Protein Structure and Function PDF

Peptides and proteins Sahar Abdelmoneim  Learning objectives: By the end of this lecture all students should be able to: Describe how amino acids are connected with each other to form peptides and proteins Explain the primary, secondary, tertiary, and quaternary structure of pr...

Peptides and proteins Sahar Abdelmoneim  Learning objectives: By the end of this lecture all students should be able to: Describe how amino acids are connected with each other to form peptides and proteins Explain the primary, secondary, tertiary, and quaternary structure of proteins. Identify the types of secondary structure. Describe the kind of the forces that stabilize each order of protein structure. Classify the proteins according to their shape Define the term denaturation Identify the main functions of Proteins  Structure of amino acids: An a-amino acid consists of a central carbon atom, called the α carbon, linked to an amino group, a carboxylic acid group, a hydrogen atom, and a R group. R group called a side chain. The side chain is different for each amino acid. Structure of amino acids: Amino group H Carboxyl group NH2 C COOH R Side chain  The Peptide Bond: Amino acids are linked together by peptide bonds. Peptide bond formation is a condensation reaction leading to the polymerization of amino acids into peptides and proteins. The Peptide Bond: The Peptide Bond:  Properties of peptides bonds Peptide bonds are planar , rigid ,flexible. rigidity means chemical hydrolysis require extreme conditions. Types of Peptides and proteins Dipeptide two amino acids. Tripeptide three amino acids. Polypeptide many amino acids. At some point a polypeptide becomes a protein (at 50 joined amino acids).  Proteins Are polymers of amino acids linked by peptide bond. Levels of protein structure  Primary structure Sequence of amino acids in the protein Peptide bond  Secondary structure There are two very important secondary structures of proteins, are: 1. α-helix 2. β-pleated sheet Both depend on hydrogen bonding Secondary structure Protein Secondary Structure: α Helix  Protein Secondary Structure: b-Pleated Sheet O H O H O H O H C C C C C C N C C N C C N C C N N C N C N C N C H O H O H O H O O H O H O H O H C C N C C N C C N C C N N C C N C C N C C N C C H O H O H O H O O H O H O H O H C C C C C C N C C N C C N C C N N C N C N C N C H O H O H O H O  Tertiary Structure Tertiary structure results from the folding of α helices and β pleated sheets Interactions Involved in Tertiary Structure 1. Hydrophobic/hydrophilic interactions 2. Hydrogen bonding 3. Disulfide linkages Interactions Involved in Tertiary Structure  Quaternary Structure Association of more than one polypeptides Each unit called subunit Subunits (monomers) can be identical (homopolymeric) or different (heteropolymeric) Interactions Involved are: Hydrophobic/hydrophilic interactions Hydrogen bonding  Classification of proteins (According to shape) Globular Fibrous Spherical Long , thin fibers Insulin, Hemoglobin, Keratin, Elastin, Collagen Enzymes, Antibodies, Myoglobin  Structure of Globular Proteins Globular proteins are round structures. Like their name, globular proteins have a round, spherical formation. This is because the hydrophobic parts of the protein fold inwards while the hydrophilic parts become arranged around the external surface. Globular proteins are water soluble. As only the hydrophilic components of the protein are on the outer surface, globular proteins are soluble in water.  Function of Globular Proteins Due to their round shape and soluble nature, globular proteins play a wide variety of vital metabolic roles in the human body. The following types of proteins are usually globular proteins:  1. Enzymes. All enzymes are globular proteins as their round shape can be altered appropriately to fit their target sites with high specificity. Examples include digestive enzymes such as amylase, pepsin, and lipase which break down starch, protein, and fats respectively  2. Transport proteins. Due to their soluble nature, globular proteins function well as transport proteins as they can cross cell membranes. An example is haemoglobin, which transports oxygen. 3. Messengers proteins. Their solubility also makes globular proteins suitable as messenger proteins, otherwise known as hormones. They regulate the body’s metabolic processes. An example would be insulin, which regulates blood sugar levels.  Haemoglobin Haemoglobin is made up of 4 globular subunits. Haemoglobin is a quaternary protein, made up of 4 tertiary globular subunits. Two of these subunits consist of α chains, while the other two contains β chains. Each of these globular subunit is linked to haem. Each globular unit is covalently bonded to haem. Haem is not a protein, therefore it is called a “prosthetic group”. Haem contains iron, which oxygen binds to. Haemoglobin is considered a “conjugated protein”. As haemoglobin is a protein that is associated with non- protein structures, it is called a conjugated protein.  Fibrous Proteins Structure of Fibrous Proteins Fibrous proteins are long chains. They are made up of repeated amino acid sequences that form long polypeptide chains. These chains twist together to form fibrous proteins. Fibrous proteins are water insoluble. As the hydrophobic parts of the polypeptide chains are not folded away from the external environment, fibrous proteins are not soluble in water.  Function of Fibrous Proteins Structural proteins are usually fibrous proteins. As fibrous proteins are stable and insoluble structures, they are not suitable to function as metabolic proteins. Rather, they act well as structural proteins which support and protect tissues. Examples include keratin which provide structure to hair and nails, and collagen, a type of connective tissue in the body.  Collagen Collagen is a strong protein due to the types of bonds in its structure. The proteins in collagen are joined together by hydrogen and covalent bonding, both of which are extremely strong and stable bonds. Collagen fibres provide support and tensile strength to many structures. Collagen is present in the body as fibres, which consist of many collagen fibrils folded around each other. There are many different types of collagen, and they can be found virtually everywhere in the body, including skin, muscles, tendons and bones.  Denaturation Is the breakdown of all non-covalent bonds in a protein, causing loss of the shape and function. But does not alter its primary structure  Denaturation caused by: Physical agent (temperature). Chemical agent (pH or salt concentration). Biological agent (bacteria)  Functions of Proteins Structure and support (collagen). Transport (hemoglobin) Control and regulation (hormones). Protection and Defense (antibodies) Most important function is catalysis (Enzymes) Thanks