Lipid absorption
After the breakdown of polymer lipid molecules, the resulting monomers are absorbed in the upper part of the small intestine in the initial 100 cm. Normally, 98% of dietary lipids are absorbed.
1. Short fatty acids (no more than 10 carbon atoms) are absorbed and pass into the blood without any special mechanisms. This process is important for infants because... milk contains mainly short- and medium-chain fatty acids. Glycerol is also absorbed directly.
2. Other digestion products (long-chain fatty acids, cholesterol, monoacylglycerols) form micelles with a hydrophilic surface and a hydrophobic core with bile acids. Their sizes are 100 times smaller than the smallest emulsified fat droplets. Through the aqueous phase, the micelles migrate to the brush border of the mucosa. Here the micelles disintegrate and the lipid components diffuse into the cell and are then transported to the endoplasmic reticulum.
Bile acids can also enter enterocytes here and then go into the blood of the portal vein, but most of them remain in the chyme and reach the ileum, where they are absorbed using active transport.
Resynthesis
Resynthesis of ATP from ADP cannot occur spontaneously; this process is associated with a number of energy-producing transformations (p.
Resynthesis of ATP from ADP cannot occur spontaneously. Let us recall here that when the terminal phosphate group is cleaved from ATP at physiological concentrations of all substances, about 8000 cal/yol are released.
Resynthesis of ATP molecules occurs due to the energy of oxidation of certain sugars. During this process, cross bridges are alternately destroyed and restored, and the mechanism of this phenomenon is not fully understood.
This resynthesis of glucose from other sources satisfies energy needs and is essential for the functioning of some types of cells, especially blood cells and nervous tissue in animals. The situation is similar in microorganisms; from carbon sources containing four and three carbon atoms, they synthesize hexoses, pentoses and structural polysaccharides. So, the question essentially comes down to how biosynthesis occurs; glucose or hexose phosphate from pyruvate.
For the resynthesis of lipoids in the intestinal wall, in addition to higher fatty acids and glycerol, phosphoric acid is also needed, as well as organic nitrogenous bases - choline or colamine. These compounds are partially absorbed from the intestinal cavity, since they are formed during the hydrolysis of dietary lipoids, and are partially delivered to the intestinal epithelial cells through the bloodstream from other tissues.
For the resynthesis of lipoids in the intestinal wall, in addition to higher fatty acids and glycerol, phosphoric acid is also needed, as well as organic nitrogenous bases - choline or colamine. These compounds are partially absorbed from the intestinal cavity, since they are formed during the hydrolysis of dietary lipoids, and are partially delivered to the intestinal epithelial cells through the bloodstream from other tissues.
First of all, ATP resynthesis is ensured by transphosphorylation of ADP with creatine phosphate.
Due to the possibility of resynthesis of glutathione, which requires the energy expenditure of ATP, the cycle can be repeated many times, transporting significant amounts of amino acids.
Thus, ATP resynthesis is associated with a number of energy-producing transformations (p.
Hydrolytic cleavage and resynthesis of the peroxide bond in the processes of synthesis and hydrolysis of peroxyesters is impossible. In this regard, the reaction under study should be considered as a special case of O-acylation, during which hydroxyl is removed from the carboxyl group, and hydrogen is removed from the hydroperoxide group.
The processes of synthesis and resynthesis of protein and nucleic acids occur more energetically in tumor cells due to decaying, energy-rich organophosphorus compounds, the formation of which is associated with the process of oxidative phosphorylation in tissues.
However, after each repeated resynthesis, a slight increase in viscosity is observed compared to the previous experiment. This may be due to the effect of pressure on the enzymes.
Therefore, when determining the resynthesis of starch after 7–10 days of warm storage, a decrease in the amount of sugars and an increase in starch are always observed. When determined at an earlier time, it is not always possible to establish changes in the ratio of starch and sugars.
The possibility of resynthesis of ATP on a large scale due to the oxidation of non-carbohydrate substances has also been proven. We now know that oxidative phosphorylation easily occurs during the oxidation of various substances in the Krebs tricarboxylic acid cycle (see p.
The possibility of resynthesis of ATP on a large scale due to the oxidation of carbohydrates in the pentose cycle has also been proven (p.
Resynthesis of lipids in enterocytes
Lipid resynthesis is the synthesis of lipids in the intestinal wall from exogenous fats that enter here; endogenous fatty acids can also be used, therefore resynthesized fats differ from food fats and are closer in composition to “their” fats. The main task of this process is to bind fatty acids received from food with alcohol - glycerol or cholesterol. This, firstly, eliminates their detergent effect on membranes and, secondly, creates their transport forms for transport through the blood to tissues.
Fatty acid activation
The fatty acid entering the enterocyte (as well as any other cell) is necessarily activated through the addition of coenzyme A. The resulting acyl-SCoA participates in the reactions of the synthesis of cholesterol esters, triacylglycerols and phospholipids.
Fatty acid activation reaction
Resynthesis of cholesterol esters
Cholesterol is esterified using acyl-SCoA and the enzyme acyl-SCoA:cholesterol acyltransferase (ACAT).
Reesterification of cholesterol directly affects its absorption into the blood. Currently, possibilities are being sought to suppress this reaction to reduce the concentration of cholesterol in the blood.
Cholesterol ester resynthesis reaction
Resynthesis of triacylglycerols
There are two ways to resynthesize TAG:
The first pathway, the main one - 2-monoacylglyceride - occurs with the participation of exogenous 2-MAG and FA in the smooth endoplasmic reticulum of enterocytes: the multienzyme complex of triacylglycerol synthase forms TAG.
Monoacylglyceride pathway for TAG formation
Since 1/4 of the TAG in the intestine is completely hydrolyzed, and glycerol is not retained in the enterocytes and quickly passes into the blood, a relative excess of fatty acids arises for which there is not enough glycerol. Therefore, there is a second, glycerol phosphate, pathway in the rough endoplasmic reticulum. The source of glycerol-3-phosphate is the oxidation of glucose. The following reactions can be distinguished:
- Formation of glycerol-3-phosphate from glucose.
- Conversion of glycerol-3-phosphate to phosphatidic acid.
- Conversion of phosphatidic acid to 1,2-DAG.
- Synthesis of TAG.
Glycerol phosphate pathway for TAG formation
Resynthesis of phospholipids
Phospholipids are synthesized in the same way as in other cells of the body (see “Synthesis of phospholipids”). There are two ways to do this:
The first route is using 1,2-DAG and the active forms of choline and ethanolamine to synthesize phosphatidylcholine or phosphatidylethanolamine.
Resynthesis of phospholipids from DAG using the example of phosphatidylcholine
The second way is based on phosphatidic acid synthesized in situ.
During contraction, ATP supplies the necessary energy for the formation of the actomyosin complex, and during muscle relaxation, it provides energy for the active transport of calcium ions into the reticulum. To maintain the contractile function of a muscle, the concentration of ATP in it must be at a constant level of 2 to 5 mmol/kg.
Therefore, during muscle activity, adenosine triphosphoric acid must be restored at the same rate as it is broken down during contraction, which is carried out by separate biochemical mechanisms of its resynthesis.
The energy sources for ATP resynthesis in skeletal muscles and other tissues are energy-rich phosphate-containing substances. They are present in tissues (creatine phosphate, adenosine diphosphate) or are formed during the catabolism of glycogen, fatty acids and other energy substrates. In addition, as a result of aerobic oxidation of various substances, proton gradient energies arise on the mitochondrial membrane.
Resynthesis of adenosine triphosphate can be carried out in reactions without the participation of oxygen ( anaerobic mechanisms ) or with its participation ( aerobic mechanism ). Under normal conditions, ATP resynthesis in muscles occurs predominantly aerobically. During intense physical work, when the delivery of oxygen to the muscles is difficult, anaerobic mechanisms of ATP resynthesis are also activated. In human skeletal muscles, three types of anaerobic and one aerobic pathway for the reduction of adenosine triphosphate have been identified.
Anaerobic mechanisms include creatine phosphokinase (phosphogenic or alactic), glycolytic (lactate) and myokinase mechanisms.
The aerobic mechanism of ATP resynthesis consists of oxidative phosphorylation occurring in mitochondria, the number of which in skeletal muscles increases significantly during aerobic training. The energy substrates of aerobic oxidation are: glucose, fatty acids, partially amino acids, as well as intermediate metabolites of glycolysis (lactic acid) and fatty acid oxidation (ketone bodies).
Each mechanism has different energy capabilities, which are evaluated according to the following criteria: maximum power, deployment rate, metabolic capacity and efficiency.
Maximum power is the highest rate of ATP production in a given metabolic process. It limits the maximum intensity of work performed by the mechanism used.
Deployment speed is the time it takes to reach the maximum power of a given adenosine triphosphate resynthesis pathway from the start of operation.
Metabolic capacity is the total amount of ATP that can be produced in the ATP resynthesis mechanism used due to the amount of energy substrate reserves. Capacity limits the amount of work performed. Metabolic efficiency is the portion of energy that is stored in the high-energy bonds of adenosine triphosphate. It determines the efficiency of the work performed and is assessed by the overall value of the efficiency factor, which represents the ratio of all usefully expended energy to its total amount released during the current metabolic process.
The overall efficiency when converting the energy of metabolic processes into mechanical work depends on two indicators:
- efficiency of phosphorylation;
- efficiency of chemomechanical coupling (efficiency of converting ATP into mechanical work).
The efficiency of chemomechanical coupling in the processes of aerobic and anaerobic metabolism is approximately the same and amounts to 50%.
The efficiency of phosphorylation is highest in the alactic anaerobic process - about 80%, and the lowest in anaerobic glycolysis - on average 44%. In the aerobic process it is approximately 60%.
Thus, anaerobic mechanisms have a high maximum power and efficiency of ATP production, but a short retention time and low capacity, due to small reserves of energy substrates. For example, the maximum power of the creatine phosphokinase reaction develops already at 0.5-0.7 s of intense work and is maintained for 10-15 s in untrained people and 25-30 s in highly trained athletes and is 3.8 kJ/kg per minute.
The glycolytic mechanism of ATP resynthesis is characterized by low efficiency. Most of the energy remains in the molecules of the resulting lactic acid. The concentration of the latter is directly dependent on the power and duration of operation, and can only be released by aerobic oxidation.
Glycolysis is the main pathway of energy production in submaximal power exercises, the maximum duration of which is from 30 s to 2.5 min (middle-distance running, swimming 100 and 200 m, etc.).
The glycolytic mechanism of energy generation serves as the biochemical basis for the body’s special speed endurance.
The myokinase reaction occurs in muscles with a significant increase in the concentration of ADP in the sarcoplasm. This situation occurs with severe muscle fatigue, when other routes of resynthesis are no longer possible.
Thus, anaerobic mechanisms are fundamental in the energy supply of short-term, high-intensity exercise .
When adapting to intense exercise, the activity of enzymes of anaerobic mechanisms and reserves of energy mechanisms increases: the content of creatine phosphate in skeletal muscles can increase by 1.5-2 times, and the glycogen content by almost 3 times.
General principle of lipid biosynthesis
The formation of FAs and their derivatives begins in the cytoplasm. The second part of biosynthesis, the elongation of the molecular chain, also continues in the cell, but the “production workshop shifts” inside the mitochondria. At each step, the compound is enriched in two C atoms, which is reminiscent of the beta-oxidation process, only in reverse.
In more detail, synthesis, for example, of palmitic acid, occurs directly in the cytoplasm. Mitochondria, on the other hand, use a ready-made “semi-finished product” to produce complete fatty acids, consisting of 18 or more carbon atoms. Mitochondria are not able to carry out the entire biosynthesis independently from “A” to “Z”. The reason is banal - “low level of qualifications”. Returning to technical terminology, mitochondria have a very low ability to incorporate labeled acetic acids into long chain lipid structures.
