Part 4. Chapter 19 Proteins: Structure: primary Secondary Tertiary Quaternary Loss of protein structure Tests for proteins (lab) Sections:

Transcription

1 Part 4 Chapter 19 Proteins: Structure: primary Secondary Tertiary Quaternary Loss of protein structure Tests for proteins (lab) Sections:

2 Proteins Structures: Primary Proteins are polypeptides of 50 or more amino acids that has biological activity. Each protein in our cells has a unique sequence of amino acids that determines its 3-D structural and biological function. The primary structure of a protein is the particular sequence of the amino acid held together by peptide bonds..

3 Proteins Structures: Primary Isulin s primary structure is two polypeptide chains. There are 21 amino acids in Chain A and 30 in chain B.

4 Protein Structures: Secondary The secondary structure of a protein describes the type of structure that forms when amino acids form H-bonds within a single polypeptide chain or between multiple polypeptides chains. The three most common secondary structures are: Alpha helix Beta-pleated sheets Triple helix In alpha helix, H-bonds form between the hydrogen atoms of the N-H groups in the amide bonds, and the oxygen atoms in the carbonyl groups of the amide bonds that are four amino acids away in the next turn of the alpha helix.

5 Protein Structures: Secondary In a β-pleated sheet, polypeptide chains are held together side by side by hydrogen bonds that form between oxygen atoms of the carbonyl group in one section of the polypeptide chain, and the hydrogen atom in the N-H groups of the amide bond in a nearby section of the polypeptide chain. The hydrogen bonds holding the sheets tightly in place account for the strength and durability of fibrous proteins such as silk. Amino acids in α-helix structures typically have shorter R group attached and amino acids in β-pleated sheet have longer R groups attached.

6 Protein Structures: Secondary

7 Collagen Collagen, which is the most abundant protein in the body, makes up 25-35% of all protein in vertebrates. It is found in: Connective tissue Blood vessels Skin Tendons Ligaments The cornea of the eye cartilage. Its strong structure is a result of three a-helices woven together like a braid to form a triple helix.

8 Protein Structures: Tertiary The tertiary structure of a protein involves attractions and repulsions between the R groups of the amino acids in the polypeptide chain. As interactions occur between different parts of the peptide chain, segments of the chain twist and bend until the protein acquires a specific three-dimensional shape. A Phone cord is similar to the α-helix secondary structure. The phone cord is likely to fold up on itself. The tertiary structure of a protein refers to the way the secondary structure folds back upon itself or twist around to form a 3-D structure. The secondary coil structure is still there, but the tertiary tangle has been superimposed on it.

9 Protein Structure: Tertiary Example: myoglobin The helix is folded up on itself. The blue tube is drawn in to more easily see the helix outline. Bonding between the side R groups are responsible for holding the tertiary structure together

10 Protein Structure: Quaternary When several polypeptide chains called subunits bind to form a larger complex, it is referred to as a quaternary structure. Many proteins are biologically active as tertiary structures, but some proteins require two or more tertiary structures to be biologically active. Quaternary structures are held together by bonding between the side chain, R, groups on the amino acids.

11 Protein Structure: Quaternary Example: Hemoglobin Each color is a different tertiary structure

12 Protein Structure

13 Loss of Protein Structure There are two different way that proteins can lose their structure. Both involve the loss of biological function, which means it no longer acts as it did in its natural state. 1. If the secondary, tertiary, or quaternary structure, but not primary is altered or disrupted it is referred to as denaturation. 2. If the primary structure is broken down it is known as hydrolysis. The proteins are ultimately converted into amino acids. In denaturation, the weak hydrogen, electrostatic, and disulfide bonds are broken. The bonds are weak enough that denaturation can occur from something as gentle as pouring a protein. It can also occur from heating, whipping, and ultraviolet radiation, or from using strong acids and bases, alcohol, or detergent. Examples: The egg albumin in egg whites are denatured and coaulates when they are whipped with an egg beater.

14 Tests for Proteins and Amino Acids Xanthoproteic Test: If the protein contains a benzene ring, it will react with concentrated nitric acid used in the test to give a yellow product. Proteins in the skin react with nitric acid to stain your skin yellow for a few days. Phenylalanine Tryptophan Tyrosine Biuret Test: Copper (II) sulfate are added to an alkaline (basic) solution that contains peptides or proteins. A violet color is produced when at least two peptide bonds are present in the peptide or protein. This means that the test will not be positive for amino acids or dipeptides. H H + N H C O C H N CH 3 C O C O - H H H