Protein Structure and Function Review: Fibrous vs. Globular Proteins

– These functions include structural support, storage, transport of other substances, intercellular signaling, movement, and defense against foreign substances. – Enzymes are proteins in a cell that regulate metabolism by selectively accelerating chemical reactions. Proteins are instrumental Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Proteins are the most structurally complex molecules known. – Each type of protein has a complex and unique shape or conformation. All protein polymers are constructed from the same set of 20 monomers, called amino acids. Polymers of proteins are called polypeptides. A protein consists of one or more polypeptides folded and coiled into a specific conformation. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Amino acids consist of four components around a central carbon atom: – a hydrogen atom – a carboxyl group – an amino group – a variable R group (or side chain). Differences in R groups produce the 20 different amino acids. A polypeptide is a polymer of amino acids connected in a specific sequence Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Amino acids – the monomers There are 20 different amino acids found in proteins. Each has a different “R” group (side chain).

The twenty amino acids found in living organisms, organized according to characteristics of R-groups. Such properties contribute to proteins folding into various shapes

Amino acids (monomers) are linked to form polypeptide chains (polymers) by dehydration synthesis (condensation reax) Additional amino acids can be added by condensation reaction. The covalent bond that forms between amino acids is called a peptide bond. Polypeptide chains have an N terminus and a C terminus, where peptide bond forms between amino acids.

A functional proteins consists of one or more polypeptides that have been precisely twisted, folded, and coiled into a unique shape. It is the order of amino acids that determines what the three-dimensional conformation will be. A protein’s function depends on its specific conformation Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 5.17

The primary structure of a protein is its unique sequence of amino acids. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 5.18

Fig Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Even a slight change in primary structure can affect a protein’s conformation and ability to function.

The secondary structure of a protein results from hydrogen bonds at regular intervals along the polypeptide backbone. – Typical shapes – coils (an alpha helix) – folds (beta pleated sheets) Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 5.20

The structural properties of silk are due to beta pleated sheets. – The presence of so many hydrogen bonds makes each silk fiber stronger than steel. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 5.21

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Tertiary structure is determined by bonds among R groups and between R groups and the polypeptide backbone. – hydrogen bonds – ionic bonds between charged R groups – van der Waals interactions among hydrophobic R groups – disulfide bridges, strong covalent bonds Fig. 5.22

Quarternary structure results from the aggregation of two or more polypeptide subunits. – Collagen is a fibrous protein of three polypeptides that are supercoiled like a rope. This provides the structural strength for their role in connective tissue. – Hemoglobin is a globular protein with two copies of two kinds of polypeptides. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 5.23

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 5.24

Fibrous Proteins Include those which function as structural proteins and which play a role in motility and contraction typically water-insoluble built up from single repeating elements of secondary structure rope-like proteins that provide strength and framework to tissues

Fibrous Protein Example 1: Collagen most abundant protein in vertebrates (~ 20 % of all proteins in human body) found in cartilage, tendons, bones, teeth, skin, and blood vessels extremely strong

Fibrous Protein Example 2: α-Keratin soft or hard fibrous protein highly insoluble in water composed of multiple α-helices twisted into thicker filaments

Globular Proteins Include most transport proteins, enzymes, and hormones typically water-soluble, roughly spherical and tightly folded hydrophilic nature – polar residues = on the surface – hydrophobic residues = on the interior many diverse structures are possible

Globular Protein Example 1: Hemoglobin each subunit of hemoglobin is a globular protein with an embedded heme group. In adult humans, the most common hemoglobin protein is a tetramer consisting of four polypeptide chains The heme group consists of an iron atom held in a ring, This iron atom is the site of oxygen binding.

Globular Protein Example 2: Enzyme Pepsin  a protease (protein- digesting enzyme), which is active in the stomach  consists of 327 amino acid residues  has a deep cleft, the bottom of which contains a pair of aspartate residues on either side of the cleft which break peptide bonds in proteins by the addition of water: -H to one side and -OH to the other.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig Alterations in pH, salt concentration, temperature, or other factors can unravel or denature a protein.