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MindMap Gallery Protein digestion and absorption and amino acid metabolism
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Protein digestion and absorption and amino acid metabolism
Mind map of protein digestion and absorption and amino acid metabolism. Amino acids are the basic building blocks of protein and one of their important physiological functions. They are used as raw materials to participate in the synthesis of intracellular proteins.
Edited at 2023-10-31 22:42:52
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Protein digestion and absorption and amino acid metabolism
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Protein digestion and absorption and amino acid metabolism
Nutritional value and digestion and absorption of protein
Amino acids are the basic building blocks of proteins and one of their important physiological functions. They are used as raw materials to participate in the synthesis of intracellular proteins.
Protein in the body is in a dynamic balance of constant synthesis and decomposition, and amino acid metabolism is the central component of protein catabolism.
Function: Maintain the growth, renewal and repair of cell tissues Participate in a variety of important physiological activities in the body Can be used as an energy source to oxidize energy
The metabolic status of proteins in the body can be described by nitrogen balance
Definition: Nitrogen balance refers to the relationship between daily nitrogen intake and output
Egg intake basically comes from protein in food
Excretion of nitrogen mainly comes from nitrogenous compounds in feces and urine
Significance: Since the average nitrogen content of protein is 16%, we can indirectly understand the status of protein synthesis and catabolism in the body.
There are three situations of human body nitrogen balance:
The total balance of nitrogen, that is, the amount of nitrogen ingested is equal to the amount of nitrogen excreted, is common in normal adults
Positive nitrogen balance: that is, the amount of nitrogen taken in is greater than the amount of nitrogen excreted. It is common in children, pregnant women and patients in the recovery period. Their manifestations are increased weight.
Negative nitrogen balance: that is, the amount of nitrogen taken in is less than the amount of nitrogen excreted. It is common in patients with hunger, severe burns, bleeding and wasting diseases.
Due to the difference in composition between food protein and human protein, it is impossible to fully utilize it after digestion and absorption. Therefore, the minimum physiological requirement of protein per day for normal adults is 30~50g. The recommended daily protein requirement for normal adults is 80g
Nutritional essential amino acids determine the nutritional value of protein
There are nine kinds of amino acids that cannot be synthesized in the human body (amino acids that are needed by the body but cannot be synthesized by itself and must be provided by food, and are called essential amino acids in nutrition): leucine, isoleucine, threonine, valine, Lysine, methionine, phenylalanine, tryptophan and histidine
Although arginine can be synthesized in the human body, the amount of synthesis is not large. If the supply is insufficient for a long time or the demand increases, it can also cause a negative nitrogen balance. Therefore, some people classify arginine as an essential nutritional amino acid.
The nutritional value of protein refers to the utilization rate of food protein in the body
The nutritional value mainly depends on the type and proportion of essential amino acids in food protein.
When a variety of proteins with low nutritional value are mixed and eaten, the essential amino acids can complement each other, thereby improving the nutritional value of the protein. This effect is called the complementary effect of food proteins.
Why do eggs need to be cooked before eating? Because at high temperatures, protein denaturation changes the spatial conformation, which is beneficial to digestion and absorption, and can also eliminate potential bacteria and viruses.
Exogenous proteins are consumed into oligopeptides and amino acids and then absorbed
Proteins are digested into oligopeptides and amino acids in the stomach and small intestine
Physiological significance:
Transform from large molecules to small molecules for easy absorption
Eliminate species specificity and antigenicity to prevent allergic and toxic reactions
Proteins are hydrolyzed into peptides and amino acids in the stomach
Pepsinogen (exopeptidase) is secreted by the chief cells of the gastric mucosa and is converted into active pepsin after activation by hydrochloric acid.
Pepsin can also activate pepsinogen and convert it into pepsin, which is called autocatalysis. The optimal pH of pepsin is 1.5~2.5
Acidic gastric juice can denature proteins and is conducive to protein hydrolysis. Pepsin has poor specificity for peptide bonds and mainly hydrolyzes peptide bonds formed by aromatic amino acids, methionine, leucine and other amino acid residues.
The milk coagulation effect of pepsin can cause the casein and calcium ions in the milk to form milk clots, which prolongs the residence time of the milk in the stomach and is beneficial to the digestion of proteins in the milk.
Proteins are hydrolyzed into oligopeptides and amino acids in the small intestine
Protein digestion occurs mainly in the small intestine
Proteases that function in the small intestine are divided into endopeptidases and exopeptidases
Endopeptidases can specifically hydrolyze some peptide bonds inside proteins. Endopeptidases include trypsin, chymotrypsin and elastase. These enzymes have certain specificity for peptide bonds composed of different amino acid residues.
Exopeptidase specifically hydrolyzes the peptide bond at the end of a protein or polypeptide. Exopeptidase mainly includes carboxypeptidase and aminopeptidase. The exopeptidase in pancreatic juice is mainly carboxypeptidase, which can be divided into carboxypeptidase A and carboxypeptidase A. Peptidase B, they hydrolyze one amino acid at a time starting from the carboxyl terminus of the peptide chain
Carboxypeptidase A mainly hydrolyzes terminal peptide bonds composed of various amino acid residues other than proline, arginine, and lysine; carboxypeptidase B mainly hydrolyzes terminal peptide bonds composed of basic amino acid residues.
The significance of zymogen activation:
Protects pancreatic tissue from autodigestion by proteases
Ensure that enzymes perform catalytic functions in their specific sites and environments
Zymogen can also be considered as the storage form of enzyme
Amino acids and oligopeptides are absorbed through active transport mechanisms
After proteins are digested into amino acids and oligopeptides, they are mainly absorbed in the small intestine through active transport mechanisms.
Mechanism: The carrier protein on the cell membrane of the small intestinal mucosa can form a triplet with amino acids or oligopeptides and sodium ions to transport the amino acids or oligopeptides and sodium ions into the cells. The sodium ions are then excreted out of the cells with the help of a sodium pump. This process requires the consumption of ATP.
7 types of carrier proteins: neutral amino acid transporter, acidic amino acid transporter, basic amino acid transporter, imamino acid transporter, β-amino acid transporter, dipeptide transporter, tripeptide transporter
Undigested and absorbed proteins become putrefactive in the lower colon
Undigested proteins and unabsorbed digestive products are decomposed by intestinal bacteria in the lower part of the colon, which is called protein putrefaction.
In fact, putrefaction is a metabolic process of intestinal bacteria itself, mainly anaerobic decomposition.
Gut bacteria produce amines through decarboxylation
Histidine, lysine, tryptophan, tyrosine and phenylalanine generate histamine, cadaverine, tryptamine, tyramine and phenylethylamine respectively through decarboxylation
Histamine and cadaverine have the effect of lowering blood pressure, and tyrosine has the effect of raising blood pressure.
Tyramine and phenylethylamine are called pseudoneurotransmitters because of their structural similarity to catecholamines.
When false neurotransmitters increase, they can competitively interfere with the normal function of catechins, hinder nerve impulse transmission, and cause abnormal inhibition in the brain. This may be one of the causes of hepatic encephalopathy.
Gut bacteria produce ammonia through deamination
source:
1 Unabsorbed amino acids can generate ammonia through deamination under the action of intestinal bacteria, which is one of the important sources of intestinal ammonia.
2. Urea in the blood penetrates into the intestines and is hydrolyzed by intestinal bacterial urease to generate ammonia.
If the intestine produces too much ammonia, liver function is damaged, etc., it will lead to an increase in blood ammonia concentration and even ammonia poisoning.
The basis for acidic enema: lowering the pH of the intestine, ammonia gas is converted into ammonia ions and excreted in the form of ammonium salt, which can reduce the absorption of ammonia
Corruption produces other harmful substances
Tyrosine – phenol, cysteine – hydrogen sulfide, tryptophan – indole (cause of odor when using the toilet)
General metabolism of amino acids
Protein breaks down into amino acids in the body
About 1% to 2% of the proteins in the adult body are degraded every day, and about 70% to 80% are reused to synthesize new proteins, mainly proteins in skeletal muscles.
Proteins degrade at different rates
The degradation rates of different proteins vary with physiological needs.
The rate of protein degradation is expressed as half-life, which is the time required to reduce its concentration to 50% of the starting value.
The half-life of the enzyme protein is only 30 minutes
There are two important pathways for protein degradation in eukaryotic cells
Proteins are degraded in lysosomes through an ATP-independent pathway
Independent of ATP and ubiquitin Utilize cathepsins in lysosomes to degrade exogenous proteins, membrane proteins and long-lived proteins
The main function of lysosomes is digestion. They are digestive organs within cells. They contain a variety of proteases called cathepsins.
Proteins are degraded in the proteasome through an ATP-dependent pathway
Dependent on ATP and ubiquitin Degrade abnormal and short-lived proteins
Ubiquitin mainly plays a labeling role and is involved in three enzymes. When the catalytic reaction is completed, ATP is consumed at the same time.
The degradation of a protein requires multiple ubiquitination reactions to form ubiquitin chains, and the ubiquitinated protein is degraded by the proteasome
Proteasomes exist in the nucleus and cytoplasm and mainly degrade abnormal proteins and short-lived proteins.
Exogenous amino acids and endogenous amino acids constitute the amino acid metabolic pool
The amino acids produced by the degradation of tissue proteins in the body and the non-essential amino acids synthesized in the body are endogenous amino acids. They are distributed throughout the body together with the amino acids digested and absorbed from food proteins. Their participation in metabolism is called the amino acid metabolic pool.
The main function of amino acids in the body is to synthesize peptides and proteins, and can also be converted into other nitrogen-containing compounds.
Amino acid catabolism begins with deamination
The main reaction of amino acid catabolism is deamination, which is the process in which amino acids remove α-amino groups to generate corresponding α-keto acids.
Amino acids are deaminated through transamination
Transamination is catalyzed by transaminase
Transamination is the reversible transfer of the amino group of α-amino acid to α-keto acid under the catalysis of aminotransferase. The result is that the amino acid is deaminated to form the corresponding α-keto acid, and the original α-keto acid is converted for another amino acid
Aminotransferase, also called aminotransferase, is most abundant in the liver and myocardium.
Transamination is not only a catabolic process of amino acids, but also an important pathway for the synthesis of certain amino acids in the body.
Most amino acids can undergo transamination except lysine, threonine, proline and hydroxyproline
According to the activity of aminotransferase in serum, it is one of the reference indicators for disease diagnosis and prognosis.
Physiological significance of transamination: Transamination is not only an important way to deaminate most amino acids in the body, but also an important way for the body to synthesize non-essential amino acids.
Aminotransferases have the same coenzyme and mechanism of action
The core of aminotransferase is the phosphate ester of vitamin B6, that is, pyridoxal phosphate, which is combined with the ε-amino group of lysine in the active center of aminotransferase.
L-glutamic acid dehydrogenase catalyzes the oxidative deamination of L-glutamic acid
L-glutamate dehydrogenase
This enzyme is widely found in tissues such as liver, kidney and brain and is a non-aerobic dehydrogenase.
ATP and GTP are allosteric inhibitors of this enzyme
ADP and GDP are allosteric activators
It is the only enzyme that can utilize both NAD and NADP to accept reducing equivalents.
This method requires the combined action of aminotransferase and L-glutamic acid dehydrogenase, that is, the transamination is coupled with the oxidative deamination of L-glutamic acid, which is called transamination and deamination, also known as combined deamination
Is the main way of deamination of amino acids
It is also the main way in which non-essential amino acids are synthesized in the body.
Amino acids are deainated catalyzed by amino acid oxidase
Mainly performed in liver, kidney and brain tissue
The amino acid peptide chain skeleton can be converted or decomposed
Alpha-keto acids can be completely oxidized and decomposed to provide energy
Amination of alpha-keto acids produces nutritionally non-essential amino acids
Alpha cupric acid can be converted into sugars and lipids
Ammonia metabolism
Ammonia produced by metabolism in the body and ammonia absorbed by the digestive tract enter the blood to form blood ammonia. Under normal circumstances, blood pressure level is 47~65umol/L, and ammonia is toxic.
There are three important sources of blood ammonia
Ammonia can be produced by both amino acid deamination and amine decomposition
Ammonia produced by amino acid deamination is the main source of ammonia in the body
Intestinal bacteria produce ammonia
Proteins and amino acids can produce amines under the putrefactive action of intestinal bacteria
Hydrolysis of urea in the intestine by bacterial urease can also produce ammonia
Clinically, weakly acidic dialysate is used for colon dialysis in patients with high blood ammonia. The purpose of prohibiting the use of alkaline soapy water enema is to reduce the absorption of ammonia, because the absorption of ammonia is affected by the intestinal pH value.
Ammonia secreted by renal tubular epithelial cells mainly comes from glutamine
Glutamine is hydrolyzed into glutamate and ammonia under the catalysis of glutaminase. Alkaline urine hinders the secretion of ammonia in renal tubular cells.
Clinically, alkaline diuretics should not be used in patients with ascites due to liver cirrhosis to avoid an increase in blood ammonia.
Ammonia is transported in the blood as alanine and glutamine
Physiological significance:
1 Ammonia must be transported to the liver in a non-toxic way through the blood to synthesize urea or transported to the kidneys and excreted from the body in the form of ammonium salts
2 Liver provides glucose to muscles
Ammonia is transported from skeletal muscle to the liver via the alanine-glucose cycle
Skeletal muscle mainly uses pyruvate as an amino receptor, and generates alanine through transamination. After alanine enters the blood, it is transported to the liver. Ammonia is used to synthesize urea, and pyruvate is used to generate glucose through the gluconeogenesis pathway.
Through the alanine-glucose cycle, the amino groups of amino acids in skeletal muscle tissue are transported to the liver in the form of pyruvate. At the same time, the liver provides skeletal muscles with glucose to generate pyruvate.
Ammonia is transported from tissues such as the brain and skeletal muscles to the liver or kidneys via glutamine
Glutamine is another form of ammonia transporter, primarily from tissues such as the brain and skeletal muscle to the liver or kidneys
Physiological significance: Glutamine is not only the detoxification product of ammonia, but also the storage and transportation form of ammonia.
Glutamine can also provide an amino group to convert aspartic acid into asparagine. Therefore, asparaginase is used clinically to hydrolyze asparagine into aspartic acid, thereby reducing asparagine in the blood and achieving the purpose of treating leukemia.
The main metabolic route of ammonia is the synthesis of urea in the liver
Directions:
1 Urea is mainly synthesized in the liver, and only a small amount of ammonia is excreted in the urine in the form of amine salts in the kidneys.
Synthesis of non-essential amino acids and other nitrogen-containing compounds
Synthesize glutamine
Urea is synthesized through the amphicycline cycle
The ornithine cycle, also known as the urea cycle, uses tissue sectioning technology and isotope tracing technology to find that both ornithine and citrulline catalyze the synthesis of urea from ammonium salts.
Arginine is an important intermediate compound. Arginase can catalyze the hydrolysis of arginine to produce urea and ornithine.
Reaction steps in the ornithine cycle in the liver
NH3, CO2 and ATP condense to form carbamoyl phosphate
It is carried out in mitochondria and is catalyzed by carbamoyl phosphate synthase I to generate carbamoyl phosphate. This is an irreversible reaction. This reaction can only be activated in the presence of the allosteric activator N-acetyl glutamate.
This reaction consumes two molecules of ATP to provide the driving force for the synthesis of amide bonds and anhydride bonds.
Carbamoyl phosphate reacts with ornithine to form citrulline
Performed in mitochondria, catalyzed by ornithine formyltransferase
Citrulline reacts with aspartic acid to form argininosuccinic acid
The reaction is catalyzed by argininosuccinate synthase, powered by ATP. Aspartate provides the second nitrogen atom in the urea molecule, and the reaction proceeds in the cytosol.
Argininosuccinic acid is cleaved to produce arginine and fumaric acid
Catalyzed by argininosuccinate lyase, the reaction occurs in the cytosol
The amino groups of various amino acids in the body can participate in the synthesis of urea in the form of aspartic acid.
Arginine is hydrolyzed to release urea and regenerated into ornithine
Catalyzed by arginase, the reaction occurs in the cytosol
Urea synthesis is regulated by bulk dietary protein and two key enzymes
High-protein meals increase urea production
Urea synthesis is affected by dietary protein
AGA activates CPS-I to initiate urea synthesis
CSP-I is a key enzyme in the initiation of the ornithine cycle
Argininosuccinic acid synthesis
Argininosuccinate synthase has the lowest activity and is the key enzyme after the initiation of urea synthesis. It can regulate the synthesis rate of urea.
Impairment of urea production can cause hypertensive ammonia or ammonia poisoning
Severe damage to liver function or hereditary defects in urea synthesis-related enzymes can lead to disorders of urea synthesis and an increase in blood ammonia concentration, which is called hyperammonemia.
Clinical symptoms: vomiting, anorexia, intermittent ataxia, lethargy and even coma
Hyperammonemia can cause brain dysfunction, called ammonia poisoning
Metabolism of individual amino acids
Decarboxylation of amino acids requires decarboxylase catalysis
Some amino acids can generate corresponding amines through decarboxylation
The enzyme that catalyzes the decarboxylation reaction is called decarboxylase. The coenzyme of amino acid decarboxylase is pyridoxal phosphate.
Amine oxidase is a flavoprotein and is most active in the liver
Decarboxylation of glutamate to gamma-aminobutyric acid
L-glutamic acid decarboxylase is highly active in brain and kidney tissues
CABA is an inhibitory neurotransmitter that has an inhibitory effect on the central nervous system (the most widely distributed inhibitory neurotransmitter in the central nervous system of vertebrates)
Decarboxylation of histidine to produce histamine
Histidine is deaminated by histidine decarboxylase to produce histamine.
Histamine is widely distributed in the body, with higher levels in the breast, lung, liver and gastric mucosa, and is mainly found in mast cells.
effect
Histamine is a potent vasodilator and increases capillary permeability
Histamine can also promote gastric mucosal cells to secrete pepsinogen and gastric acid
Histamine can cause smooth muscle contraction, causing bronchospasm and leading to asthma
Antihistamines are used against allergies and to treat bronchial asthma and gastritis
Tryptophan undergoes hydroxylation and decarboxylation to form 5-hydroxytryptamine
5-HT is widely distributed in various tissues in the body. In addition to nervous tissue, it also exists in the stomach, intestines, platelets and breast cells.
effect:
5-hydroxytryptamine is a neurotransmitter that has inhibitory effects and directly affects nerve conduction
Serotonin has a strong vasoconstrictive effect in peripheral tissues
Decarboxylation of certain amino acids can produce polyamines
Ornithine decarboxylase is the key enzyme for polyamine synthesis. Spermine and spermidine are important substances that regulate cell growth. Polyamines are important substances that regulate cell growth.
In clinical practice, polyamine levels in patients' blood or urine are often measured as one of the biochemical indicators for auxiliary diagnosis of tumors and changes in disease conditions.
Certain amino acids produce one-carbon units during catabolism
Tetrahydrofolate participates in one-carbon unit metabolism as a carrier of one-carbon units
One-carbon unit refers to an organic group containing one carbon atom produced during the catabolism of certain amino acids.
Including methyl (–CH3), methylene (–CH2–), methine (=CH–), formyl (–CHO) and iminomethyl (–CH=NH), etc.
One-carbon units cannot exist free, and tetrahydrofolate is the carrier of one-carbon units.
One-carbon units produced from amino acids are interconvertible
One-carbon units mainly come from the catabolism of serine, glycine, histidine and tryptophan
The main function of the one-carbon unit is to participate in the synthesis of purine and pyrimidine
The one-carbon unit produced during the catabolism of amino acids can be used as a raw material for the synthesis of purine and pyrimidine
application
The application of sulfa drugs can inhibit the synthesis of dihydrofolate by certain bacteria, thereby inhibiting bacterial reproduction, but has little effect on the human body.
The use of folic acid analogs such as methotrexate can inhibit the production of FH4, thereby inhibiting the production of nucleic acids and playing an anti-tumor role.
Metabolism of sulfur-containing amino acids can produce a variety of biologically active substances
Sulfur-containing amino acids include methionine, cysteine, and cystine
Methionine participates in methyl transfer reaction
Methionine transmethylation is related to the methionine cycle
Modify the structure of DNA to control gene expression/modify non-nutritional substances to inactivate them
Catalyzed by adenosyltransferase, SAM is called active methionine, which is the most important direct methyl donor in the body
Physiological significance of the methionine cycle:
1N'5-CH3-FH4 provides a methyl group to generate methionine, and then provides a methyl group through SAM to carry out methylation reactions that are widespread in the body
Promote FH4 regeneration
Insufficient vitamin B12 can cause megaloblastic anemia, and elevated levels of homocysteine in the blood may be an independent risk factor for atherosclerosis and coronary heart disease.
Methionine-based creatine is synthesized and provides a methyl group
Creatine and creatine phosphate are important compounds for energy storage and utilization. Creatine phosphate, as a storage form of energy, is abundant in cardiac muscle, skeletal muscle and brain tissue.
Creatine is synthesized with glycine as the backbone, the amidine group provided by arginine, and the methyl group provided by S-adenosylmethionine. The liver is the main organ that synthesizes creatine
Creatine kinase is mainly composed of two subunits, namely M subunit and B subunit, which together constitute three isoenzymes: MM skeletal muscle, MB cardiac muscle, and BB brain.
When myocardial infarction occurs, the activity of MB-type creatine kinase in the blood increases, so it can be used as one of the auxiliary diagnostic indicators of myocardial infarction.
The end metabolite of creatine and creatine phosphate is creatinine
Creatinine is produced primarily in skeletal muscle through the nonenzymatic reaction of creatine phosphate. Measurement of creatinine in blood helps diagnose renal insufficiency
Cysteine is related to the production of various physiologically active substances
Cysteine and cystine are interconvertible
The disulfide bond formed between two cysteine residues plays an important role in maintaining the stability of protein spatial conformation and its function.
There are many important enzymes in the body, such as succinate dehydrogenase, lactate dehydrogenase, etc. Their activity is directly related to the sulfhydryl group of cysteine, so it is called sulfhydryl enzyme.
Cysteine can be converted into taurine
Sulfuric acid is one of the components of conjugated bile acids
Cysteine can generate active sulfate radicals
Cysteine is the main source of sulfate in the body
PASA has active chemical properties and can provide sulfate radicals during liver biotransformation to generate sulfate esters from certain substances.
Aromatic amino acid metabolism requires oxygenase catalysis
Aromatic amino acids include phenylalanine, tyrosine and tryptophan. Tyrosine can be produced by hydroxylation of phenylalanine.
Phenylalanine and tyrosine metabolism are both related and different
Hydroxylation of phenylalanine to tyrosine
The main metabolic pathway of phenylalanine is to generate tyrosine through hydroxylation, and the reaction is catalyzed by phenylalanine hydroxylase
Phenylalanine hydroxylase mainly exists in tissues such as the liver. It is a monooxygenase and its coenzyme is tetrahydrobiopterin.
Inherited metabolic diseases: The accumulation of phenylpyruvic acid is toxic to the central nervous system, leading to brain development disorders and mental retardation in children.
Tyrosine is converted into catecholamines and melanin or completely oxidized and decomposed
Tyrosine is catalyzed by tyrosine hydroxylase to produce dopa in the adrenal medulla and nervous tissue.
Dopamine is a neurotransmitter, and the production of dopamine is reduced in the brains of patients with Parkinson's disease.
Another pathway of tyrosine metabolism is the synthesis of melanin
Patients with congenital tyrosinase deficiency have white skin and hair due to their inability to synthesize melanin, which is called albinism.
When the enzyme that catabolizes homogentisic acid in the body is congenitally defective, the decomposition of homogentisic acid is blocked, and alkaptonuria may occur.
Tyrosine catabolism produces pyruvate and acetoacetyl COA
The decomposition of branched chain amino acids has a similar metabolic process
Branched-chain amino acids include valine, leucine, and isoleucine, and catabolism occurs primarily in skeletal muscle
In addition to being used as raw materials for protein synthesis, amino acids can also be converted into neurotransmitters, hormones and other important nitrogen-containing physiologically active substances.