structure of proteins and posttraslational modification of proteins and its significance

Assembly of spatial structure of proteins.Posttranslational modification of proteins and its

significance

Tural Abdullayev Group 30

Assembly of spatial structure of proteins

• 4 orders of protein structure:1. Primary structure

− The amino acids in a polypeptide chain

2. Secondary structure− The folding of short ( 3 – 10 residues) contiguous segments of

polypeptide into geometrically ordered units

3. Tertiary structure− Three – dimensional assembly of secondary structural units to

form larger functional units

4. Quaternary structure − Association of individual polypeptide chain subunits in a

geometrically specific manner


• Primary structure– Unique sequence of amino acid– Peptide bonds between Amino Acids– Sequence determined by DNA– Chang in primary structure can affect

a protein’s conformation and ability to function


• Secondary structure:– Has hydrogen bonds– Has 2 structures

1. α – helix− It has a rigid, stable conformation that maximizes hydrogen

bonds between each carbonyl oxygen atom and amid hydrogen (N – H) of amino acid residue located down the chain

− Trans side chains of amino acids project outward from the helix

2. β – sheets − The hydrogen bonding usually occurs between regions of

neighboring polypeptide strands aligned parallel to each other

− Carbonyl oxygen of one peptide bonded is hydrogen – bonded to the nitrogen

− Optimal hydrogen binding occurs when the sheet is bent to form β – pleated sheets

• Tertiary structure:– Types of interaction:

1. Hydrogen bonds2. Ionic bonds3. Hydrophobic interactions

– Usually inside the protein

4. Covalent bonds– Disulfide bridge formed between

the Sulfhydryl groups (SH) of cysteine amino acids



• Quarternary structure– Aggregation of two or more polypeptide

subunits– Forms 2 types of proteins:

1. Globular2. Fibrous

– Not found in all proteins– Water soluble– Compact ,spherical

The biological function of some molecules is determined by multiple polypeptide chains – multimeric proteins.

Subunits are held together by non covalent interactions.

Eg: Hemoglobin has the subunit composition a2b2

Denaturation

• A change in a shape of the protein that disturbs its function

• Alterations in the environment disrupt bonds and forces of attraction– pH– Salt concentration– Temperature

Posttranslational modification of proteins and its significance

• After the proteins emerge from the ribosome, they undergo posttranslational modification

• Modifications may occur on:1. Amino acid side chains2. Protein’s C – terminal3. Protein’s n – terminal

• Initial Methionine – is removed by specific proteases– Initial Methionine is not the N – terminal of all mature proteins

• Amino acid residues within the peptide chain can be modified enzymatically to alter the activity or stability of the proteins:– Direct it to a subcellular compartment, or prepare it for

secretion from the cell• Amino acid residues are modified enzymatically by the

addition of various types of functional groups– For example: N – terminal amino acid is acetylated, and methyl

groups can be added to lysine residues



• Proline and lysine residues can be modified by Hydroxylation– In collagen, hydroxylation lead to stabilization of the protein

• Carboxylations – important for the function of proteins involved in blood

coagulation• Formation of γ – carboxylglutamate allows these proteins to

chelate Ca2+ , a step in Clot Formation

• ADP – ribose group– Can be transferred from NAD + to certain proteins

• Addition and Removal of Phosphate groups– Serve to regulate the activity of many proteins

• Glycosylation – Addition of – CH groups– Is a common modification– Mainly occurs on proteins that are going to be secreted or

incorporated into lysosomes or cellular membranes– Also can promote the protein folding and improve stability


structure of proteins and posttraslational modification of proteins and its significance

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