8.1. - 8.2. Proteins Flashcards
The four macromolecules…
Proteins, lipids, carbohydrates and nucleic acids.
Proteins…
Polymers of 20 different amino acids.
Made of one or multiple polypeptide chains (a single, unbranched chain of amino acids folded into specific 3D shapes, defined by the sequence of the amino acids).
The alpha-carbon is asymmetrical.
Amino acids can exist in two isomeric forms…
D-amino acid (right).
L-amino acid (left) which is found in organisms.
Formed during condensation fractions of amino acids.
Amino acids are covalently linked using peptide bonds. They are fixed as the peptide bond allows no rotation.
Four structures of proteins…
Primary: the sequence of amino acids. This determines the secondary and tertiary structures.
Secondary: hydrogen bonding causes amino acids to fold into different structures:
- Alpha helix: the right-handed coil resulting from hydrogen bonding between N-H groups on one amino acid and C=O groups on another amino acid.
- Beta pleated: two or more polypeptide chains are aligned, hydrogen bonds form between the chains.
Tertiary: caused by hydrogen bonds, ionic bonds and disulphide bridges. Produces a 3D structure. The outer surface presents functional groups that interact with other molecules.
Quaternary: interactions of subunits by hydrophobic interactions, van der waals etc. Each subunit has its own unique tertiary structure.
Denaturing…
Exposing secondary and tertiary structures to heat or extreme pH’s can lead to denaturing as the structure breaks down.
However, when cooled, the original structure reforms, showing that the information to specify protein structure is contained within the primary structure.
Vital amino acids…
Methionine: the start codon.
Proline: causes kinks in chains of amino acids.
Cysteine: links amino acids together through disulphide bonds (SH group in the side chain creates a very strong bond that can withstand harsh extracellular conditions).
Enzymes…
Biological catalysts that speed up rates of reactions, by reducing activation energies, without getting used up themselves.
They are usually very specific, only fitting one substrate (enzyme-substrate complex).
Enzymes lower the energy barrier by bringing reactants closer together.
Activation energy changes the reactants into unstable versions with higher free energy.
This energy can come from heating the system as reactants have more kinetic energy.
Enzyme-substrate complex…
Enzymes can form an enzyme-substrate complex with one substrate.
The enzyme-substrate complex forms, enzyme and substrate are held together by covalent and hydrogen bonds and electrical attraction.
The enzyme may change when bound to the substrate, but returns to its original form after the reaction has occured.
The binding of a substrate depends on hydrogen bonds, attraction and repulsion of electrically charged groups and hydrophobic interactions.
Many enzymes will also change their shape when binding to the substrate (induced fit model).
Three ways enzymes work…
Enzymes change the orientation of the substrate molecule, bringing together the atoms that will bond - ensuring perfect alignment to allow bonds to form.
Enzymes stretch bonds in the substrate molecule, making them unstable. This encourages them to react together.
Enzymes can temporarily add chemical groups to substrates to make them easier to react.
Lysozyme…
A digestive enzyme (from the lysosomes) that acts as an antibiotic because it catalyses the cutting of polysaccharide chains in bacterial cell walls, causing them to rupture.
It catalyses the hydrolysis reaction that is spontaneous but not instantaneous.
Regulation of reactions…
The first reaction is the commitment step and the others then happen in sequence.
Feedback inhibition (end-production inhibition) means that the final product in the reaction acts as an inhibitor to shut down the pathway.
Enzymes are most active at specific pH’s and specific pH’s influence the ionisation of functional groups.
