NEET MDS Lessons
Biochemistry
Glycogen Metabolism
The formation of glycogen from glucose is called Glycogenesis
Glycogen is a polymer of glucose residues linked mainly by a(1→ 4) glycosidic linkages. There are a(1→6) linkages at branch points. The chains and branches are longer than shown. Glucose is stored as glycogen predominantly in liver and muscle cells
Glycogen Synthesis
Uridine diphosphate glucose (UDP-glucose) is the immediate precursor for glycogen synthesis. As glucose residues are added to glycogen, UDP-glucose is the substrate and UDP is released as a reaction product. Nucleotide diphosphate sugars are precursors also for synthesis of other complex carbohydrates, including oligosaccharide chains of glycoproteins, etc.
UDP-glucose is formed from glucose-1-phosphate and uridine triphosphate (UTP)
glucose-1-phosphate + UTP → UDP-glucose + 2 Pi
Cleavage of PPi is the only energy cost for glycogen synthesis (1P bond per glucose residue)
Glycogenin initiates glycogen synthesis. Glycogenin is an enzyme that catalyzes glycosylation of one of its own tyrosine residues.
Physiological regulation of glycogen metabolism
Both synthesis and breakdown of glycogen are spontaneous. If glycogen synthesis and phosphorolysis were active simultaneously in a cell, there would be a futile cycle with cleavage of 1 P bond per cycle
To prevent such a futile cycle, Glycogen Synthase and Glycogen Phosphorylase are reciprocally regulated, both by allosteric effectors and by covalent modification (phosphorylation)
Glycogen catabolism (breakdown)
Glycogen Phosphorylase catalyzes phosphorolytic cleavage of the →(1→4) glycosidic linkages of glycogen, releasing glucose-1-phosphate as the reaction product.
Glycogen (n residues) + Pi → glycogen (n-1 residues) + glucose-1-phosphate
The Major product of glycogen breakdown is glucose -1-phosphate
Fate of glucose-1-phosphate in relation to other pathways:
Phosphoglucomutase catalyzes the reversible reaction:
Glucose-1-phosphate → Glucose-6-phosphate
Functions of lipids
1. They are the concentrated fuel reserve of the body (triacylglycerols).
2. Lipids are the constituents of membrane structure and regulate the membrane permeability (phospholipids and cholesterol).
3. They serve as a source of fat soluble vitamins (A, D, E and K).
4. Lipids are important as cellular metabolic regulators (steroid hormones and prostaglandins).
5. Lipids protect the internal organs, serve as insulating materials and give shape and smooth appearance to the body.
The basic characteristics of enzymes includes
(i) Almost all the enzymes are proteins and they follow the physical and chemical reactions of proteins (ii) Enzymes are sensitive and labile to heat
(iii) Enzymes are water soluble
(iv) Enzymes could be precipitated by protein precipitating agents such as ammonium sulfate and trichloroacetic acid.
The Effects of Enzyme Inhibitors
Enzymes can be inhibited
- competitively, when the substrate and inhibitor compete for binding to the same active site or
- noncompetitively, when the inhibitor binds somewhere else on the enzyme molecule reducing its efficiency.
The distinction can be determined by plotting enzyme activity with and without the inhibitor present.
Competitive Inhibition
In the presence of a competitive inhibitor, it takes a higher substrate concentration to achieve the same velocities that were reached in its absence. So while Vmax can still be reached if sufficient substrate is available, one-half Vmax requires a higher [S] than before and thus Km is larger.
Noncompetitive Inhibition
With noncompetitive inhibition, enzyme molecules that have been bound by the inhibitor are taken out
- enzyme rate (velocity) is reduced for all values of [S], including
- Vmax and one-half Vmax but
- Km remains unchanged because the active site of those enzyme molecules that have not been inhibited is unchanged.
Classification of Fatty Acids and Triglycerides
Short-chain: 2-4 carbon atoms
Medium-chain: 6-12 carbon atoms
Long-chain: 14-20 carbon atoms
Very long-chain: >20 carbon atoms
• are usually in esterified form as major components of other lipids
A16-carbon fatty acid, with one cis double bond between carbon atoms 9 and 10 may be represented as 16:1 cisD9.
Double bonds in fatty acids usually have the cis configuration. Most naturally occurring fatty acids have an even number of carbon atoms
Examples of fatty acids
|
18:0 |
stearic acid |
|
18:1 cisD9 |
oleic acid |
|
18:2 cisD9,12 |
linoleic acid |
|
18:3 cisD9,12,15 |
linonenic acid |
|
20:4 cisD5,8,11,14 |
arachidonic acid |
There is free rotation about C-C bonds in the fatty acid hydrocarbon, except where there is a double bond. Each cis double bond causes a kink in the chain,
PHOSPHOLIPIDS
These are complex or compound lipids containing phosphoric acid, in addition to fatty acids, nitrogenous base and alcohol
There are two classes of phospholipids
1. Glycerophospholipids (or phosphoglycerides) that contain glycerol as the alcohol.
2. Sphingophospholipids (or sphingomyelins) that contain sphingosine as the alcohol
Glycerophospholipids
Glycerophospholipids are the major lipids that occur in biological membranes. They consist of glycerol 3-phosphate esterified at its C1 and C2 with fatty acids. Usually, C1 contains a saturated fatty acid while C2 contains an unsaturated fatty acid.
In glycerophospholipids, we refer to the glycerol residue (highlighted red above) as the "glycerol backbone."
Glycerophospholipids are Amphipathic
Glycerophospholipids are sub classified as
1. Phosphatidylethanolamine or cephalin also abbreviated as PE is found in biological membranes and composed of ethanolamine bonded to phosphate group on diglyceride.
2. Phosphatidylcholine or lecithin or PC which has chloline bonded with phosphate group and glycerophosphoric acid with different fatty acids like palmitic or hexadecanoic acid, margaric acid, oleic acid. It is a major component of cell membrane and mainly present in egg yolk and soy beans.
3. Phosphatidic acid (phosphatidate) (PA)
It consists of a glycerol with one saturated fatty acid bonded to carbon-1 of glycerol and an unsaturated fatty acid bonded to carbon-2 with a phosphate group bonded to carbon-3.
4.Phosphatidylserine (PS)
This phospholipid contains serine as an organic compound with other main components of phospholipids. Generally it found on the cytosolic side of cell membranes.
5. Phosphoinositides
It is a group of phospholipids which are negatively charged and act as a a minor component in the cytosolic side of eukaryotic cell membranes. On the basis of different number of phosphate groups they can be different types like phosphatidylinositol phosphate (PIP), phosphatidylinositol bisphosphate(PIP2) and phosphatidylinositol trisphosphate (PIP3). PIP, PIP2 and PIP3 and collectively termed as phosphoinositide.
6. Cardiolipin :
lt is so named as it was first isolated from heart muscle. Structurally, a cardiolipin consists of two molecules of phosphatidic acid held by an additional glycerol through phosphate groups. lt is an important component of inner mitochondrial membrane. Cardiolipin is the only phosphoglyceride that possesses antigenic properties.
MAGNESIUM
The normal serum level of Magnesium is 1.8 to 2.2. mg/dl.
Functions of Magnesium
(a) Irritability of neuromuscular tissues is lowered by Magnesium
(b) Magnesium deficiency leads to decrease in Insulin dependent uptake of glucose
(c) Magnesium supplementation improves glucose tolerance
Causes such as liver cirrhosis, protein calorie malnutrition and hypo para thyroidism leads to hypomagnesemia
The main causes of hypermagnesemia includes renal failure, hyper para thyroidism, rickets, oxalate poisoning and multiple myeloma.