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1.7Chemistry of Life

Proteins — AP BIOLOGY Revision Notes & Topical Questions

5 min read 9 parts Lesson 7 of 7 2 quizzes

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Lesson summary

1.7 Proteins — Notes — AP Biology notes: Describe the structure and function of proteins.Structure — proteins are linear chains of amino acids joined by covalent peptide bondsRoles in cellsEnzymes.Cell membrane proteins such as carriers.Hormones.Immunoproteins such as immunoglobu…

  • Importance
  • R group categories
  • Dipeptide and polypeptide
  • Hydrolysis
  • Biuret test
  • Primary structure

Section 1 of 9Functions of proteins

Functions of proteins
Structure
  • Structure proteins are linear chains of amino acids joined by covalent peptide bonds
Roles in cells
  • Enzymes.
  • Cell membrane proteins such as carriers.
  • Hormones.
  • Immunoproteins such as immunoglobulins.
  • Transport proteins such as hemoglobin.
  • Structural proteins such as keratin and collagen.
  • Contractile proteins such as myosin.
Importance
  • Importance all expressed genes code for specific proteins, so all reactions necessary for life depend on protein function

Section 2 of 9Amino acid structure

Amino acid structure
Structure
  • Structure a central carbon is bonded to an amino group (NH₂), a carboxyl group (COOH), a hydrogen atom and a variable R group
  • There are 20 amino acids common to all living organisms.
  • The R group is what differs between amino acids.
R group categories
  • R group categories hydrophobic (nonpolar), hydrophilic (polar) or ionic
    • (acidic or basic)
  • The interactions of R groups determine the structure and function of that region of the protein.
  • In water, hydrophobic R groups usually cluster in the interior and hydrophilic R groups face the surface.

Section 3 of 9Peptide bonds

1
The hydroxyl (–OH) of the carboxyl group of one amino acid is lost.
2
A hydrogen atom is lost from the amino group of the next amino acid.
3
The remaining carbon (with its double-bonded oxygen) bonds to the nitrogen of the second amino acid.
4
A peptide bond forms by dehydration synthesis and a water molecule is released.
Peptide bonds
Dipeptide and polypeptide
  • Dipeptide and polypeptide two amino acids make a dipeptide, and three or more make a polypeptide
Hydrolysis
  • Hydrolysis adding water breaks peptide bonds and releases amino acids
R groups
  • R groups they are not involved in forming the peptide bond
Biuret test
  • Biuret test biuret reagent turns from blue to lilac, and it needs two or more peptide bonds, so free amino acids give a negative result
    • (alkali plus CuSO₄)

Section 4 of 9Primary and secondary structure

1
Primary structure
  • Primary structure the sequence of amino acids, held by peptide bonds and determined by the gene (DNA)
2
Effect of sequence
  • Effect of sequence the sequence, type and number of amino acids determine the shape and function of a protein
  • A single change in the sequence can alter function, as in sickle-cell disease where valine replaces glutamic acid.
3
Secondary structure
  • Secondary structure local folding of the chain into an α-helix or β-pleated sheet
  • It is held by hydrogen bonds between the –NH of one amino acid and the –C=O of another in the backbone.
  • In an α-helix the bonds form between every fourth peptide bond.
  • In a β-pleated sheet two parts of the chain lie parallel and bond with each other.
  • Hydrogen bonds are weak, so high temperature and pH changes break them easily.

Section 5 of 9Tertiary and quaternary structure

Tertiary structure
  • Tertiary structure the overall three-dimensional shape of one polypeptide, common in globular proteins
  • Hydrogen bonds between R groups.
  • Disulfide bonds between cysteine amino acids.
  • Ionic bonds between charged R groups.
  • Weak hydrophobic interactions between nonpolar R groups.
Quaternary structure
  • Quaternary structure interactions between two or more polypeptide chains working as one functional macromolecule
  • Each chain is a subunit.
  • Hemoglobin has four subunits and carries oxygen.
Levels
  • Levels three levels relate to a single chain and the fourth to proteins with two or more chains, and all four levels determine function

Section 6 of 9Denaturation and folding

Denaturation
  • Denaturation high temperature, extreme pH or high salt concentration disrupts hydrogen and ionic bonds, so the protein unfolds and loses function
Chaperone proteins
  • Chaperone proteins they help other proteins fold correctly
Size range
  • Size range polypeptides range from 3 amino acids (glutathione) to more than 34 000 (titin)

Section 7 of 9More from the specification

1Fatty acid
  • A hydrocarbon chain with a carboxyl group.
2Glycosidic bond
  • The covalent link between two sugar units made by condensation.
3Monomer
  • A small molecule that links with others of its kind to build a polymer.
4Glucose
  • A simple sugar; the unit that starch, glycogen and cellulose are built from.
5Starch
  • An insoluble polysaccharide of glucose used by plants for energy storage.
6Lipids
  • Concentrated energy, insulation, cell membranes.
7Glycogen
  • The polysaccharide in which glucose is stored in animals.
8Nucleotide
  • The unit of DNA made of a phosphate, a deoxyribose sugar and a base.
9Carbohydrate
  • A molecule of carbon, hydrogen and oxygen, including sugars, starch, glycogen and cellulose.
10Polysaccharide
  • Many sugars joined, such as starch, glycogen, cellulose or chitin.

Section 8 of 9Key terms

TermMeaning
Amino acidMonomer with an amino group, carboxyl group and R group.
R groupVariable side chain that gives each amino acid its properties.
Peptide bondCovalent bond between amino acids.
Primary structureThe amino acid sequence.
Secondary structureα-helix and β-pleated sheet held by backbone hydrogen bonds.
Tertiary structure3-D shape from R group interactions.
Quaternary structureStructure of two or more polypeptides together.
DenaturationLoss of shape and function.

Section 9 of 9Exam technique

1
Forgetting that R groups are not involved in peptide bond formation.
2
Saying secondary structure is held by R group bonds — it is held by backbone hydrogen bonds.
3
Confusing quaternary structure with tertiary — quaternary involves two or more polypeptides.
4
Saying denaturation breaks peptide bonds — it disrupts weaker bonds and shape.

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