2.1.2g Proteins Flashcards
What are proteins
Proteins are polymers
What are the monomers in proteins
Amino acids
What is formed when 2 amino acids join together
A dipeptide (joined tg by a peptide bond)
What is formed when 2 or more amino acids join together
A polypeptide (joined tg by a peptide bond)
What are proteins composed of
Long chains of recurring monomers called amino acids
They are made up of 1 or more polypeptides
Structure of amino acids
All amino acids share same general structure, w a central CARBON atom bound to:
- An amine group (-NH2) attached to C atom
- A carboxylic acid group (-COOH)
- A hydrogen atom (H)
- A variable side chain (R) (there are 20 diff R groups, therefore there are 20 diff types of amino acids)
See slide 3/5 for diagram of amino acids
Practice drawing structure
What is the smallest amino acid
Glycine
- the R group is a hydrogen atom
(diagram on slide 5)
What are the bonds between amino acids & how are they joined together
- Amino acids are covalently joined together by peptide bonds in a condensation reaction to form dipeptides & polypeptides
- A water molecule of water is released during this reaction
What is the covalent bond between amino acids called
A peptide bond - for this reason, long chains of covalently bonded amino acids are called polypeptides
How can polypeptide chains be broken down
Via hydrolysis reactions, which adds a water molecule to break the peptide bond
See slide 7, pg26 for diagram of the formation of a dipeptide
What are peptide bonds formed between
The amine and carboxylic acid groups of adjacent amino acids
This is a C-N bond
How is a water molecule formed in the formation of dipeptides
The amine group loses a hydrogen atom (H) & the carboxylic acid loses a hydroxyl (OH) - this forms water (H2O)
What are the levels of protein structure
- PRIMARY structure
- SECONDARY structure
- TERTIARY structure
- QUATERNARY structure
(have pg 27 open for dias)
What is the PRIMARY structure of a protein
This is the specific sequence & number of amino acids in the polypeptide chain
- The primary structure of protein is determined by the gene
- Shape of molecule is linear
- Diff proteins have diff sequences of amino acids in their primary structure. A change in just 1 may change the structure of the whole protein
(see slide 11 for diagram)
What is the SECONDARY structure of a protein
- The polypeptide chain doesnt remain flat/straight. Hydrogen bonds form between nearby amino acids in the chain, making it automatically coil into an alpha helix or beta pleated sheet
- The secondary structure is the shape that the chain of amino acids forms, either: alpha helix OR beta pleated sheet
- Both alpha helices & beta pleated sheets result from hydrogen bonds forming between non-adjacent amine & carboxylic group
(see slide 12 for diagram)
What is the TERTIARY structure of a protein
- The coiled or folded chain of amino acids is often coiled and folded further. More bonds form between diff parts of the polypeptide chain.
- Tertiary structure is the 3D shape of the protein & is formed from further twisting & folding
- Not all proteins have a tertiary structure
(see slide 13/14)
What are the different bonds that maintain the TERTIARY structure of proteins
- Disulfide bridges
- Ionic bonds
- Hydrogen bonds
- Hydrophilic/Hydrophobic interactions
SECONDARY structure of proteins: alpha helix & beta pleated sheets differences
- Alpha helix - STRONG, helical shape
- Beta pleated sheet - WEAK, strength achieved through layering & bonds between layers
How does TERTIARY structure determine function
A polypeptide chain will fold differently due to the interactions (& hence the bonds that form) between R groups
Each of the 20 amino acids that make up proteins has a unique R group. Therefore, many different interactions can occur, creating a vast range of protein configurations & therefore functions
What is the QUATERNARY structure of a protein
- Some proteins are made of several diff polypeptide chains held tg by bonds. The quaternary structure is the way these polypeptide chains are assembled tg.
- The quaternary structure is the interaction between 2 or more polypeptides.
- It is the protein’s final 3D structure
- This structure only exists in proteins consisting therefore of 2 or more polypeptides
(see s16-17)
What is a good example of a protein consisting of 2 or more polypeptides & therefore has a QUATERNARY structure
Haemoglobin (Hb, shown on s16)
- Hb becomes a biologically active molecule upon the establishment of it quaternary structure
- Hb is made of 4 polypeptide chains, bonded tg & its role is to carry iron and oxygen around the body
- Hb has a globular quaternary structure
Another eg is collagen - has a fibrous quaternary structure
See slide 18 for comparison between primary, secondary, tertiary
Protein folding
Primary –> secondary –> tertiary –> quaternary
See s20-21 for summarised diagrams of primary, secondary, tertiary, quaternary
What is a conjugated protein
A globular protein that contains a non-protein component called a prosthetic group
There are different kinds of prosthetic groups, they contain an iron ion (Fe2+)
What 2 things can proteins be
Globular or Fibrous
What are simple proteins
Proteins without a prosthetic group
What are globular proteins
Round and compact
eg. enzymes
What are fibrous proteins
Composed of long & narrow strands and thus can be used to form fibres for structural roles
eg. keratin
Characteristics of globular proteins
- Roll up to form balls
- Soluble in water
- Usually have metabolic roles
- egs. haemoglobin, insulin, enzymes (amylase)
Characteristics of fibrous proteins
- Regular, repetitive sequence of amino acids
- Form fibres
- Insoluble & strong
- Fairly unreactive (unlike many globular proteins)
- Usually have structural roles
- egs. collagen, keratin, elastin
see slide 27 for comparison between diagram of fibrous and globular
Collagen as an eg of fibrous protein
- Properties: very strong but flexible molecule. (minerals can bind to the protein to increase rigidity)
- Function: a connective tissue found in bone, skin & muscle
Keratin as an eg of fibrous protein
- Properties: either flexible (as it is in skin) or hard/tough (as it is in nails)
- Structure: contains a large proportion of the sulphur containing amino acid, cysteine. This results in formation of many strong disulphide bonds. Degree of disulphide bonds determine flexibility of the protein
- Function: Found in skin, hair, nails
Elastin as an eg of fibrous protein
- Properties: it is elastic, so allows tissues to return to og shape after stretched.
- Function: found in elastic connective tissue, sa skin, walls of blood vessel, in alveoli of lungs
Why are globular proteins soluble in water but fibrous proteins are not?
Globular proteins have Hydrophobic R groups in the centre of the molecule, not in contact with the water. The Hydrophilic R groups are on the outside of the molecule, in contact w the water. Hydrophilic R groups are attracted to water
Fibrous proteins have Hydrophobic R groups on the outside of the molecule
Bonds in the TERTIARY structure: Ionic bonds
Attractions between negatively-charged R groups & positively-charged R groups on different parts of the molecule (weaker than disulfide, easily broken)
Bonds in the TERTIARY structure: Disulfide bonds
Whenever two molecules of the amino acids cysteine come close tg, the sulfur atom in one cysteine bonds to the sulfur in the other cysteine, forming a disulfide bond (strong, not easily broken)
Bonds in the TERTIARY structure: Hydrophobic & hydrophilic interactions
When hydrophobic R groups are close tg in the protein, they tend to clump tg. This means that hydrophilic R groups are more likely to be pushed to the outside, which affects how the protein folds up into its final structure
Bonds in the TERTIARY structure: Hydrogen bonds
These weak bonds from between slightly positively-charged hydrogen atoms in some R groups & slightly negatively-charged atoms in other R groups on the polypeptide chain
What bonds are involved in the PRIMARY structure of proteins
Held tg by peptide bonds between amino acids
What bonds are involved in the SECONDARY structure of proteins
Held tg by hydrogen bonds
What bonds are involved in the QUATERNARY structure of proteins
This tends to be determined by the tertiary structure of the individual polypeptide chains being bonded tg.
Bc of this, it can be influenced by all the bonds (peptide, hydrogen, ionic, disulfide, hydrophobic/lic)
Examples of globular proteins
- Haemoglobin
- Insulin
- Amylase
Insulin as an eg of globular proteins: What is it?
A hormone secreted by the pancreas
Insulin as an eg of globular proteins: Function?
- It helps regulate the blood glucose level
- Its solubility is important - means it can be transported in the blood to the tissues where it acts
Insulin as an eg of globular proteins: Structure?
An insulin molecule consists of 2 polypeptide chains, which are held tg by disulfide bonds
Amylase as an eg of globular proteins: What is it? + function
An enzyme the catalyses the breakdown of starch in the digestive system
(fyi, most enzymes are globular proteins)
Amylase as an eg of globular proteins: Structure
- Made of a single chain of amino acids
- Its secondary structure contains both alpha-helix & beta-pleated sheet sections
Haemoglobin as an eg of globular proteins: What is it?
A globular conjugated protein - meaning its a protein with a non-protein group attached
Haemoglobin as an eg of globular proteins: Function
Carries oxygen around the body in the red blood cells
Haemoglobin as an eg of globular proteins: Structure?
- The non-protein part is called a prosthetic group
- Each of the 4 polypeptide chains in haemoglobin has a prosthetic group called haem
- A haem group contains iron, which oxygen binds to
- It has hydrophobic R groups on inside, hydrophilic R groups on outside
- Quaternary protein
Structure of globular proteins
- The hydrophilic R groups on the amino acids tend to be pushed to the outside of the molecules.
- This is caused by the hydrophobic & hydrophilic interactions in the protein’s tertiary structure
- This makes globular proteins soluble, so theyre easily transported in fluids
