Dr. Tarek El Sewedy Department of Medical Laboratory Technology Faculty of Allied Medical Sciences Faculty of Allied Medical Sciences

Structure And Function of proteins

By the end of this lecture, students will learn: 1.Protein structure, function, reactions. INTENDED LEARNING OUTCOMES

Lecture Content Levels of Protein structure: Primary, Secondary, Tertiary and Quaternary. Protein structure Classification of proteins. Hydrolysis of proteins. Denaturation of proteins.

Proteins Proteins are Macromolecules consisting of long sequences of amino acids in peptide linkage. Protein accounts for almost 20% of total body weight. The human body is made up of approximately 100 trillion cells - each one has a specific function. Each cell has thousands of different proteins, which together make the cell do its job - the proteins are tiny machines within the cell. One gram of protein or carbohydrate contains 4 calories, while one gram of fat has 9 calories.

Levels of protein structure There are 4 levels of protein structure: 1. Primary 2. Secondary 3. Tertiary 4. Quaternary

1. Primary structure of proteins Describes the order of the amino acids joined together to make the protein (exact sequence of amino acids before folding). The end of the peptide chain with the -NH 2 group is known as the N-terminal, and the end with the - COOH group is the C-terminal

2. Secondary Structure It is the simple folding of a protein to create simple structures. The secondary structure of a protein or polypeptide is due to hydrogen bonds forming between amide and carboxyl groups. There are two possible types of secondary structure: 1. Alpha helix, the hydrogen bonding causes the polypeptide to twist into a helix. 2. Beta sheet the hydrogen bonding enables the polypeptide to fold back and forth upon itself like a pleated sheet.

Tertiary structure Refers to the three-dimensional structure of the entire polypeptide chain. result from four different bonds: 1. Ionic interactions 2. Hydrogen bonds 3. van der Waals forces 4. Disulphide bond

Quaternary Structure It is the interaction between several chains of polypeptide subunits. Not all proteins have quaternary structure, since they might be functional as monomers. The quaternary structure is stabilized by the same range of interactions as the tertiary structure. Hemoglobin is an example of a heterotetramer

Proteins could be classified by 1. Shape 2.Function Globular Fibrous 1. Catalytic as enzyme 2. Structural as collagen, keratin 3. storage as ferritin (store iron). 4. Protective as immunoglobulins 5. Regulatory : hormones as insulin 6. Communication as neurotransmitters 7. Motion as actin/myosin; in muscle 8. Transporter proteins as hemoglobin. 9. Carrier as albumin. 3. structure  Simple  Conjugated Glycoprotein: Immunogloulin. Metalloprotein: Hemoglobin Nucleoprotein : RNA bound protein Phosphoprotein: casein Lipoprotein : Low density lipoprotein (LDL), HDL.

Water insoluble and found as structural materials, e.g. collagen, keratin. Fibrous proteins Globular proteins Compact, roughly spherical, water soluble and comprise all other types of protein as albumins and globulins.

Hydrolysis of proteins Hydrolysis of proteins results in breaking down the peptide bonds to give amino acids thus it disrupts the primary structure of protein. Hydrolysis can be achieved by Enzymes as Proteases. Biological role of hydrolysis: 1. Convert inactive prohormones into active hormones. ex; Proinsulin (inactive) → Insulin (active). 2. Digestion of protein by enzyme as trypsin and pepsin.

Inactive form Active form Hydrolysis

Protein denaturation It is a process in which proteins can lose their structures and function, without breaking the peptide bonds by denaturing agents such as: 1.Heat, U.V radiation. 2.Heavy metal as mercury. 3.Soaps. 4.Organic acids as acetic acid. 5.Strong acids and bases as sulfuric acid and sodium hydroxide. Note: Denaturation disrupts 2ry,3ry,4ry structure of protein not 1ry structure.

All proteins have unique shapes that define their roles and interaction with other proteins. Environmental factors and genetic mutations can affect a protein's structure, or three-dimensional shape, causing it to misfold. Misfolded proteins can no longer perform their functions leading to various diseases. Misfolded proteins often clump together, forming aggregates. The aggregates are toxic to some cells such as neurons and lead to diseases such as Alzheimer's Disease.

رحمه مسعد عبد اللطيف نورهان أشرف السيد

ASSIGNMENTS Selected students are requested to prepare slides about one of the following topics (To be delivered before next lecture): Digestion of proteins Essential amino acids. Non essential amino acids Physical properties of amino acids. Chemical properties of amino acids. Disease resulting from disturbance in amino acid metabolism. Ketone bodies and amino acids Translation of RNA

Principles of Biochemistry, Donald J. Voet, Judith G. Voet, Charlotte W. pratt; Willey, 3 rd ed. Suggested readings: