polypeptides Flashcards
Define polypeptide
a linear organic polymer consisting of a large chain of amino acids, forming part of / all of a protein molecule.
How many proteins to DNA code for in the human body?
25000 - 50000
What are the functions of proteins?
- transport
- regulation
- growth
- repair
- enzyme reactions
- cell + tissue culture
What are the levels of structure pf a polypeptide ?
- Primary
- Secondary
- Tertiary
- Quarternary
Explain what primary structure is
- sequence of single chain of amino acids = linear
- single change in 1° = profound biological change in overall structure + function
Explain secondary structure
- polypeptide chains dont lie straight
- can for either an alpha helix / beta fold
Describe an alpha helix (secondary)
- spring-like
- H-bonds between C=O and -NH groups to stablise the structure
Describe beta fold (secondary)
- pleated sheets
- H bonds in parallel chains
Explain tertiary structure
- chains to produce 3D shapes
- chemical + hydrophobic reactions between R-groups keeps structure
- either forms fibrous proteins or globular proteins
Explain what a fibrous protein is, give examples
- long, coiled chains
- keratin = hair skin and nails (disulfide bonds due to cysteine - strong inflexible + insoluble)
- collagen = high quantities in connective tissues for tensile strength (tendons) also resilient sheets support skin + internal organs
- elastin = resilience + elasticity to tissues and organs (alveoli + blood vessels)
Explain what a globular protein is
- spherical shape
- water soluble (hydrophobic R groups on inside and hydrophilic on the outside)
- functions where solubility helps: immunity, muscle contraction and chemical reactions.
- e.g. enzymes, haemoglobin
What is an example of a globular protein and what is its function?
INSULIN
- hormone = blood glucose regulation
- soluble; transported in the bloodstream
- precise shape; fits into specific receptors on membranes
Explain what a conjugated protein is
- a globular protein containing non-protein compound = prosthetic group, attached by covalent bonding and weak interactions
- lipids combine with proteins = lipoproteins + glycoproteins
What is an example of a conjugated protein and what is its function?
HAEMOGLOBIN
- red oxygen-carrying pigment found in RBC
- Quaternary
- 2 alpha + 2 beta subunits - each containing haem group (non-polypeptide)
- Iron II in haem group - reversibly combining with O molecule
- main function - transport O2 around body - from lungs to cells where they release it for respiration
What types of bonding are there between R groups of the protein to hold the tertiary structure?
- ionic bonding (between oppositely charged R groups - stronger than h bonds)
- disulfide bonds / bridges (strongest bond out of all of them - between R groups containing sulfur atoms)
- hydrophobic + hydrophilic interactions (between polar and non polar r groups)
- hydrogen bonding (weakest bonds)
Explain quaternary structure
- made up of several polypeptide chains held together by ionic bonds, hydrophobic and hydrophilic interactions, hydrogen bonds and disulfide bonds
- forms biologically active molecule e.g. haemoglobin = 4 chains together
Describe the structure of an amino acid
- carbon atom (C) surrounded by:
- one amine group (H2N)
- one carboxylic group (COOH)
- one hydrogen (H)
- one R-group (determines type of amino acid)
Describe the process of synthesis of a dipeptide
CONDENSATION REACTION
- two amino acids joined together by a peptide bond
- C from carboxylic group forms a peptide bond with N from amine group (C=O + N-H is an amide group)
- OH from carboxylic group and H from amide group produces water molecule
- amino acid + amino acid –> dipeptide + water
Describe the process of synthesis of a polypeptide
- repeated condensation reactions forming peptide bonds between amino acids
- R-group interactions between the different r-groups of different amino acids
Describe the process of breaking down a dipeptide
HYDROLYSIS
- the addition of water results in a break of the peptide bond
- OH joins to form a carboxylic group while the H joins to form the amine group
- dipeptide + water –> 2 amino acids
Describe the process of breaking down a polypeptide
- repeated hydrolysis to break peptide bonds and form many individual amino acids
What elements make up proteins?
C H O N S
What is the test for proteins?
BIURET TEST
- protein solution with equal volume of biuret reagent
- positive result = blue to lilac
- N atoms in peptide bond form purple complex with copper (II) ions in biuret reagent
What is an example of an enzyme that is a conjugated protein?
CATALASE
- 4 haem groups each containing iron II which bonds with hydrogen peroxide and speeds up its breakdown.
- hydrogen peroxide = byproduct of metabolism but is damaging to cells
