Biochemistry of the liver

portal vein

hepatic artery

bile duct

sinusoids

bile canaliculi

central vein

LIVER STRUCTURELIVER STRUCTURE

Liver’s functions.

1. Liver is a main organ which is responsible for dividing of nutritional substances in our organism (for example, glucose, triacylglicerides and ketone bodies).

2. Hepatocytes synthesizes as lot of blood plasma proteins and lipoproteins, low-weight bioactive substances (creatin, 25-oxicalciferol, hem), cholesterol.

3. Synthesis of urea (final product of nitrogen metabolism) also takes place in the liver.

4. Liver synthesizes bile acids and excrete a bile into intestinal tract. This process plays a very important role in lipids digestion and excretion of cholesterin and some products of metabolism into intestine.

5. Liver play a big desintoxification role, inactivates endogenic and exogenic substances (drugs, some hormones, different toxins).

6. Liver is a depo for iron, some another metals, vitamines A, D, E, B12, folic acid.

Role of the liver in carbohydrate metabolism. From intestine glucose pass into the liver, where

most part of it undergone the phosphorillation. Glucose-6-phosphate formed in result of this reaction, which catalyzed by two enzymes – hexokinase and glucokinase.

Glucose-6-phosphate is a key product of carbohydrates metabolism. In the liver this substance can metabolized into different ways depend of liver’s and whole organism’s necessity.

Glucose

The fate of glucose molecule in the cell

Glucose-6-phosphate

Pyruvate

Glycogen

Ribose, NADPH

Pentose phosphate pathway supplies

the NADPH for lipid synthesis and

pentoses for nucleic acid synthesis

Glycogenesis (synthesis of glycogen) is

activated in well fed, resting state

Glycolysis is activated if

energy is required

Glycogenolysis (degradation of

glycogen)

Gluconeogenesis is activated if glucose is

required

TCA cycle

1. Synthesis of glycogen. Content in the liver – 70-100g 2. Glucose-6-phosphatase catalize dephosphorillation of

glucose-6-phosphate and formation of free glucose3. Excess of glucose-6-phosphate, which not used for synthesis

of glycogen will follow to form free glucose4. Glucose-6-phosphate decomposed to H2O and CO2, and free

energy for hepatocytes formed.5. Part of glucose-6-phosphate oxidized in pentosophosphate

cycle. 6. Hepatocytes content full set of gluconeogenesis necessary

enzymes. So, in liver glucose can be formed from lactate, pyruvate, amino acids, glycerol.

7. Gluconegenesis from lactate takes place during intensive muscular work. Lactate formed from glucose in muscles, transported to the liver, new glucose formed and transported to the muscles

Role of the liver in lipid metabolism.In the liver all processes of lipid metabolism take place. Most

important of them are following:Lipogenesis (synthesis of fatty acids and lipids). Substrate for this

process – acetyl-CoA, formed from glucose and amino acids, which are not used for another purposes

Liver more active than another tissues synthesizes saturated and monounsaturated fatty acids. Fatty acids then used for synthesis of lipids, phospholipids, cholesterol ethers.

Liver play a central role in synthesis of cholesterin, because near 80 % of its amount is synthesized there. Biosynthesis of cholesterin regulated by negative feedback. When the level of cholesterin in the meal increases, synthesis in liver decreases, and back to front. Besides synthesis regulated by insulin and glucagon.

Liver is a place of ketone bodies synthesis. These substances formed from fatty acids after their oxidation, and from liver transported to another tissues, first of all to the heart, muscles, kidneys and brain

Role of the liver in protein metabolism.Liver has full set of enzymes, which are necessary for

amino acids metabolism. Amino acids from food used in the liver for following pathways:

1. Protein synthesis.2. Decomposition for the final products.3. Transformation to the carbohydrates and lipids.4. Interaction between amino acids.5. Transformation to the different substances with amino

group.6. Release to the blood and transport to another organs

and tissues.

Liver synthesizes 100 % of albumins, 90 % of α1-globulines, 75 % of α2-globulines, 50 % of β-globulins, blood clotting factors, fibrinogen, protein part of blood lipoproteins, such enzyme as cholinesterase.

Liver can synthesize non-essential amino acids.

Liver synthesizes purine and pyrimidine nucleotides, hem, creatine, nicotinic acid, choline, carnitine, polyamines.

Role of the liver in detoxification processes.A xenobiotics is a compound that is foreign to the body. The

principal classes of xenobiotics of medical relevance are drugs, chemical cancerogens, and various compounds that have found their way into our environment by one route or another (insecticides, herbicides, pesticides, food additions, cosmetics, domestic chemical substances).

Some internal substances also have toxic properties (for example, bilirubin, free ammonia, bioactive amines, products of amino acids decay in the intestine).

Moreover, all hormones and mediatores must be inactivated.Reactions of detoxification take place in the liver. Big molecules like bilirubin excreted with the bile to intestine

and leaded out with feces. Small molecules go to the blood and excreted via kidney with urine.

REACTION ENZYME LOCALIZATION

PHASE I

Hydrolysis

Reduction

Oxidation

EsterasePeptidase Epoxide hydrolase

Azo- and nitro-reduction Carbonyl reductionDisulfide reductionSulfoxide reduction

Alcohol dehydrogenaseAldehyde dehydrogenaseAldehyde oxidaseXanthine oxidaseMonoamine oxidaseDiamine oxidaseFlavin-monooxygenasesCytochrome P450

Microsomes, cytosol, lysosomes, blood lysosomes Microsomes, cytosol

Microflora, microsomes, cytosolCytosol, blood, microsomesCytosolCytosol

CytosolMitochondria, cytosolCytosolCytosolMitochondriaCytosolMicrosomesMicrosomes

PHASE II

Glucuronide conjugationSulfate conjugationGlutathione conjugationAmino acid conjugationAcetylationMethylation

MicrosomesCytosol, microsomesCytosolMitochondria, cytosolMitochondria, microsomesCytosol, microsomes, blood

General ways of xenobiotics biotransformation and their localization in cell

The metabolism of xenobiotics has 2 phases:

In phase 1, the major reaction involved is hydroxylation, catalyzed by members of a class of enzymes referred to as monooxygenases or cytochrome P-450 species. These enzymes can also catalyze deamination, dehalogenation, desulfuration, epoxidation, peroxidation and reduction reaction. Hydrolysis reactions and non-P-450-catalyzed reactions also occur in phase 2.

Cytochrom P450

The highest concentration – in endoplasmic reticulum of hepatocytes (microsomes).

Hem containing protein.

Catalyzes monooxigenation of oxygen atom into substrate; another oxygen atom is reduced to water

Electrons are transferred from NADPH to cytochrome P450 through flavoprotein NADPH-cytochrome P450 reductase.

[1] In the resting state, the heme iron istrivalent. Initially, the substrate binds near the heme group.

[2] Transfer of an electron from FADH2 reduces the iron to the divalent form that is able to bind an O2 molecule.

[3] Transfer of a second electron and achange in the valence of the iron reduce the bound O2 to the peroxide.

[4] A hydroxyl ion is now cleaved from this intermediate. Uptake of a proton gives rise to H2O and the reactive form of oxygen mentioned above. In this ferryl radical, the iron is formally tetravalent.

[5] The activated oxygen atom inserts itself into a C–H bond in the substrate, thereby forming an OH group.

[6] Dissociation of the product returns theenzyme to its initial state.

In phase 2, the hydroxylated or other compounds produced in phase 1 are converted by specific enzymes to various polar metabolites by conjugation with glucuronic acid, sulfate, acetate, glutathione, or certain amino acids, or by methylation.

In certain cases, phase 1 metabolic reaction convert xenobiotics from inactive to biologically active compounds. In these instances, the original xenobiotics are referred to as prodrugs or procarcinogens. In other cases, additional phase 1 reactions convert the active compounds to less active or inactive forms prior to conjugation. In yet other cases, it is the conjugation reactions themselves that convert the active product of phase 1 to less active or inactive species, which are subsequently excreted in the urine or bile. In a very few cases, conjugation may actually increase the biologic activity of a xenobiotics.

There are at least 5 types of phase 2 reactions:1. Glucuronidation. UDP-glucuronic acid is the

glucuronyl donor, and a variety of glucuronyl transferases, present in both the ER and cytosol, are the catalysts. Molecules such as bilirubin, thyroxin, 2-acetylaminofluorene (a carcinogen), aniline, benzoic acid, meprobromate (a tranquilizer), phenol, crezol, indol and skatol, and many steroids are excreted as glucuronides. The glucuronide may be attached to oxygen, nitrogen, or sulfur groups of substrates.

2. Sulfation. Some alcohols, arylamines, and phenols are sulfated. The sulfate donor in these and other biologic sulfation reactions is adenosine 3´-phosphate-5´-phosphosulfate (PAPS); this compound is called active sulfate

3. Conjugation with Glutathione. Glutathione (γ-glutamylcysteinylglycine) is a tripeptide consisting of glutamic acid, cysteine, and glycine. Glutathione is commonly abbreviated to GSH; the SH indicates the sulfhydryl group of its cysteine and is the business part of the molecule. A number of potentially toxic electrophilic xenobiotics (such as certain carcinogens) are conjugated to the nucleophilic GSH. The enzymes catalyzing these reactions are called glutathione S-transferases and are present in high amounts in liver cytosol and in lower amounts in other tissues.

Acetylation. These reactions is represented by X + Acetyl-CoA → Acetyl-X + CoA, where X represents a xenobiotic. These reactions are catalyzed by acetyltransferases present in the cytosol of various tissues, particularly liver. The different aromatic amines, aromatic amino acids, such drug as isoniazid, used in the treatment of tuberculosis, and sulfanylamides are subjects to acetylation. Polymorphic types of acetyltransferases exist, resulting in individuals who are classified as slow or fast acetylators, and influence the rate of clearance of drugs such as isoniazid from blood.

5. Methylation. A few xenobiotics (amines, phenol, tio-substances, inorganic compounds of sulphur, selen, mercury, arsenic) are subject to methylation by methyltransferases, employing S-adenosylmethionine as methyl donor. Also catecholamines and nicotinic acid amid (active form of vitamin PP) are inactivated due to methylation.

Very important way of detoxification is ureogenes (urea synthesis). Free ammonia, which formed due to metabolism of amino acids, amides and amines, removed from organism in shape of urea.

Muscle structure

Proteins of muscles

3 types:•proteins of sarcoplasma•proteins of miofibrils•proteins of stroma

Proteins of Sarcoplasma•Miogen fraction

(enzymes of glycolysis etc.)•Albumins•Globulins•Myoglobin (chromoprotein, provides the red color to muscles, responsible for oxygen storage)

Proteins of Stroma

•collage•keratin•elastinare constituents of connective tissue of vessel walls, nerves, sarcolema.

Proteins of Miofibrils•Myosin (56-60 %)•Actin (20-25 %)•Tropomyosin (10-15 %)•Troponin complex (4-6 %)

Structure of filaments and myofibrils

Sarcoplasma of striated muscle fibers contains myofibrils oriented along which are built of 2 types protein filaments: thick and thin

•Muscle contraction is carried out due to the sliding of thick and thin filaments•Chemical energy – ATP hydrolysis•Contraction is regulated by Ca2+ concentration

Structure of Thick Filament•Thick filaments consist of myosin

molecules•Myosin molecule built of 2 heavy (200000 Da) and 4 light (16000-25000 Da) chains•Heavy chains are coiled around each other and form the “tail” of the molecule•2 light chains form the globular head of the molecule•The head has ATP-ase properties

About 400 molecules of myosin are combined in the thick filament

About half of molecules is directed to one end of filament, another half – to another end

The structure of thin filaments are proteins actin, tropomyosin andtroponinTwo forms of actin: globular G-actin and fibrillarF-actin.Globular actin molecules noncovalently connected to form F-actin.Two F-actin chains screwed into a spiral.In the groove spiral F-actin is located tropomyosin.With one molecule of tropomyosin contact 7 pairs of actin.At 1 tropomyosin molecule is 1 molecule of globular protein troponin.

•Two forms of actin: globular G-actin and F-actin fibrillar.Globular actin molecules nonсovalently connected to form F-actin.Two F-actin chains screwed into a spiral.Troponin is composed of 3 subunits (C, I, T).

Myofibrils contain approximately 2500 filaments.In a thick thread miozynovu contributes 6 thin

•structural unit of myofibrils - sarcomereboth ends of thick filaments myosin freethin filament with one end attached to the Z-disc

The sliding filament model describes themechanism involved in muscle contraction.

[1 ] In the initial state, the myosin heads areattached to actin. When ATP is bound, theheads detach themselves from the actin (the“plasticizing” effect of ATP).[2 ] The myosin head hydrolyzes the bound ATP to ADP and Pi, but initially withholds the two reaction products. ATP cleavage leads to allosteric tension in the myosin head.[3 ] The myosin head now forms a newbond with a neighboring actin molecule.[4 ] The actin causes the release of the Pi,and shortly afterwards release of the ADP as well. This converts the allosteric tension in the myosin head into a conformationalchange that acts like a rowing stroke.

Red fibers provide for their ATP requirementsmainly (but not exclusively) from fatty acids, which are broken down via β-oxidation,the tricarboxylic acid cycle, and the respiratory chain (right part of the illustration). The red color in these fibers is due to the monomeric heme protein myoglobin, which theyuse as an O2 reserve. Myoglobin has a much higher afinity for O2 than hemoglobin and therefore only releases its O2 when there isa severe drop in O2 partial pressure