Biomolecules — proteins Flashcards
How are polypeptides & proteins synthesised?
- gene from DNA is transcripted into mRNA
- mRNA moves out of nucleus, through the nuclear envelope, via the nuclear pore
- In cytoplasm, rRNA & ribosomal subunits assemble to form ribosomes
- mRNA is translated into polypeptide chain by ribosomes
- there are Receptor sites on RER which are complementary to the shape of the synthesised polypeptide
- polypeptide folds to form protein
Types of Protein
- Primary
- Secondary
- tertiary
- quaternary (NO R AFTER “qua”)
Functional groups in an amino acid
- Amino group (—NH2)
- Carboxyl group (—COOH)
- Side chain (represented by R)
Glycine structure properties
Glycine
Structure:
H3N+ CH2COO-
R-Group: H atom
It is Non-polar and hydrophobic
Properties of amino acids
Physical
- Insoluble in organic solvents. Can dissolve in water to form electrically neutral zwitterions
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Chemical properties: - Amphoteric can act as buffers
Struc:
- Has both acidic (—COOH) and basic (—NH2) groups
- so can resist slight pH changes
Formation & Breaking of Peptide Bond
- Peptide bond formed between amino & carboxyl groups
- in a condensation rxctn
- w/ removal of 1 H2O
What is primary Struc of a protein?
Primary struc is the specific number & sequence of amino acids joined by peptide bonds in a polypeptide chain
- Beginning of pp chain: N terminus / free amino group
- End of pp chain: C terminus / free carboxyl group
__ - mRNA to code pp chain was transcribed from a specific DNA sequence
- pp chain is coded from a specific mRNA sequence
- pp chains synthesised in specific order from N to C terminus during translation
What is secondary struc of a protein?
- Secondary struc is the repeated coiling and folding of a polypeptide chain.
- It is maintained by H bonds between the NH and CO group in a peptide bond in 2 amino acids.
- H bonds at regular intervals
- (In a-helix, H bond every 4 amino acids)
2 main forms of secondary structure:
- a-helix
- b-pleated sheets
a-helix structure
- Unbranched polypeptide tightly coiled into spiral
- Each turn consist of 3.6 amino acids
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STRUC: - Held by intra-chain H bonds between NH and CO group in peptide bonds of 2 amino acids that are 4 amino acids away
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FUNC - The many H bonds make a-helix stong, inelastic and flexible
Eg: a-keratin in hair & nails
__ - R groups of a.a project outwards of a-helix to prevent steric interference between backbone of a-helix and residues
B-pleated sheets structure
- made of extended adjacent regions of a single polypeptide chain arranged in a parallel or anti-parallel manner
Parallel:
- Neighbouring polypeptide Segments run in same N to C direction
Anti-parallel
- Neighbouring polypeptide Segments run in opposite N to C direction
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STRUCT
- Held tgt by intra-chain and interchain H bonds between amino and carboxyl group in 2 peptide bonds of adjacent regions
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FUNCTION
- Many H bonds make B-pleated sheets stable & rigid
- B-pleated sheet has high tensile strength
- so it can’t be stretched
What is tertiary Struc of a protein?
- it is the further coiling & folding of secondary structure to form a specific 3D conformation
- Tertiary structure held by H bonds, ionic bonds, disulfide bonds &/or H.I between R groups of ONE polypeptide chain
H bonds in tertiary protein
- Relatively weak bonds
- H bonds formed between an electro -ve atom (like N or O) and an electro +ve H atom attached to another electro -ve atom
- .
- Can be break with heat or changes in pH
Ionic bonds in tertiary protein
- Relatively weak bonds
- Electrostatic (FO) attraction between +ve charged & -ve charged R-groups of diff aa
- pH changes & molecular vibrations due to high temp break H bonds R groups
For change in pH,
- Added H+ and OH- ions combined with
Disulfide bonds in tertiary proteins
- Very strong bonds
- formed between sulfhydryl groups (—SH) in R-groups of the aa cysteine
- Can only be broken by reducing agents
H.I in tertiary proteins
__- Relatively weak bonds
- in aq medium, polypeptide will fold such that non-polar, hydrophobic molecules are facing each other & away from aq medium
- H. I between non-polar R-groups
Quaternary struc protein
- Involves >1 polypeptide chains joined via H bonds, ionic bonds, HI and/or disulfide bonds between R-groups
Haemoglobin struc to function
Struc:
- Globular protein, small and compact
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Function:
- Pack as many haemoglobin molecules into 1 RBC to bind to as much oxygen as possible
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Struc:
- Hydrophilic aa on exterior of protein
- Hydrophobic aa on interior of protein
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FUNC:
- Allows aa to form H bonds with water so haemoglobin is soluble in water
- good transport protein in blood
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Struc:
- haemoglobin has haem groups with Fe2+ in the centre
Functions
- Iron ion can bind reversible to oxygen
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Struc:
- The hydrophobic pockets of all 4 subunits have a haem group w/ Fe2+
- 1 mole of haemoglobin has 4 haem groups in total to bind to 4 mol of oxygen
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FUNCTION
- increase haemoglobin‘s oxygen carrying capacity
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Struc:
- Quaternary struc is held by weak ionic bonds, hydrogen bonds & H.I w/o disulfide bonds
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Function:
- Allows for cooperative binding of oxygen
- where binding of 1 molecule of O2 causes a slight change in confirmation of haemoglobin
- to allow other O2 molecules to bind quicker
Collagen struc to function
Struc
- Hydrophobic aa are on the exterior
- Hydrophilic aa are in the interior
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Function:
- Collagen is insoluble in water
- & metabolically inactive
- good structural protein
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Struc:
- Glycine makes up 1/3 of a tropocollagen chain
- Glycine has the smallest R-group so it fits in the centre of a tropocollagen chain
- this allows for cooling of the 3 pp chains
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Function
- makes Tropocollagen rigid & has high tensile strength
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Struc:
- Extensive H bonds between OH groups of pp chains within the tropocollagen
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Fucntion
- Gives tropocollagen high tensile strength
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Struc
- tropocollagen molecules have a staggered arrangement
- With Covalent crosslinks between carboxyl & amino ends tropocollagen molecules
- to form collagen fibrils
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Functions
- High tensile strength & rigidity
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Struc
- Many collagen fibrils assemble to form collagen fibres
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Function
- Many fibrils make collagen fibres stronger
