Proteins: Amino Acids and Protein Structure PDF

Amino Acids and Proteins  Objectives Describ Describe the generalised structure of an e amino acid. Describ Describe the formation and breakage of a e peptide bond. Compa Compare the levels of protein structures. re Outline the molecular structure o...

Amino Acids and Proteins  Objectives Describ Describe the generalised structure of an e amino acid. Describ Describe the formation and breakage of a e peptide bond. Compa Compare the levels of protein structures. re Outline the molecular structure of Outline haemoglobin as an example of a globular protein. Outline the molecular structure of collagen as Outline an example of a fibrous protein. Proteins - Polymers of amino acids arranged in a linear sequence (20 different amino acids) Importanc e structur enzyme transpo movem al catalysi defence rt ent support s Amino Acids - Proteins ❖ Amino acids are the building blocks of proteins. Proteins are natural polymers of successive amino acids. ❖ There are 20 different amino acids that make up human proteins 4  Amino Acids Amino acids have both a carboxyl group α -COOH an amino group -NH2 5 Amino Acids 6 bonds glycine optical bonds  Bond ⮚ When two amino acids combine, there is a formation of an amide/peptide and a loss of a water molecule 7 Peptide bond formation  The Zwitterion A zwitterion is a molecule that has both positive and negative charges 1 0 Amino Acids are Amphoteric Amino acids are amphoteric. They can behave as both an acid and a base, since they have both a proton donor group and a proton acceptor group. In neutral aqueous solutions the proton typically migrates from the carboxyl group to the amino group, leaving an ion with both a (+) and a (-) charge. 1 1 Zwitterion and pH Zwitterion and pH C4H10 C3H8O C2H6O alanine Except for glycine, all amino acids have a chiral carbon atom. The amino acids found in proteins are all laevorotatory or L forms. glycine Determining the enantiomers Essential Amino Acids ► Of the 20 amino acids that make up proteins 10 of them can be synthesized by the human body. ► The other 10 amino acids must be acquired from food sources. These amino acids are known as essential amino acids ► arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine 2 2 Levels of protein structure  Bonds that stabilise proteins bonds  Proteins- Levels of Structure ❖ Amino acids can undergo condensation reactions in any order, thus making it possible to form large numbers of proteins. ❖ Structurally, proteins can be described in four ways. 1.Primary 2.Secondary 3.Tertiary 4.Quaternary 2 6 Primary and secondary structure of proteins ► https://www.youtube.com/watch?v=a56C hCHTa3w  Primary Structure ❖ The primary structure of a protein is defined by the linear sequence of amino acids, which form the protein. ❖ This sequence is determined by the base pair sequence in the DNA used to create it. The sequence for bovine insulin is shown below 2 8  Secondary Structure ► The secondary structure describes the way that the chain of amino acids folds itself due to intramolecular hydrogen bonding ► The two common structures are : 2 9 3 0 Tertiary and quaternary structure of proteins ► https://www.youtube.com/watch?v=1Fl_r SgQ5_8  Tertiary Structure The tertiary structure maintains the three- dimensional shape of the protein. The amino acid chain (in the helical, pleated or random coil form) links itself in places to form the unique twisted or folded shape of the protein. 3 2  Tertiary Structure There are four ways in which parts of the amino acid side chains interact to stabilize its tertiary shape. They include: Covalent bonding, for example disulphide bridges formed when two cysteine molecules combine in which the –SH groups are oxidized Hydrogen bonding between polar groups on the side chain. Ionic bonds (Salt bridges ) formed between –NH2 and –COOH groups Hydrophobic interactions between non- polar side chains. 3 3  Quaternary Structure ❖ Many proteins are not single strands ❖ The quaternary structure refers to how the protein subunits interact with each other and arrange themselves to form a larger aggregate protein complex. 3 4 https://www.youtube.com/watch?v=PPJ7C3hcnPw  Protein denaturation and testing  Denaturing Proteins ► The natural or native structures of proteins may be altered, and their biological activity changed or destroyed by treatment that does not disrupt the primary structure. ► Following denaturation, some proteins will return to their native structures under proper conditions; but extreme conditions, such as strong heating, usually cause 3 irreversible change. 7 Denaturing Proteins 3  Protein Tests: ►Biuret Biuret reagent is a light blue solution containing Cu2+ ion in an alkaline solution. Biuret turns purple when mixed with a solution containing protein. ► The purple colour is formed when copper ions in the biuret reagent react with the peptide bonds of the polypeptide chains to form a complex. 3 9 Review - Objectives 1 2 3 Describe the Describe the Compare the generalised formation levels of structure of and protein an amino breakage of structures. acid. a peptide bond. Evaluation All of the following are considered “weak” interactions in proteins, except: A) hydrogen bonds. B) hydrophobic interactions. C) ionic bonds. D) peptide bonds. E) van der Waals forces. Evaluation Roughly how many amino acids are there in one turn of a helix? A) 1 B) 2.8 C) 3.6 D) 4.2 E) 10 Evaluation Which of the following is least likely to result in protein denaturation? A) Altering net charge by changing pH B) Changing the salt concentration C) Disruption of weak interactions by boiling D) Exposure to detergents E) Mixing with organic solvents such as acetone  Proteins- Levels of Structure ❖ PROTEIN STRUCTURE- Amino acid structure s. Primary, secondary, tertiary and quaterna ry proteins – YouTube ❖ Proteins and their Structure – YouTube ❖ https://www.youtube.com/watch?v=MODnIk Qvyz0 4 5