7.5 Proteins
The 20 different amino acids
7.5.1: Explain the four levels of protein structure, indicating the significance of each level.
Peptide bonds link the amino acids together
Polypeptide with five amino acids.
7.5.1: Primary structure
Sequence of amino acids.
May have disulfide bridges (sulfur linkage
between two cysteines)
7.5.1: Secondary Structures
Secondary structures: alpha helix and beta pleated sheets
Alpha Helix
Beta Pleated Sheet
Hydrogen bonding stabilises the secondary structure
Alpha helix
Beta pleated sheet
Beta pleated sheet
7.5.1: Tertiary Structure
Tertiary structures:
Attractions between
alpha helices and
beta sheets –
hydrophobic
interactions between
R groups cause
folding of the
polypeptide at the
tertiary level.
Beta pleated sheet
Alpha helix
Bonding in the tertiary structure
IB Question: Explain primary structures and tertiary structures of an enzyme.
M09/4/BIOLO/HP2/ENG/TZ2/XX
IB Question: Explain primary structures and tertiary structures of an enzyme.
M09/4/BIOLO/HP2/ENG/TZ2/XX
primary structure is (number and) sequence of amino acids;
joined by peptide bonds;
tertiary structure is the folding of the polypeptide/secondary structure/alpha helix;
stabilized by disulfide/ionic/hydrogen bonds/hydrophobic interactions;
tertiary structure gives three dimensional globular shape/shape of active site; [3 max]
7.5.1: Quaternary Structure
The structure of a protein that results from the interaction of two or
more individual polypeptides to give larger functional molecules.
7.5.1: Conjugated Protein
Prosthetic group. The non-protein part of a
protein required for the protein to be
functional. E.g the heme molecule of
haemoglobin.
IB Question: Explain the four levels of protein structure [8]
IB Question: Explain the four levels of protein structure [8]
primary structure is sequence / number of amino acids;
determined by base sequence in the gene;
(largely) determines higher level structures/secondary structure/tertiary structure;
secondary structure is regular repeating patterns;
such as alpha/α helix and beta/β (pleated) sheet;
determined by H bonds (within chain);
contributes to the strength of fibrous proteins;
tertiary structure refers to overall 3-D shape;
conformation can determine function;
tertiary structure determined by R-group interactions / ionic interactions /
hydrophobic interactions / disulfide bridges / H-bonds;
quaternary structure is only found in proteins formed from more than one
polypeptide;
e.g. hemoglobin; (accept other suitable example)
quaternary structure may involve the binding of a prosthetic group;
IB QUESTION: Outline the first three levels of protein structure, including the types of bonding
within each and the significance of each level. [5]
primary structure/level: order/sequence of amino acids;
linked by peptide bonds;
determines the type/function of protein / 2º and 3º structures;
secondary structure/level: regular folding / beta-pleated sheets / spiralling /alpha-helices;
held through hydrogen bonding;
tertiary structure/level: 3-dimensional conformation of a polypeptide/protein;
held with ionic bonds, hydrogen bonds, disulfide bonds/bridges and hydrophobic bonds; (must
give at least two bonds)
determines overall shape / a named example e.g.: active sites on enzymes; [5 max]
To receive full marks the candidate must mention each of the three levels,
otherwise award [4 max].
IB Question: Bt proteins act as toxins to insects, primarily by destroying epithelial cells in the
insect’s digestive system. Below is the three-dimensional structure of one such protein.
(i) State the type of structure shown in the region marked A in the diagram above.
[1]
(ii) Outline how this structure is held together.
[2]
(iii) Region A inserts into the membrane. Deduce, with a reason, the nature of the
amino acids that would be expected to be found in this region. [2]
(i) helix / alpha helix [1]
(ii) hydrogen bonds;
between the turns of the helix (rather than between R-groups);
bonds between carboxyl and NH groups/C-O---H-N; [2 max]
(iii) non-polar amino acids/R-groups;
(inner part of phospholipid) bilayer is hydrophobic/non-polar; [2]
7.5.2: Outline the difference between fibrous and globular proteins, with reference to two
examples of each protein type.
•Globular proteins are near soluble (colloids).
•They have more compact and rounded shapes.
enzymes
antibodies
7.5.2:Fibrous Proteins
Fibrous proteins are water insoluble, long and narrow proteins.
collagen
Myosin and actin
IB Question: Distinguish between fibrous and globular proteins with reference to one example
of each protein type. [6]
fibrous proteins are strands/sheets whereas globular proteins are rounded;
fibrous proteins (usually) insoluble whereas globular proteins (usually) soluble;
globular more sensitive to changes in pH/temperature/salt than fibrous;
fibrous proteins have structural roles/globular proteins used for
named fibrous proteins e.g. keratin/fibrin/collagen/actin/myosin/silk protein;
named globular protein e.g. insulin/immunoglobulin/hemoglobin/named enzyme; [6 max]
Do not accept statements about fibrous proteins having only secondary structure and globular
proteins having only tertiary structure.
7.5.3 : Explain the significance of polar and non-polar amino acids.
Polar amino acids
HYDROPHILIC (+ve or –ve charge)
Non-polar amino acids
HYDROPHOBIC (R-groups stay close together in water)
7.5.3
The polarity of R groups plays a role in the tertiary structure of globular proteins. Thus,
polarity plays a role in shaping enzymes and their active sites.
7.5.3
Membrane proteins are firmly anchored in the phospholipid bilayer because they
have two polar ends and a non-polar center. One end of a membrane protein
contacts the watery extracellular fluid and the other end extends to the watery
cytoplasm. The non-polar center remains inside the membrane because it is
hydrophobic.
7.5.3
Protein channels facilitate the passage of polar molecules across cellular membranes because
the polar amino acids line the inside of the channel and non-polar amino acids line the
outside.
IB Question: Explain the significance of polar and non-polar amino acids
Actual question: Outline how polar amino acids and non-polar amino acids control the
position of proteins in plasma membranes. [8]
Actual IB mark scheme from a past paper
membrane is a lipid bi-layer;
membrane has hydrophobic interior / lipid hydrophobic tails oriented inward;
hydrophilic on cytoplasmic and extracellular side / lipid hydrophilic heads oriented
outward;
polar amino acids are hydrophilic/water soluble/attracted to outside of membrane;
non-polar amino acids are hydrophobic/attracted to inside of membrane;
integral proteins embedded in the membrane;
non-polar amino acids cause proteins to be embedded in membrane;
peripheral proteins associated with surface of membrane;
polar amino acids cause parts of proteins to protrude from membrane;
transmembrane proteins have both polar and non-polar amino acids;
polar amino acids create channels through which (hydrophilic) substances/ions
can diffuse;
7.5.4: State four functions of proteins, giving a named example of each
.
Transport
e.g. haemoglobin
Enzymes e.g.
lactase
Structure
e.g. collagen
Antibodies e.g. flu antibodies
Movement e.g. myosin and
actin