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.

HORMONES

A hormone is a chemical that acts as a messenger transmitting a signal from one cell to another. When it binds to another cell which is the target of the message, the hormone can alter several aspects of cell function, including cell growth, metabolism, or other function.

Hormones can be classified on three primary ways as following:

1.  Autocrine: An autocrine hormone is one that acts on the same cell that released it.

2.  Paracrine: A paracrine hormone is one that acts on cells which are nearby relative to the cell which released it. An example of paracrine hormones includes growth factors, which are proteins that stimulate cellular proliferation and differentiation.

3. Endocrine: An endocrine hormone is one that is released into the bloodstream by endocrine glands. The receptor cells are distant from the source. An example of an endocrine hormone is insulin, which is released by the pancreas into the bloodstream where it regulates glucose uptake by liver and muscle cells.

Insulin

Insulin is a polypeptide hormone synthesized in the pancreas by β-cells, which construct a single chain molecule called proinsulin. 

Insulin, secreted by the β-cells of the pancreas in response to rising blood glucose levels, is a signal that glucose is abundant.

Insulin binds to a specific receptor on the cell surface and exerts its metabolic effect by a signaling pathway that involves a receptor tyrosine kinase phosphorylation cascade.

The pancreas secretes insulin or glucagon in response to changes in blood glucose.

Each cell type of the islets produces a single hormone: α-cells produce glucagon; β-cells, insulin; and δ-cells, somatostatin.

Insulin secretion

When blood glucose rises, GLUT2 transporters carry glucose into the b-cells, where it is immediately converted to glucose 6-phosphate by hexokinase IV (glucokinase) and enters glycolysis. The increased rate of glucose catabolism raises [ATP], causing the closing of ATP-gated K+ channels in the plasma membrane. Reduced efflux of K+ depolarizes the membrane, thereby opening voltage-sensitive Ca2+ channels in the plasma membrane. The resulting influx of Ca2+ triggers the release of insulin by exocytosis.

Insulin lowers blood glucose by stimulating glucose uptake by the tissues; the reduced blood glucose is detected by the β-cell as a diminished flux through the hexokinase reaction; this slows or stops the release of insulin. This feedback regulation holds blood glucose concentration nearly constant despite large fluctuations in dietary intake.

Insulin counters high blood glucose

Insulin stimulates glucose uptake by muscle and adipose tissue, where the glucose is converted to glucose 6-phosphate. In the liver, insulin also activates glycogen synthase and inactivates glycogen phosphorylase, so that much of the glucose 6-phosphate is channelled into glycogen.

Diabetes mellitus, caused by a deficiency in the secretion or action of insulin, is a relatively common disease. There are two major clinical classes of diabetes mellitus: type I diabetes, or insulin-dependent diabetes mellitus (IDDM), and type II diabetes, or non-insulin-dependent diabetes mellitus (NIDDM), also called insulin-resistant diabetes. In type I diabetes, the disease begins early in life and quickly becomes severe. IDDM requires insulin therapy and careful, lifelong control of the balance between dietary intake and insulin dose.

Characteristic symptoms of type I (and type II) diabetes are excessive thirst and frequent urination (polyuria), leading to the intake of large volumes of water (polydipsia)

Type II diabetes is slow to develop (typically in older, obese individuals), and the symptoms are milder.

Glycolysis enzymes are located in the cytosol of cells.  Pyruvate enters the mitochondrion to be metabolized further

Mitochondrial compartments: The mitochondrial matrix contains Pyruvate Dehydrogenase and enzymes of Krebs Cycle, plus other pathways such as fatty acid oxidation. 

Pyruvate Dehydrogenase catalyzes oxidative decarboxylation of pyruvate, to form acetyl-CoA

FAD (Flavin Adenine Dinucleotide) is a derivative of the B-vitamin riboflavin (dimethylisoalloxazine-ribitol). The flavin ring system undergoes oxidation/reduction as shown below. Whereas NAD⁺ is a coenzyme that reversibly binds to enzymes, FAD is a prosthetic group, that is permanently part of the complex. 

FAD accepts and donates 2 electrons with 2 protons (2 H):

Thiamine pyrophosphate (TPP) is a derivative of  thiamine (vitamin B₁). Nutritional deficiency of thiamine leads to the disease beriberi. Beriberi affects especially the brain, because TPP is required for carbohydrate metabolism, and the brain depends on glucose metabolism for energy

Acetyl CoA, a product of the Pyruvate Dehydrogenase reaction, is a central compound in metabolism. The "high energy" thioester linkage makes it an excellent donor of the acetate moiety

For example, acetyl CoA functions as:

• input to the Krebs Cycle, where the acetate moiety is further degraded to CO₂.
• donor of acetate for synthesis of fatty acids, ketone bodies, and cholesterol.

ATPs  formed in TCA cycle from one molecule of Pyruvate

1. 3ATP            7. 3ATP          5. 3 ATP                     

 8. 1 ATP         9. 2 ATP          11.3 ATP         Total =15 ATP.

 ATPS formed from one molecule of Acetyl CoA =12ATP

ATPs formed from one molecule of glucose after complete oxidation

One molecule of glucose -->2 molecules of pyruvate

['By glycolysis] ->8 ATP

2 molecules of pyruvate [By TCA cycle] -> 30 ATP

Total = 38 ATP

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

The Protein Buffer Systems

The protein buffers are very important in the plasma and the intracellular fluids but their concentration is very low in cerebrospinal fluid, lymph and interstitial fluids.

The proteins exist as anions serving as conjugate bases (Pr ⁻ ) at the blood pH 7.4 and form conjugate acids (HPr) accepting H⁺ .  They have the capacity to buffer some H₂CO₃  in the blood.