Biol 1551 Sep 16^th lecture notes

 

Chapter 3 Biological molecules (continued)

 

7. Proteins
1. Proteins perform a variety of functions.

 

·        structure

ex. collagen – tendons, ligaments, skin

      keratin – hair, nails

·        movement

   ex. actin, myosin – muscle contraction

 

·        storage

ex. ovalbumin – egg white protein, used as source of amino acids for developing embryos.

 

      Also, plant sees are rich in storage proteins –nuts and beans

 

·        defense

ex. antibodies – biological weapon in body systems

 

·        transport

ex. hemoglobin – transport oxygen to cells

 

·        signal

ex. protein hormones

         insulin – regulate blood sugar levels

 

·        enzyme – biological catalyst (Fig 5.6, p77), facilitate chemical reactions

ex. Sucrase breaks down sucrose to glucose and fructose.

 

 

2. Function vs. structure

·        Each protein forms very specific shape.  The shape of a protein determines its specific function.

ex. An enzyme must have a shape that fits its target molecules.

 

·        denaturation

- loss of specific protein shape – loss of protein function

- Factors cause proteins to denature are:

      heat (ex. cooking egg/meat results in complete changes in

texture)

                                                pH (ex. curdling of milk protein in acidic environment)

                                                salt (too much or too little)

 

 

3. Structure of protein

 

i) amino acid (monomer unit)

structure of amino acid (Fig 3.12, p 42)

Carbon is attached to four different components

                        - hydrogen

                        - amino group

                        - carboxyl group

                        - R group

 

There are 20 different kind of R group to make up 20 different amino acid (Appendix 2).

 

Some amino acids are hydrophilic.

Some amino acids are hydrophobic.

 

ii) Peptide

Peptide bond is formed between the carboxyl group of one amino acid and the amino group of another amino acid (Fig3.13, p43)

 

dipeptide – two amino acids

polypeptide (protein)

 

 

iii) Primary structure (Fig 3.15, p45)

·        Sequence of amino acids in polypeptide chain

·        Arrangement of amino acids (amino acid sequence) ultimately determines 3-D structure of protein.

 

iv) Secondary structure (Fig 3.16, p45)

·        pattern generated by regularly spaced hydrogen bonds

o       alpha helix

o       pleated sheet

 

 

v) Tertiary structure (Fig 3.17, p45)

·        3-D structure of polypeptide generated mainly by interactions between R groups of amino acids

o       ionic bonds

o       hydrogen bonds

o       polar forces (clustering of hydrophobic Rs)

o       S-S bonds (covalent bonds formed between SH group of cysteine)

 

 

vi) Quaternary structure (Fig 3.18, p45)

·        formed by interaction among 2 or more polypeptide chains

·        each chain is referred to as subunit

·        ex. Hemoglobin consists of four subunits.