7.5: PROTEINS

Proteins

Function

Structure

Function• 7.5.4: State four functions of proteins,

giving a named example of each. [Obj. 1] • Proteins are the workhorses of the cell and are used

for structural support/structure, movement/ transport of substances, signaling, movement, and defense.

• The general function of proteins includes a main component of hair, nails, bones, muscles, ligaments and tendons.

1. Enzymes: Facilitate chemical reactions

2. Transport: Allows substances to move in/out of the cell or to different parts of the body. Haemoglobin transports oxygen from the lungs to other tissues in the body.

3. Chemical Messengers: Some hormones are proteins such as insulin, which helps to regulate blood glucose levels.

4. Movement: Myosin found in muscle fibers causes contraction of the muscle which results in movement.

5. Defense: Immunoglobulin acts as an antibody.

6. Structural: Collagen strengthens bones, skin and tendons.

Insulin Function

Structure• 7.5.1: Explain the four levels of protein

structure, indicating the significance of each level. [Obj. 3]

• Proteins are composed of long chains of amino acids

• Polymers of amino acids are called polypeptides.• Humans need 20 amino acids.• 12 are produce from the body• 8 are essential and must be obtained from foods

Amino Acids• Amino acids are organic molecules

possessing both carboxyl and amino groups.

• The general formula for an amino acid: The alpha carbon is in the center, bonded

• to an amino group, a carboxyl group, a hydrogen atom, and a variable R group.

• The R group is also known

as the side chain.

The R group differs for each amino acid.

A peptide bond joins amino acids together to form polypeptides.

• The physical and chemical properties of the side chain

• determine how a particular amino acid will behave.

• When two amino acids are positioned so that the• carboxyl group of one is adjacent to the amino

group• of the other, an enzyme can begin the

dehydration• reaction that will form the peptide bond (type of• covalent bond).

Four Levels of Protein Structure

• When a cell creates a polypeptide, the chain automatically folds to achieve the shape it needs to carry out its function.

• This shape is held together by a variety of different bonds between parts of the chain.

Primary Structure.• The primary structure of a protein is the

sequence of amino acids.

• Even a slight change in the order of amino acids can affect the protein’s ability to function.

Secondary Structure• 30 or more amino acids begins to coil up into a

helix or bend into a pleated sheet• They are the result of hydrogen bonds at regular

intervals along the polypeptide backbone• One main type of secondary structure is the α

helix, a coil held together by hydrogen bonding between every fourth amino acid.

• The other main type of secondary structure is the β pleated sheet, where two or more regions of polypeptide chain lie parallel to each other.

alpha - helix: the most common, most stable and best-known type being an alpha - helix. Here, the hydrogen bonds form between neighboring amino acids or such of the vicinity, so that the chain winds up to a helix. Stable, however, does not mean rigid. Due to the thermal movement all atoms within a molecule move against one another

Alpha Helix

Alpha helix

• alpha - helix: the most common, most stable and best-known type being an alpha - helix. Here, the hydrogen bonds form between neighbouring amino acids or such of the vicinity, so that the chain winds up to a helix. Stable, however, does not mean rigid. Due to the thermal movement all atoms within a molecule move against one another

beta - pleated-sheets: Hydrogen bonds may also develop between the amino acids of different polypeptide chains that are arranged in an antiparallel or parallel way. This produces beta - pleated-sheets.

Tertiary Structure• consists of irregular

contortions because of interactions between side chains.

• The helix folds into a 3D structure determined by the R groups

Quarternary Structure• polypeptides can be coiled into a triple helix

or bunched into a roughly spherical shape.

7.5.3: Explain the significance of polar and non-polar amino acids. [Obj. 3]

• Controlling the position of proteins in membranes: The non-polar amino acids cause proteins to be embedded in membranes while polar amino acids cause portions of the proteins to protrude from the membrane. 

• Creating hydrophilic channels through membranes: Polar amino acids are found inside membrane proteins and create a channel through which hydrophilic molecules can pass through. 

• Specificity of active site in enzymes: If the amino acids in the active site of an enzyme are non-polar then it makes this active site specific to a non-polar substance. On the other hand, if the active site is made up of polar amino acids then the active site is specific to a polar substance. 

Fibrous and Globular proteins• 7.5.2: Outline the difference between fibrous

and globular proteins, with reference to two examples of each protein type. [Obj. 2]

• Protein shape can be categorised as either fibrous or globular.

• Fibrous proteins tend to be elongated, physically tough and insoluble in water. Collagen found in the skin and keratin found in hair are examples of fibrous proteins.

• Globular proteins tend to be compact, rounded and water soluble. Haemoglobin and enzymes are examples of globular proteins.

What Determines Protein Conformation?

• pH, salt concentration, temperature, and other aspects of a protein’s environment can affect what happens to a protein.

• Changes in its environment can cause a protein to become denatured and biologically inactive.

• Denaturation: A protein has lost its natural shape and can no longer perform its normal function due to extreme temperature or pH.

Denatured• Denaturation agents can disrupt the bonds

that hold the protein together.

• Excessive heat can also overpower the weak interactions that stabilize the shape of the protein.

• Some proteins can return to normal after conditions are fixed.