Glycogenolysis

Breakdown of  glycogen to glucose is called glycogenolysis. The Breakdown of glycogen takes place in liver and muscle. In Liver , the end product of glycodgen breakdown is glucose where as in muscles the end product is Lactic acid Under the combined action of Phosphorylase  (breaks only –α-(1,4) linkage )and Debranching enzymes (breaks only α-(1,6) linkage )glycogen is broken down to glucose.

BIOLOGICAL BUFFER SYSTEMS 

Cells and organisms maintain a specific and constant cytosolic pH, keeping biomolecules in their optimal ionic state, usually near pH 7. In multicelled organisms, the pH of the extracellular fluids (blood, for example) is also tightly regulated. Constancy of pH is achieved primarily by biological buffers : mixtures of weak acids and their conjugate bases 

Body fluids and their principal buffers

Body fluids                     Principal buffers

Extracellular fluids        {Biocarbonate buffer Protein buffer } 

Intracellular fluids         {Phosphate buffer, Protein }

Erythrocytes                 {Hemoglobin buffer}

FACTORS AFFECTING ENZYME ACTIVITY

Velocity or rate of enzymatic reaction is assessed by the rate of change in concentration of substrate or product at a given time duration. Various factors which affect the activity of enzymes include:

1. Substrate concentration

2. Enzyme concentration

3. Product concentration

4. Temperature 5. Hydrogen ion concentration (pH)

6. Presence of activators

7. Presence of inhibitor

Effect of substrate Concentration :  Reaction velocity of an enzymatic process increases with constant enzyme concentration and increase in substrate concentration.

Effect of enzyme Concentration: As there is optimal substrate concentration, rate of an enzymatic reaction or velocity (V) is directly proportional to the enzyme concentration.

Effect of product concentration In case of a reversible reaction catalyzed by a enzyme, as per the law of mass action the rate of reaction is slowed down with equilibrium. So, rate of reaction is slowed, stopped or even reversed with increase in product concentration

Effect of temperature: Velocity of enzymatic reaction increases with temperature of the medium which they are most efficient and the same is termed as optimum temperature.

Effect of pH: Many enzymes are most efficient in the region of pH 6-7, which is the pH of the cell. Outside this range, enzyme activity drops off very rapidly. Reduction in efficiency caused by changes in the pH is due to changes in the degree of ionization of the substrate and enzyme.

Highly acidic or alkaline conditions bring about a denaturation and subsequent loss of enzymatic activity

Exceptions such as pepsin (with optimum pH 1-2), alkaline phosphatase (with optimum pH 9-10) and acid phosphatase (with optimum pH 4-5)

Presence of activators Presence of certain inorganic ions increases the activity of enzymes. The best examples are chloride ions activated salivary amylase and calcium activated lipases.

Effect of Inhibitors The catalytic enzymatic reaction may be inhibited by substances which prevent the formation of a normal enzyme-substrate complex. The level of inhibition then depends entirely upon the relative concentrations of the true substrate and the inhibitor

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

Parathyroid Hormone

Parathyroid hormone (PTH), parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands.

It acts to increase the concentration of calcium (Ca2+) in the blood, whereas calcitonin (a hormone produced by the parafollicular cells of the thyroid gland) acts to decrease calcium concentration.

PTH acts to increase the concentration of calcium in the blood by acting upon the parathyroid hormone 1 receptor (high levels in bone and kidney) and the parathyroid hormone 2 receptor (high levels in the central nervous system, pancreas, testis, and placenta).

Effect of parathyroid hormone in regulation of serum calcium.

Bone -> PTH enhances the release of calcium from the large reservoir contained in the bones. Bone resorption is the normal destruction of bone by osteoclasts, which are indirectly stimulated by PTH forming new osteoclasts, which ultimately enhances bone resorption.

Kidney -> PTH enhances active reabsorption of calcium and magnesium from distal tubules of kidney. As bone is degraded, both calcium and phosphate are released. It also decreases the reabsorption of phosphate, with a net loss in plasma phosphate concentration. When the calcium:phosphate ratio increases, more calcium is free in the circulation.

Intestine -> PTH enhances the absorption of calcium in the intestine by increasing the production of activated vitamin D. Vitamin D activation occurs in the kidney. PTH converts vitamin D to its active form (1,25-dihydroxy vitamin D). This activated form of vitamin D increases the absorption of calcium (as Ca2+ ions) by the intestine via calbindin.