LIPIDS

The lipids are a heterogeneous group of compounds, including fats, oils, steroids, waxes, and related compounds, which are related more by their physical than by their chemical properties.

Lipids are non-polar (hydrophobic) compounds, soluble in organic solvents.

Most membrane lipids are amphipathic, having a non-polar end and a polar end

Lipids are important in biological systems because they form the cell membrane, a mechanical barrier that divides a cell from the external environment.

Lipids also provide energy for life and several essential vitamins are lipids.

Lipids can be divided in two major classes, nonsaponifiable lipids and saponifiable lipids.

A nonsaponifiable lipid cannot be broken up into smaller molecules by hydrolysis, which includes triglycerides, waxes, phospholipids, and sphingolipids.

A saponifiable lipid contains one or more ester groups allowing it to undergo hydrolysis in the presence of an acid, base, or enzyme.

Nonsaponifiable lipids include steroids, prostaglandins, and terpenes

Nonpolar lipids, such as triglycerides, are used for energy storage and fuel.

Polar lipids, which can form a barrier with an external water environment, are used in membranes.

Polar lipids include glycerophospholipids and sphingolipids.

Fatty acids are important components of all of these lipids.

CLINICAL SIGNIFICANCE OF ENZYMES

The measurement of enzymes level in serum is applied in diagnostic application

Pancreatic Enzymes

Acute pancreatitis is an inflammatory process where auto digestion of gland was noticed with activation of the certain pancreatic enzymes. Enzymes which involves in pancreatic destruction includes α-amylase, lipase etc.,

1.  α-amylase (AMYs) are calcium dependent hydrolyase class  of metaloenzyme that catalyzes the hydrolysis of 1, 4- α-glycosidic linkages in polysaccharides. The normal values of amylase is in range of 28-100 U/L. Marked increase of 5 to 10 times the upper reference limit (URL) in AMYs activity indicates acute pancreatitis and severe glomerular impairment.

2.  Lipase is single chain glycoprotein. Bile salts and a cofactor called colipase are required for full catalytic activity of lipase. Colipase is secreted by pancreas. Increase in plasma lipase activity indicates acute pancreatitis and carcinoma of the pancreas.

Liver Enzymes

Markers of Hepatocellular Damage

1.  Aspartate transaminase (AST) Aspartate transaminase is present in high concentrations in cells of cardiac and skeletal muscle, liver, kidney and erythrocytes. Damage to any of these tissues may increase plasma AST levels.

The normal value of AST for male is <35 U/ L and for female it is <31 U/L.

2.  Alanine transaminase (ALT) Alanine transaminase is present at high concentrations in liver and to a lesser extent, in skeletal muscle, kidney and heart. Thus in case of liver damage increase in both AST and ALT were noticed. While in myocardial infarction AST is increased with little or no increase in ALT.

The normal value of ALT is <45 U/L and <34 U/L for male and female respectively

Markers of cholestasis

1.  Alkaline phosphatases

Alkaline phosphatases are a group of enzymes that hydrolyse organic phosphates at high pH. They are present in osteoblasts of bone, the cells of the hepatobiliary tract, intestinal wall, renal tubules and placenta.

Gamma-glutamyl-transferase (GGT) Gamma-glutamyl-transferase catalyzes the transfere of the γ–glutamyl group from peptides. The activity of GGT is higher in men than in women. In male the normal value of GGT activity is <55 U/L and for female it is <38 U/L.

2.  Glutamate dehydrogenase (GLD) Glutamate dehydrogenase is a mitochondrial enzyme found in liver, heart muscle and kidneys.

Muscle Enzymes

1.  Creatine Kinase Creatine kinase (CK) is most abundant in cells of brain, cardiac and skeletal.

2.  Lactate Dehydrogenase

Lactate dehydrogenase (LD) catalyses the reversible interconversion of lactate and pyruvate.

Titration of a weak acid with a strong base

• A weak acid is mostly in its conjugate acid form

• When strong base is added, it removes protons from the solution, more and more acid is in the conjugate base form, and the pH increases

• When the moles of base added equals half the total moles of acid, the weak acid and its conjugate base are in equal amounts. The ratio of CB / WA = 1 and according to the HH equation, pH = pKa + log(1) or pH = pKa.

• If more base is added, the conjugate base form becomes greater till the equivalance point when all of the acid is in the conjugate base form.

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

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

The input to fatty acid synthesis is acetyl-CoA, which is carboxylated to malonyl-CoA.

The ATP-dependent carboxylation provides energy input. The CO₂ is lost later during condensation with the growing fatty acid. The spontaneous decarboxylation drives the condensation. 

 fatty acid synthesis
acetyl-CoA + 7 malonyl-CoA + 14 NADPH → palmitate + 7 CO₂ + 14 NADP⁺ + 8 CoA

ATP-dependent synthesis of malonate:
8 acetyl-CoA + 14 NADPH + 7 ATP → palmitate + 14 NADP⁺ + 8 CoA + 7 ADP + 7 P_i

Fatty acid synthesis occurs in the cytosol. Acetyl-CoA generated in the mitochondria is transported to the cytosol via a shuttle mechanism involving citrate