges/disulfide bonds; hydrophobic interactions.

Primary structure determines the position of R groups; R groups determine which bonds can form during folding; this changes the tertiary/three-dimensional shape; the active site/binding site is altered so the protein may no longer function.

Globular are spherical/fibrous are long; globular are soluble/fibrous are insoluble; globular have hydrophilic R groups outside/fibrous have R groups distributed throughout (any 2)

Quaternary structure with four polypeptide chains; each contains a haem group with iron; so one haemoglobin transports four oxygen molecules; hydrophilic R groups face outwards making it soluble in blood plasma.

Indicative content (any 6): haemoglobin is globular/spherical/water-soluble; quaternary structure with four polypeptide chains; each chain has a haem group/prosthetic group with iron; binds four oxygen molecules per haemoglobin; hydrophilic R groups outside making it soluble in plasma; suited to oxygen transport in blood; collagen is fibrous/long/insoluble; three polypeptide chains in triple helix held by hydrogen bonds; many molecules cross-link to form fibres; provides high tensile strength; suited to structural/supportive roles in skin/tendons/ligaments.

A specific region of an enzyme with a shape complementary to its substrate.

Each enzyme has a particular tertiary structure; this gives the active site a specific shape; only a substrate with a complementary shape can bind/form an enzyme-substrate complex.

Both involve a complementary shape between active site and substrate; lock and key has a rigid active site/induced fit has a flexible active site; in induced fit the active site changes shape on substrate binding; induced fit explains how bonds in the substrate are strained, lowering activation energy.

The minimum energy required to start/initiate a chemical reaction.

Enzymes lower the activation energy; they do this by straining bonds in the substrate (induced fit)/holding substrates in the correct orientation; so more substrate molecules have enough energy to react at a given temperature.

Intracellular enzymes catalyse reactions inside the cell; extracellular enzymes are secreted/catalyse reactions outside the cell.

Intracellular: catalase/DNA polymerase/respiratory enzymes; extracellular: amylase/trypsin/lipase/protease.

At the start, substrate concentration is highest; so reaction proceeds at maximum speed; as time goes on substrate is used up/products accumulate, reducing rate.

High temperatures break/disrupt hydrogen and ionic bonds in the tertiary structure; the enzyme denatures; active site shape changes/is no longer complementary to substrate; substrate can no longer bind/fewer enzyme-substrate complexes form.

Temperature; pH; enzyme concentration; volume of solutions/same apparatus (any 3)

Calculating the initial rate $$\text{rate}=\frac{\text{volume}}{\text{time}}=\frac{18}{30}$$ $$\text{rate}=0.6{\text{ cm}}^{3}{\text{ s}}^{-1}$$ The initial rate of reaction is 0.6 cm³ s⁻¹.