VITAMINS

Based on solubility Vitamins are classified as either fat-soluble (lipid soluble) or water-soluble. Vitamins A, D, E and K are fat-soluble

Vitamin C and B is water soluble.

B-COMPLEX VITAMINS

Eight of the water-soluble vitamins are known as the vitamin B-complex group: thiamin (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), vitamin B6 (pyridoxine), folate (folic acid), vitamin B12, biotin and pantothenic acid.

The amino acids buffer system

Amino acids contain in their molecule both an acidic (− COOH) and a basic (− NH₂) group. They can be visualized as existing in the form of a neutral zwitterion in which a hydrogen atom can pass between the carboxyl and amino groups. 

By the addition or subtraction of a hydrogen ion to or from the zwitterion, either the cation or anion form will be produced 

Thus, when OH⁻ ions are added to the solution of amino acid, they take up H⁺ from it to form water, and the anion is produced. If H⁺ ions are added, they are taken up by the zwitterion to produce the cation form. In practice, if NaOH is added, the salt H₂N - CH₂ - COONa would be formed. and the addition of HCl would result in the formation of amino acid hydrochloride.

K_eq, K_w and pH

As H₂O is the medium of biological systems one must consider the role of this molecule in the dissociation of ions from biological molecules. Water is essentially a neutral molecule but will ionize to a small degree. This can be described by a simple equilibrium equation:

H₂O <-------> H⁺ + OH^-

This equilibrium can be calculated as for any reaction:

K_eq = [H⁺][OH^-]/[H₂O]

Since the concentration of H₂O is very high (55.5M) relative to that of the [H⁺] and [OH^-], consideration of it is generally removed from the equation by multiplying both sides by 55.5 yielding a new term, K_w:

K_w = [H⁺][OH^-]

This term is referred to as the ion product. In pure water, to which no acids or bases have been added:

K_w = 1 x 10^-14 M²

As K_w is constant, if one considers the case of pure water to which no acids or bases have been added:

[H⁺] = [OH^-] = 1 x 10^-7 M

This term can be reduced to reflect the hydrogen ion concentration of any solution. This is termed the pH, where:

pH = -log[H⁺]

Glycolysis Pathway

The reactions of Glycolysis take place in the cytosol of cells.

Glucose enters the Glycolysis pathway by conversion to glucose-6-phosphate. Initially, there is energy input corresponding to cleavage of two ~P bonds of ATP. 

1. Hexokinase catalyzes:  glucose + ATP → glucose-6-phosphate + ADP

ATP binds to the enzyme as a complex with Mg⁺⁺.

The reaction catalyzed by Hexokinase is highly spontaneous 

2. Phosphoglucose Isomerase catalyzes: 

glucose-6-phosphate (aldose) → fructose-6-phosphate (ketose)

The Phosphoglucose Isomerase mechanism involves acid/base catalysis, with ring opening, isomerization via an enediolate intermediate, and then ring closure .

3. Phosphofructokinase catalyzes: 

fructose-6-phosphate + ATP  → fructose-1,6-bisphosphate + ADP

The Phosphofructokinase reaction is the rate-limiting step of Glycolysis. The enzyme is highly regulated. 

4. Aldolase catalyzes: 

fructose-1,6-bisphosphate   → dihydroxyacetone phosphate + glyceraldehyde-3-phosphate

The Aldolase reaction is an aldol cleavage, the reverse of an aldol condensation.

5. Triose Phosphate Isomerase (TIM) catalyzes

dihydroxyacetone phosphate (ketose) → glyceraldehyde-3-phosphate (aldose)

Glycolysis continues from glyceraldehydes-3-phosphate

The equilibrium constant (K_eq) for the TIM reaction favors dihydroxyacetone phosphate, but removal of glyceraldehyde-3-phosphate by a subsequent spontaneous reaction allows throughput. 

6. Glyceraldehyde-3-phosphate Dehydrogenase catalyzes:

glyceraldehyde-3-phosphate + NAD⁺ + P_i  → 1,3,bisphosphoglycerate + NADH + H⁺

This is the only step in Glycolysis in which NAD⁺ is reduced to NADH

A cysteine thiol at the active site of Glyceraldehyde-3-phosphate Dehydrogenase has a role in catalysis . 

7. Phosphoglycerate Kinase catalyzes:

1,3-bisphosphoglycerate + ADP  →  3-phosphoglycerate + ATP

This transfer of phosphate to ADP, from the carboxyl group on 1,3-bisphosphoglycerate, is reversible

8. Phosphoglycerate Mutase catalyzes:  3-phosphoglycerate → 2-phosphoglycerate

Phosphate is shifted from the hydroxyl on C3 of 3-phosphoglycerate to the hydroxyl on C2.  

9. Enolase catalyzes:  2-phosphoglycerate  → phosphoenolpyruvate + H₂O

This Mg⁺⁺-dependent dehydration reaction is inhibited by fluoride. Fluorophosphate forms a complex with Mg⁺⁺ at the active site .

10. Pyruvate Kinase catalyzes:  phosphoenolpyruvate + ADP  → pyruvate + ATP

This transfer of phosphate from PEP to ADP is spontaneous. 

Balance sheet for high energy bonds of ATP: 

• 2 ATP expended
• 4 ATP produced (2 from each of two 3C fragments from glucose) 
• Net Production of 2~ P bonds of ATP per glucose

Proteins are complex macromolecules composed of amino acids that perform diverse biological functions. Understanding their structure-function relationships is crucial for medical applications and biochemistry.

Levels of Protein Structure

Primary Structure

• Linear sequence of amino acids connected by peptide bonds
• Determines all higher levels of organization
• Coded by DNA sequence

Secondary Structure

• Local folding patterns stabilized by hydrogen bonds
• Alpha helix: Right-handed spiral structure
• Beta sheet: Extended polypeptide chains arranged side by side
• Beta turn: Connects different secondary structural elements

Tertiary Structure

• Three-dimensional folding of entire polypeptide chain
• Stabilized by:
  • Hydrogen bonds
  • Disulfide bridges
  • Van der Waals forces
  • Electrostatic interactions
  • Hydrophobic interactions

Quaternary Structure

• Assembly of multiple polypeptide subunits
• Present only in proteins with more than one polypeptide chain
• Examples: Hemoglobin (4 subunits), antibodies

Protein Classification

Based on Structure

1. Fibrous Proteins

   • Elongated, insoluble
   • Structural functions
   • Examples: Collagen, keratin, elastin

2. Globular Proteins

   • Compact, soluble
   • Functional proteins
   • Examples: Enzymes, antibodies, hormones

Based on Composition

1. Simple Proteins

   • Composed only of amino acids
   • Examples: Albumin, globulins

2. Conjugated Proteins

   • Contain non-protein prosthetic groups
   • Glycoproteins: Contain carbohydrates
   • Lipoproteins: Contain lipids
   • Nucleoproteins: Contain nucleic acids
   • Phosphoproteins: Contain phosphate groups
   • Metalloproteins: Contain metal ions

- There are two important phospholipids, Phosphatidylcholine and Phosphatidylserine found the cell membrane without which cell cannot function normally.

- Phospholipids are also important for optimal brain health as they found the cell membrane of brain cells also which help them to communicate and influence the receptors function. That is the reason food stuff which is rich in phospholipids like soy, eggs and the brain tissue of animals are good for healthy and smart brain.

- Phospholipids are the main component of cell membrane or plasma membrane. The bilayer of phospholipid molecules determine the transition of minerals, nutrients, and drugs in and out of the cell and affect various functions of them.

- As phospholipids are main component of all cell membrane, they influence a number of organs and tissues, such as the heart, blood cells and the immune system. As we grown up the amount of phospholipids decreases and reaches to decline.

- Phospholipids present in cell membrane provide cell permeability and flexibility with various substances as well its ability to move fluently. The arrangement of phospholipid molecules in lipid bilayer prevent amino acids, carbohydrates, nucleic acids, and proteins from moving across the membrane by diffusion. The lipid bi-layer is usually help to prevent adjacent molecules from sticking to each other.

- The selectivity of cell membrane form certain substances are due to the presence of hydrophobic and hydrophilic part molecules and their arrangement in bilayer. This bilayer is also maintained the normal pH of cell to keeps it functioning properly.

- Phospholipids are also useful in the treatment of memory problem associated with chronic substances as they improve the ability of organism to adapt the chronic stress.