NEET MDS Lessons
Biochemistry
Nomenclature for stereoisomers: D and L designations are based on the configuration about the single asymmetric carbon in glyceraldehydes
For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group.
Most naturally occurring sugars are D isomers.
D & L sugars are mirror images of one another. They have the same name. For example, D-glucose and L-glucose
Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc. The number of stereoisomers is 2 n, where n is the number of asymmetric centers. The six-carbon aldoses have 4 asymmetric centers, and thus 16 stereoisomers (8 D-sugars and 8 L-sugars
An aldehyde can react with an alcohol to form a hemiacetal
Similarly a ketone can react with an alcohol to form a hemiketal
Pentoses and hexoses can cyclize, as the aldehyde or keto group reacts with a hydroxyl on one of the distal carbons
E.g., glucose forms an intra-molecular hemiacetal by reaction of the aldehyde on C1 with the hydroxyl on C5, forming a six-member pyranose ring, named after the compound pyran
The representations of the cyclic sugars below are called Haworth projections.
Fructose can form either:
- a six-member pyranose ring, by reaction of the C2 keto group with the hydroxyl on C6
- a 5-member furanose ring, by reaction of the C2 keto group with the hydroxyl on C5.
Cyclization of glucose produces a new asymmetric center at C1, with the two stereoisomers called anomers, α & β
Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1 extending either:
- below the ring (α)
- above the ring (β).
Because of the tetrahedral nature of carbon bonds, the cyclic form of pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar
Keq, Kw 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:
Keq = [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, Kw:
Kw = [H+][OH-]
This term is referred to as the ion product. In pure water, to which no acids or bases have been added:
Kw = 1 x 10-14 M2
As Kw 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+]
Thiamin: Vitamin B1
Thiamin, or vitamin B1, helps to release energy from foods, promotes normal appetite, and is important in maintaining proper nervous system function.
RDA (Required Daily allowance) Males: 1.2 mg/day; Females: 1.1 mg/day
Thiamin Deficiency
Symptoms of thiamin deficiency include: mental confusion, muscle weakness, wasting, water retention (edema), impaired growth, and the disease known as beriberi.
The Phosphate Buffer System
This system, which acts in the cytoplasm of all cells, consists of H2PO4– as proton donor and HPO4 2– as proton acceptor :
H2PO4– = H+ + H2PO4–
The phosphate buffer system works exactly like the acetate buffer system, except for the pH range in which it functions. The phosphate buffer system is maximally effective at a pH close to its pKa of 6.86 and thus tends to resist pH changes in the range between 6.4 and 7.4. It is, therefore, effective in providing buffering power in intracellular fluids.
Glucagon
Glucagon, a peptide hormone synthesized and secreted from the α-cells of the islets of Langerhans of pancreas, raises blood glucose levels. The pancreas releases glucagon when blood sugar (glucose) levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is released into the bloodstream. Glucagon and insulin are part of a feedback system that keeps blood glucose levels at a stable level.
Regulation and function
Secretion of glucagon is stimulated by hypoglycemia, epinephrine, arginine, alanine, acetylcholine, and cholecystokinin.
Secretion of glucagon is inhibited by somatostatin, insulin, increased free fatty acids and keto acids into the blood, and increased urea production.
Applications of the Henderson-Hasselbalch equation
• Calculate the ratio of CB to WA, if pH is given
• Calculate the pH, if ratio of CB to WA is known
• Calculate the pH of a weak acid solution of known concentration
• Determine the pKa of a WA-CB pair
• Calculate change in pH when strong base is added to a solution of weak acid. This is represented in a titration curve
• Calculate the pI
Role of Coenzymes
The functional role of coenzymes is to act as transporters of chemical groups from one reactant to another.
Ex. The hydride ion (H+ + 2e-) carried by NAD or the mole of hydrogen carried by FAD;
The amine (-NH2) carried by pyridoxal phosphate