Amino acids, Proteins Lipids or Fats and Steroids ( Biomolecules )

Amino acid

Amino acids are organic compounds that combine to form proteins. They are the building block of proteins. They are called monomers of proteins. Some characteristics of proteins are as follows;

• There is a presence of at least one acidic carboxylic group (- COOH- ) and one basic amino group ( -NH₂-).
• Colourless, crystalline solids.
• Water soluble and insoluble in organic solvents.
• There are 20 amino acids in nature.
• Eg; alanine, glycine, etc. The simplest one is glycine.

Formation of Peptide Bond;

When two amino acids are joined together by the union ofα- carboxyl group (-COOH ) of an amino acid with theα- amino group ( -NH₂ ) of other amino acids, a peptide bond is formed and a molecule of water is eliminated. This process is known as peptide linkage. The polypeptide chain is formed when amino acids are joined together in a long chain.

Types of amino acids

20 types of amino acids are divided into two categories.

1. Essential amino acids: Essential amino acids cannot be made by the body. As a result, they must come from food. The essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
2. Non- essential amino acids: They are produced in our body even if we do not get it from the food we eat. The 12 non-essential amino acids are alanine, serine, glycine, glutamine, tyrosine, asparatic acid, cysteine, arginine, histidine, proline, glutamic acid and asparagine.

Functions

• Acts as the building block of protein.
• Tyrosine gives rise to thyroxine and adrenaline hormones and melanin pigments.
• Repair body tissues.
• Perform many other body functions.

Proteins

Proteins are polymers of amino acids covalently linked by peptide bonds into a chain. These most the most complex chemical compounds formed of C, H, O, N, S, P. Its characteristic element is nitrogen.

Categories of proteins.

Depending upon their chemical nature, they are divided into three categories,

1. Simple proteins: Formed of peptide chains and gives amino acids on hydrolysis. Eg; globulin, histones, etc.
   
   
2. Conjugated proteins:Proteins formed in combination with non-protein substances. These are also called prosthetic groups.
   
   Glycoproteins -Protein + Glucose. Eg; mucin of saliva.
   Phosphoproteins - Amino acids + Phosphate. Eg; casein of milk.
   Lipoproteins - Amino acid + Lipid. Eg; Proteins of brains.
   Nucleoprotein - Amino acid + Nucleic acid. Eg; chromosomes of a cell.
   Chromoprotein - Amino acid + coloured pigment. Eg; haemoglobin and retina of the eye.
   
   
3. Derived proteins; Formed by partial hydrolysis of simple and conjugated proteins.
   Eg; Peptones, Proteoses.

On the basis of structure of molecules, proteins are classified as

1. Globular protein; consists of more than one polypeptide chains of alpha helix configuration folded up some definite manner which is held together by hydrogen bonds and cohesive forces. They are water soluble, and also in a salt solution and acids or bases. Eg; albumin and globulin.
2. Fibrous proteins; twisted around each other producing some fibre - like structure. These are insoluble in water or any other reagents. Eg; keratin, elastin, collagen, etc.

Structures of protein.

Structural features of proteins are usually described at four levels of complexity.

• Primary structure; The linear arrangement of amino acids in a protein and the location of covalent linkages such as disulphide bonds between amino acids. Eg; insulin.
• Secondary structure; Here polypeptide chain bends and folds due to molecular force and gives special shapes to the protein. Eg; include alpha helices and pleated sheets, which are stabilised by hydrogen bonding.
• Tertiary structure; Here, long peptide chain is coiled and variously folded by itself forming the tertiary structure having four kinds of bonds.
  Hydrogen bonds
  Ionic bonds
  Hydrophobic bond
  Disulphide bond.
• Quaternary structure; Non - covalent interactions that bind multiple polypeptides into a single, larger protein. Formed due to polymerisation of several tertiary proteins. Eg: phosphorylase.

Functions

1. It is also known as building blocks as it plays the vital role in the maintenance of body tissue, including development and repair.
2. Protein provides energy fuel. The caloric value of 1 gm of protein is 5.65 kcal while the physiological fuel value of 1 gm of proteins is 4.0 kcal.
3. Proteins act as enzymes or biocatalyst which regulate life processes.
4. Protein is involved in the creation of some hormones like insulin.
5. Proteins are antibodies or immune- globins. They neutralise the foreign bodies and develop immunity.

Lipids or Fats

Lipids are a broad group of naturally-occurring molecules which includes fats and fat-like substances. It is the second group of organic compounds that serve as food for the body. Its characteristics are,

• Water soluble.
• Soluble innon- polar organic solvents like ether, acetone, etc.
• Contains C, H, O, sometimes N or K.
• It yields fatty acids on hydrolysis.
• Combines with fatty acids to form esters.

Fatty acids

Fatty acids are straight chain organic acid. Usually, contains the even number of carbon atoms. They can be saturated ( one bond ) or unsaturated ( one or more double bond )
General Formula = R - COOH, where
R= CH3, CH2

There are two types of fatty acids;

• Unsaturated fatty acids:They have one or more double bonds between the carbon atoms. They have a very low melting point. Eg; oleic acid, linoleic acid, etc.
• Saturated fatty acids;They have no any double bond between the atoms. They have high melting point. Eg; palmitic acid, stearic acid.

Classification of Lipids

Lipids are classified into three categories,

1. Simple lipids.
   They are esters of fatty acids. They are divided into three types.
   
   • Neutral lipids:They are esters of fatty acids and glycerols hence called as glycerides. Depending upon the number of fatty acids attached to glycerol they are mono, di, or triglycerides.
     

     

   • Oils;They are rich in unsaturated fatty acids. They are liquid in state.
   • Waxes;There are esters of long chain fatty acids and alcohol. They are chemically inert without double bond. They are protective in function. The important types of waxes are plant waxes, bee's wax, lanolin ( wool fat ).
     
     
2. Complex lipids.
   They are formed in combination of either carbohydrates or phosphates or protein.
   
   • Glycolipids( Glycolipids = lipids + carbohydrate ); They are found in membranes and are the important constituent of the white matter of nerve tissues.
   • Phospholipids( phospholipids = lipids + phosphate ); Most important lipids. Their molecules are polar having hydrophilic and hydrophobic ends. Form plasma membrane.
     Eg: lecithin, cephalins, etc.
   • Lipoprotein( lipoprotein = lipids + protein molecules); Found in blood plasma.Lipid-associated with protein molecules.
     
     
3. Derived Lipids.
   Derived lipids are the substances derived from simple and compound lipids by hydrolysis. Eg; steroids, terpenes, and carotenoids.

Steroids:

Do not fatty acids, they are nonsaponifiable, and are not hydrolysed on heating. Eg, cholesterol, diosgenin.

Importance of steroids

• Cholesterol is a precursor molecule of many sex hormones.
• Cholesterol on irradiation by UV rays forms vitamin D.
• Diosgenin is used in the manufacture of anti- fertility pills.

Functions

• Lipids provide energy fuel. The caloric value of 1 gm of fats is 9.45 kcal while the physical fuel of 1 gm of fats is 9.0 kcal.
• Lipid acts as a heat insulator.
• Lipids act as a solvent for fat- soluble vitamins like A, D, E, and K.
• Absorb mechanical impact around organs like the eyeball.
• Phospholipids form a constituent of a membrane of various organs.

Reference;

https://en.wikipedia.org/wiki/Lipid
A text book of biology 8th edition.