Protein Structure and Function Review: Fibrous vs. Globular Proteins

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

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• 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.

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• 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

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Amino acids – the monomersThere 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

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Fig. 5.17

• The primary structure of a protein is its unique sequence of amino acids.

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Fig. 5.18

Fig. 5.19

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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)

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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.

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Fig. 5.21

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• 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.

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Fig. 5.23

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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.

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Fig. 5.25

Alterations in pH, salt concentration, temperature, or other factors can unravel or denature a protein.