Proteins Flashcards
How is a polypeptide formed?
Two amino acids are joined together by a peptide bond to form a dipeptide. The peptide is formed between the amino group of one amino acid and the carboxyl group of the other amino acid by a condensation reaction which catalyzed by enzymes. Dipeptides have one free amino group and one free carboxyl group at the other end which allows for further combination between other dipeptides. Repeated over and over again will form a polypeptide.
What is a functional protein?
It is one or more polypeptide chains that are twisted , folded and coiled into a molecule of three dimensional conformation.
What is the primary structure?
It refers to the specific sequence and number of amino acids in a polypeptide chains.
Maintained by hydrogen bonds and determine the type and location of the bonds in the secondary, tertiary and quaternary structure and hence the folding and function of the polypeptide.
What is the secondary structure?
Alpha-helix and beta pleated sheets.
Maintained by the hydrogen bonds between the NH2 and CO group of the polypeptide backbone.
What is the tertiary structure?
Further folding into the three dimensional structure.
Maintained by all 4 diff types of bonds.
What is the quaternary structure?
The folding of more than one polypeptide
Maintained by all 4 diff types of bonds .
What is the feature of the a helix structure?
There are 3.6 amino acid residues in one complete turn of the a helix structure.
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What is the acronym to rmb comparison between fibrous and globular protein?
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Organisation
Sequence a.a
Specificity of a.a
Length
Molecular structure
Stability
Solubility
Function
Structure of collagen.
Its amino acid sequence is Glycine-Proline-X , with X being any amino acid ( primary )
Its basic structural unit is tropocollagen made of three polypeptide chains.
In each polypeptide chain , it is a left-handed helical structure held by hydrogen bonds between the C=O group and the N-H group of the polypeptide backbone.
The three polypeptide chains are twisted around each other to form a right handed triple helix.
The polypeptide in each tropocollagen is cross-linked to another polypeptide chain via hydrogen bonds
Each tropocollagen is further cross-linked with other tropocollagens via covalent bonds.
Functions of collagen.
The polypeptide chains in tropocollagen are mainly made up of proline and glycine residues which are non-polar and hydrophobic in nature. Most peptide groups are used for inter-chain hydrogen bonds hence there are no groups left to form hydrogen bonds with water. => Collagen is insoluble in water thus chemically inert and stable, making it a stable structural component.
Glycines small size allows three helical polypeptide chains to participate in hydrogen bonding and form a tight coil. Each tropocollagen lie in parallel and are organized in collagen fibres via covalent cross-linking. => this gives collagen its great tensile strength and does not break under weight or pressure.
What are the structural features and functions of Haemoglobin.
Each subunit of haemoglobin folds and coils into a compact globular shape. => This allows more packing of haemoglobin ito RBC for more efficient transport of RBC.
Each of the 4 subunits of haemoglobin carries a prosthetic iron haem group which can reversibly bind to one oxygen molecule => this allows the uptake and release of oxygen , one RBC has the capacity to 4 haem groups increases efficiency of oxygen transport.
The binding of one haem group will cause conformational change to the other haem groups , making it easier for oxygen to bind. => Cooperative binding facilitates easier binding and release of oxygen.
The secondary structure of the polypeptide is folded such that the bulk of the hydrophobic amino acid residues are buried in the interior of the globular structure while the hydrophilic amino acid residues are on the outside. => Haemoglobin is soluble in aqueous environment of red blood cells to bind to oxygen that has been dissolved in the blood.
Why does the egg turn white when u fry?
Upon denaturation , unfolded protein will aggregate into large and random structures. Hydrophobic amino acids are now exposed to the outside , it will associate with hydrophobic amino acids of other protein molecules. Hydrophilic amino acids will attract water. The polypeptides then randomly assemble in large and insoluble structures.
Effect of pH on the structure of protein?
Change in pH alters the concentration of H+ in the medium, which alter the charges of
acid and basic R groups of amino acids.
* A change in pH may cause hydrogen bonds and ionic bonds between the R-groups to break.
* This causes polypeptide chain to unfold and disrupt the specific 3D conformation and tertiary structure of the protein, resulting in denaturation.
