Tertiary Structure of Proteins The tertiary structure defines the specific overall 3-D shape of the protein Tertiary structure is based on various types of interactions between the side-chains of the peptide chain

Stabilizing Interactions of Tertiary Structures

Globular Proteins Globular proteins fold up into compact, spherical shapes Their functions include biosynthesis, transport and metabolism For example, myoglobin is a globular protein that stores oxygen in the muscles - myglobin is a single peptide chain that is mostly  -helix - the O 2 binding pocket is formed by a heme group and specific amino acid side-chains that are brought into position by the tertiary structure

Fibrous Proteins Fibrous proteins consist of long fibers and are mainly structural proteins For example,  -keratins are fibrous proteins that make hair, fur, nails and skin - hair is made of twined fibrils, which are braids of three  -helices (similar to the triple helix structure of collagen) - the  -helices are held together by disulfide bonds  -keratins are fibrous proteins found in feathers and scales that are made up mostly of  -pleated sheets

Quaternary Structure of Proteins Quaternary structure describes the joining of two or more polypeptide subunits The subunits each have their own tertiary structure and are held together by the same forces involved in tertiary structure For example, hemoglobin is a globular protein that consists of four subunits, of two different types - each subunit contains a heme group for O 2 binding

Summary of Protein Structural Levels

Hydrolysis of Peptides and Proteins Peptide bonds are amide bonds and are resistant to hydrolysis However, they can be hydrolyzed with enzymes or with strong acid or base and heat Proteins are hydrolyzed in the stomach with both acid (HCl) and enzymes (such as pepsin) - the amino acids are then absorbed in the intestines and used to synthesize new proteins Below is the acid hydrolysis of the dipeptide Ala-Ser to form the amino acids alanine and serine

Denaturation of Proteins Denaturation causes proteins to lose their 3-D structure and so they lose their function Denaturation involves the disruption of cross-linking in the secondary, tertiary and quaternary protein structures Heat and organic compounds disrupt H-bonding and hydrophobic interactions Acids and bases disrupt H-bonding between polar R groups and break ionic bonds Heavy metal ions break S-S bonds by reacting with the sulfur Agitation such as whipping stretches chains, disrupting all types of cross-linking

Applications of Denaturation Denaturation of protein occurs when: - an egg is cooked - the skin is wiped with alcohol - heat is used to cauterize blood vessels - instruments are sterilized in autoclaves