[STPM] [BIOLOGY] [NOTES] CHAPTER 1 : BASIC CHEMISTRY OF A CELL (PART 4)
Monday, May 12, 2014 | 6:05 AM | 0 comments
Hey,let's continue our next subtopic(proteins)
Nearly every dynamic function of a living being depends on proteins.
Some protein can speed up the chemical reactions,while others play a role structural support,storage transport,cellular communication,movement and defense against foreign substances.Proteins may be composed of one or more polypeptide chains.Each polypeptide chain is a polymer consisting of many basic units of amino acids linked together,peptide bonds,through condensation reactions.
We discuss the amino acids now :)
All amino acids share a common structure.The only different is the variable R groups which determine the types of amino acids.
Amino acids are organic molecules possessing both corboxyl and amino groups.When amino acids dissolves in water,amino acids dissociate to form zwitterious,carrying a positive charge on the amino part and negative charge on the carboxyl group.Thus,amino acids show bipolar property. **still remember bipolar?**
***tips:appear in subtopic 1.1(water)***
Amino acids can be function as a pH buffer to maintain the pH level in the blood.
Can u still remember what u learn biology in form 4(chapter 4)? If forgot, never mind, here got discussed it again :)
Amino acids can be grouped into 2 groups:1.)essential amino acids
2.)non-essential amino acids
The essential amino acids are the amino acids which cannot synthesized by our body and is needed to obtain from our daily diet.
The non-essential amino acids are the amino acids which can synthesized by our body.The task is completed by liver by means of transamination.
What is polymerization of amino acids?
Polymerization of amino acids is when two amino acids are positioned so that the carboxyl group of one is adjecent to the amino group of the other,they can become joined by a dehydration reaction,with the removal of a water molecule.The resulting covalent bond is called peptide bond.Repeated over and over,this process yields a polypeptide,a polymer of many amino acids linked by peptide bonds.The polypeptide may coil and fold into a particular structure as a result of a hydrogen bonds,disulphide bonds,Van de Waal's forces or hydrophobic interaction and ionic bonds.These bonds determine the behaviour proteins and thus give a rise to form separate levels of structure.
Protein structure can be categorized into 4 types of structure:1.)primary structure.
2.)secondary structure
3.)tertiary structure
4.)Quaternary structure
Primary stucture of protein
Primary structure of protein is its unique sequence of amino acids in a linear polypeptide chain.
The sequence of amino acids in a linear polypeptide chain is determined genetically by DNA.
Secondary structure of protein
Secondary structure of a protein is the coiling and folding of a polypeptide chain that contribute to the protein’s overall shape.
One such secondary structure is alpha-helix, a delicate coil held together by hydrogen bonding between every fourth amino acids.
The other main type of secondary structure is the beta-pleated sheet, a folded sheet held together by hydrogen bonding between two or more regions of a pair of parallel polypeptides chain lying side by side.
Example: keratin, fibroin and silk.
Tertiary structure of protein
Superimposed on the patterns of secondary structure is a protein’s tertiarystructure.
Tertiary structure of protein is three dimensional, compact and globular.
The structure is maintained by the interaction of hydrogen bonds, disulphide bonds, ionic bonds and hydrophobic interaction.
Example: hormones, enzymes, antibodies and plasma proteins.
Quaternary structure of protein
Quaternary structure is the overall protein structure that results from the aggregation of several tertiary structures of proteins.
Example: haemoglobin, collagen
Classification of proteins
Structurally, proteins can be classified into three groups:
1.Fibrous proteins 2.Globular proteins 3.Intermediate proteins
Let's dicuss fibrous proteins xP
Firbrous proteins
Examples: collagen, keratin, fibrin and myosin.
Have secondary structure.
Insoluble in water.
Helical structures or pleated sheets held together by hydrogen bond.
A stable protein structure.
Globular proteins
Examples: globulin, enzymes, antibody and hormone.
Have tertiary structures.
Tightly coiled and folded to form sphere.
Structure maintained by various bonds.
Easily soluble in water.
Intermediate proteins
Example: fibrinogen.
Basically a fibrous protein but soluble in water.
If classified according to their composition, proteins are placed into two groups.
Simple proteins are considered pure proteins which do not contain any other substances except amino acids.
Conjugated protein is considered complex compound which contains protein partand a non-protein component known as prosthetic group.
Properties of proteins
Colloid formation
Globular proteins are soluble but being macromolecules, they do not dissolve completely in water but form a colloidal solution.
The electrical charges found on the bonding of the proteins prevent them from settling down and thus, they remain suspended in water.
Denaturation of proteins
Denaturation of proteins involves the loss of their specific three-dimensionalshape and is usually irreversible.
This is caused by the breaking of the bonds which hold the shape of the proteins.
Heat is the main factor that contributes to the denaturation of proteins.
Other than that, there are several factors which may denature proteins:
1.Strong acids and alkalis
2.Organic solvents and detergents
3.Heavy metals
4.Radiation
By : Gemini
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