Untitled Quiz

Protein Metabolism

• Protein metabolism is the metabolism of amino acids.
• Amino acid metabolism is part of nitrogen metabolism in the body.
• Dietary protein is the source of nitrogen entering the body.
• Dietary proteins are broken down for the formation of tissue proteins.
• There is a continuous breakdown of endogenous tissue proteins and synthesis.
• Dietary proteins are the primary source of nitrogen metabolized by the body.
• Amino acids produced by digestion enter the blood.
• Various cells take up those amino acids forming cellular pools.
• These cellular pools are used for the synthesis of proteins and other nitrogen-containing compounds, or are oxidized for energy.
• Amino acids are either directly oxidized or converted to glucose, then oxidized or stored as glycogen.
• Amino acids may be converted to fatty acids and stored as adipose triacylglycerols.
• Glycogen and triacylglycerols are oxidized during fasting
• The liver is the main site of amino acid oxidation, however, most tissues can oxidize branched-chain amino acids (leucine, isoleucine, and valine).
• Before amino acid carbon skeletons are oxidized, the nitrogen must be removed.
• Amino acid nitrogen forms ammonia, which is toxic to the body.
• In the liver, ammonia and amino groups from amino acids are converted to urea and excreted in urine.
• The process by which urea is produced is known as the urea cycle.
• The liver is responsible for producing urea.
• Branched-chain amino acids can be oxidized in many tissues, but nitrogen must always travel to the liver for disposal.
• Table 1 shows major nitrogenous urinary excretory products (urea, NH4+, creatinine, uric acid). Amounts excreted vary by gender.
• Dietary proteins undergo digestion in the stomach & intestine.
• Proteolytic enzymes (proteases) break down dietary proteins into the constituent amino acids.
• Proteases are synthesized as inactive forms known as zymogens, which are cleaved to produce active proteases
• Pepsin, in the stomach, initially digests proteins to smaller polypeptides.
• Pancreatic proteases (trypsin, chymotrypsin, elastase, carboxypeptidases) further cleave polypeptides into oligopep- tides and amino acids.
• Oligopeptides are cleaved to amino acids by enzymes from intestinal epithelial cells (aminopeptidases).
• Amino acids are absorbed into cells by active and facilitative transport systems.
• Some systems use facilitative transporters, others use sodium- linked transporters.
• Protein defects can result in disease.
• Proteins are continually synthesized and degraded in cells.
• Lysosomal proteases (cathepsins) degrade proteins.
• Cytoplasmic proteins targeted to be degraded are linked to ubiquitin and go to the proteasome.
• Released amino acids can be used to make new proteins or for energy.

Protein Digestion and Amino Acid Absorption

• Proteolytic enzymes (proteases) break down dietary proteins into their constituent amino acids.
• These proteases are usually synthesized as zymogens.
• Zymogens are secreted into the digestive tract, where they are cleaved to produce active proteases.
• In the stomach, pepsin begins the digestion of proteins by hydrolyzing them to smaller polypeptides.
• The contents of the stomach pass into the small intestine where pancreatic proteases continue the digestion.
• Pancreatic proteases (trypsin, chymotrypsin, elastase, carboxypeptidases) cleave the polypeptides into oligopeptides and amino acids.
• Further cleavage of oligopeptides into amino acids is accomplished by enzymes from intestinal epithelial cells (aminopeptidases).

Protein Digestion by Gastric Secretion

• Pepsinogen is secreted by chief cells of the stomach.
• Parietal cells of the stomach secrete HCl.
• Hydrochloric acid in the stomach alters the conformation of pepsinogen to produce pepsin, an active protease.
• The activation of pepsinogen is autocatalytic.
• Dietary proteins are denatured by the acid in the stomach.
• This denaturation makes proteins better substrates for intestinal proteases.
• Pepsin is a fairly broad specificity protease but favors cleaving peptide bonds in which the carboxyl group is provided by an aromatic or acidic amino acid.

Protein Digestion in the Small Intestine

• Most dietary proteins are not digested much in the mouth or stomach.
• Digestion of proteins is completed in the small intestine.
• Pancreatic secretions include bicarbonate which neutralizes the stomach acid and raises the pH for the optimal function of pancreatic proteases.
• Pancreatic proteases work in the inactive proenzyme form.
• Trypsinogen is cleaved and activated to trypsin by enteropeptidase from the brush border cells of the small intestine.
• Trypsin, in turn activates other pancreatic proteases.
• The enzymes (trypsin, chymotrypsin and elastase) are serine proteases that act as endopeptidases
• Other enzymes (carboxypeptidases A & B) are exopeptidases and cleave one amino acid at a time at either end of the peptide chain.
• The amino acids produced by protein digestion are absorbed through intestinal epithelial cells, entering the blood.

Summary of Amino Acid Metabolism

• The amino acid pool in the cells is formed by dietary amino acids and from the existing ones that were degraded in the cells.
• Proteins within cells have half-lives (time at which 50% of the protein is degraded).
• Intracellular degradation is by either lysosomes or the ubiquitin/proteasome system.
• The amino acids then join the intracellular amino acid pool, or are used for new protein synthesis.

Metabolic Fates of Amino Acids

• Body protein biosynthesis, small peptide biosynthesis (e.g., glutathione), non protein nitrogenous compound synthesis (e.g., creatine, urea), and energy production (e.g., gluconeogenesis, ketone bodies).
• Deamination and transamination are crucial reactions in the metabolism of amino acids.

Metabolism of Amino Acids: Deamination

• Removal of ammonia from amino acids.
• Oxidative deamination—a major pathway involving glutamate dehydrogenase in mitochondria or amino acid oxidase in peroxisomes.
• Direct deamination (non-oxidative) by dehydration or desulphydration.

Transamination

• Transfer of amino groups from one amino acid to another, using a-ketoglutarate as a common amino group acceptor.
• Alanine transaminase (ALT) and aspartate transaminase (AST) are important examples.

Deamination by Dehydration

• Conversion of serine or threonine to pyruvate via dehydration.

Deamination by Desulfhydration

• Conversion of cysteine to pyruvate via desulfhydration.

Transdeamination

• Involves both transamination followed by oxidative deamination.
• A major pathway to funnel ammonia from various amino acids with a-Ketoglutarate being the acceptor.

The Fate of Carbon Skeletons of Amino Acids

• Some amino acids are degraded to common metabolic intermediates (e.g., pyruvate, acetyl-CoA, α-ketoglutarate, oxaloacetate, and succinyl-CoA).
• Others undergo complex metabolic pathways to common intermediates.
• Some amino acids can be converted to other amino acids.

Metabolism of Common Intermediates

• Oxidation via the tricarboxylic acid (TCA) cycle for energy production.
• Synthesis of fatty acids or ketone bodies.
• Gluconeogenesis, which converts certain intermediates to glucose.

Metabolism of Ammonia

• Ammonia is a toxic byproduct of amino acid metabolism.
• Removal through various pathways including the urea cycle.
• Transamination and deamination of amino acids in the liver and kidney generates ammonia which is converted to urea, a non-toxic and water-soluble compound.
• Ammonia is also produced from other sources (e.g., physiological amines, purine nucleotides).

Metabolic Disposal of Ammonia

• Ammonia from amino acids and other compounds is converted to nontoxic forms for excretion.
• This can be accomplished by either: (a) converting it to glutamate to become glutamine, or (b) converting it into urea in the urea cycle.

Urea Formation and The Urea Cycle

• The urea cycle is a major pathway for converting ammonia to urea.
• The cycle includes several enzymes that occur within the mitochondria and cytosol of the liver.
• The urea cycle replenishes crucial intermediates for other metabolic pathways.

Regulation of Urea Cycle

• Activity of individual enzymes, including carbamoyl phosphate synthase I, ornithine transcarbamylase, and arginase.
• Regulation is affected by N-acetylglutamate which activates carbamoyl phosphate synthase I; and flux of ammonia, availability of aspartate, and ornithine (crucial components in the urea cycle).

Protein Turnover and Replenishment of the Intracellular Amino Acid Pool

• The amino acid pool within cells is formed by breakdown of existing proteins or from ingested dietary proteins.
• Proteins have a characteristic half-life, denoting the time during which half of them are degraded.
• Intracellular degradation is mainly carried out by the lysosomal pathway or the ubiquitin-proteasome pathway.

Digestion of Proteins

• The digestion of proteins begins in the stomach, where pepsinogen is activated by lowered pH to generate pepsin,
• The active pepsin then cleaves proteins into smaller polypeptides.
• Subsequent in the small intestine, by pancreatic proteases (trypsin, chymotrypsin, elastase, carboxypeptidases) the digestion of protein continues, leading to further breakdown of the polypeptides into free amino acids.
• The final products, amino acids, are then absorbed into the bloodstream from the intestinal cells.

Digestion of proteins in stomach

• Pepsinogen is secreted by chief cells of the stomach.
• Hydrochloric acid is secreted by parietal cells of the stomach.
• Activation of pepsinogen by lowered pH results in pepsin.
• Pepsin cleaves proteins into smaller polypeptides which eventually are exposed and cleaved by additional enzymes such as those in pancreatic secretions.

Digestion by Pancreatic Enzymes

• Bicarbonate in pancreatic secretions neutralizes low pH from the stomach.
• Pancreatic proteases are synthesized as zymogens.
• Pepsin, in the stomach, initially digests proteins to smaller polypeptides and other enzymes complete the digestion in the small intestine.
• These enzymes (trypsin, chymotrypsin, and elastase) act as endopeptidases cleaving peptide bonds in the interior.
• Protease enzymes such as carboxypeptidases A and B act as exopeptidases. They remove one amino acid at a time from the terminal ends of the peptide chain.
• Intestinal proteases (aminopeptidases— located on the brush border and other peptidases-intracellularly) cleave oligopeptides into individual amino acids for absorption into the bloodstream.

Absorption of Amino Acids

• Amino acids are absorbed by intestinal epithelial cells.
• They enter the bloodstream via facilitated and secondary active transport systems.

Lysosomal and Proteasome Protein Degradation

• Lysosomes contain proteases that degrade intracellular proteins.
• Damaged proteins are targeted for degradation via the ubiquitin-proteasome system.

Nitrogen Balance

• Nitrogen balance is the difference between nitrogen intake and nitrogen loss.
• Positive nitrogen balance occurs in growth, pregnancy, lactation, and recovery from illness.
• Negative nitrogen balance occurs in starvation, malnutrition, and certain diseases.
• The rate of synthesis and degradation may contribute to this balance

De Novo Synthesis of Pyrimidines

• Different from purine synthesis, the pyrimidine ring is generated before the ribose is attached.
• The sources are CO2, glutamine, and aspartate.
• The pathway starts with the synthesis of carbamoyl phosphate (by CPS II).

De Novo Synthesis of Purines

• The purine ring is assembled from components like glycine, aspartate, formate, and glutamine.
• The assembly starts on a ribose-5-phosphate (provided by PRPP) foundation.

Purine Salvage Pathways

• This pathway recycles purine bases or nucleosides obtained from daily diet or the normal turnover of nucleic acids.
• The salvage pathway generally uses less energy than if they were re-created from scratch.

Pyrimidine Salvage Pathways

• In this pathway, pre-assembled pyrimidines like uridine, cytidine, thymidine, and deoxycytidine from the food sources are reutilized to produce the corresponding nucleotides.

Degradation of Purines

• The purine ring remains intact during degradation and is converted to uric acid.
• Uric acid excretion via urine is the most common pathway.
• This pathway consists of several steps, starting from nucleotides and ending with uric acid as the final product.

Degradation of Pyrimidines

• Unlike purines, the pyrimidine ring is broken down into smaller fragments.
• The cytosol of the liver is the primary site of degradation.
• Some of the smaller fragments are reutilized, whereas others are excreted as waste products.

Diseases associated with purine degradation: Gout

• Gout is a metabolic disorder characterized by high levels of uric acid in the bloodstream and fluids.
• Uric acid may precipitate out in the joints, causing severe pain and inflammation.

Diseases associated with purine degradation: Lesch-Nyhan syndrome.

• Lesch-Nyhan syndrome is a genetic disorder affecting purine salvage pathways.
• It is caused by a deficiency in HGPRT leading to high uric acid levels.
• The deficiency causes several complications including brain damage, self-mutilation, and kidney dysfunction.

Overview of Fatty Acid Synthesis

• Involves the elongated fatty acid chain to provide ATP and NADPH.
• Major enzyme is Acetyl-CoA carboxylase (ACC).
• This enzyme catalyzes the committed step of fatty acid synthesis and converts acetyl-CoA to malonyl-CoA.

Regulation of Fatty Acid Synthesis

• Allosteric regulation by citrate, fatty acyl-CoA, hormone- mediated regulation by insulin, and glucagon.
• Long-term regulation involves changes in gene expression that affect the amounts of the enzymes responsible for fatty acid synthesis.

Fatty Acid Chain Elongation and Desaturation

• Fatty acid elongation occurs in the endoplasmic reticulum (ER) and utilizes malonyl-CoA.
• Desaturation adds double bonds to fatty acids; it occurs in the ER and involves cytochrome b5 and NADPH.
• These processes lead to a wide diversity of fatty acid structures within the organism, which is important for various biological functions such as membrane structure and hormone synthesis.

Lipogenesis

• Fatty acids are stored in adipose tissue in the form of triacylglycerols.
• The pathway begins with glycerol-3-phosphate, which acts as a substrate for the sequential addition of activated fatty acyl- CoA to form triacylglycerols (TAGs).
• G3P is formed in the liver from glycerol or DHAP from glycolysis.
• In adipose tissue, only DHAP converted to G3P is possible.

Lipolysis

• Lipolysis (breakdown of TAG) is initiated by hormones (e.g., glucagon, epinephrine).
• These hormones activate protein kinase A (PKA), which phosphorylates perilipin proteins and other enzymes.
• Phosphorylation of perilipin allows hormone-sensitive lipase (HSL) to access the TAG stored in lipid droplets.
• This leads to the release of free fatty acids and glycerol.
• Free fatty acids are carried in blood bound to albumin.
• Glycerol is taken up by the liver, which then converts it to glycerol-3-phosphate for gluconeogenesis.

Ketone Body Synthesis (Ketogenesis)

• The liver converts excess acetyl-CoA into ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone).
• Ketogenesis occurs when the rate of fatty acid oxidation exceeds the capacity of the TCA cycle to utilize the resulting acetyl-CoA.
• Key regulatory step is HMG-CoA synthase.
• The ketone bodies freely circulate in blood, providing an alternative fuel source for tissues that cannot use fatty acids directly.

Ketone Body Utilization

• Tissues (e.g., muscle, brain, heart) take up ketone bodies.
• They are reconverted into acetyl-CoA, which enters the TCA cycle, yielding ATP.
• ẞ-Hydroxybutyrate dehydrogenase is critical for the conversion of β-hydroxybutyrate to acetoacetate.
• Acetoacetate can be further metabolized to two acetyl-CoA molecules which then enter the TCA cycle and are fully oxidized to produce ATP.

Cholesterol Metabolism

• Cholesterol is the major sterol in animal tissues.
• There are several pathways in mammals for cholesterol synthesis and degradation.
• Most cells can synthesize cholesterol.
• Cholesterol synthesis occurs in the endoplasmic reticulum (ER) and includes a series of enzymatic steps.
• The most crucial step is the reduction of HMG-CoA to mevalonate, catalyzed by the enzyme HMG-CoA reductase.
• Cholesterol is transported in the blood as part of lipoproteins, which include VLDL, LDL, and HDL.

Degradation of Cholesterol

• Cholesterol is largely eliminated from the body as bile acids.
• This involves the introduction of a hydroxyl group at specific locations in the cholesterol molecule, followed by additional conversion steps—such as reduction, oxidation, and conjugation reactions.
• Secondary bile acids are formed by microbial action in the intestine, contributing to the overall regulation of cholesterol levels.

Functions of Bile Salts

• Cholesterol's conversion to bile salts is a crucial step for cholesterol excretion.
• Bile acids and bile salts are amphipathic compounds (having both hydrophobic and hydrophilic components).
• They serve as emulsifying agents in the digestive tract.
• Emulsification is a critical step in the absorption of dietary lipids.

Synthesis of Steroid Hormones

• Cholesterol serves as the starting material for the synthesis of steroid hormones.
• Different steroid hormones, including glucocorticoids, mineralocorticoids, and sex hormones (androgens and estrogens).
• The pathway of synthesis begins in the mitochondria, where cholesterol is converted to pregnenolone.
• Then the synthesis continues in the smooth endoplasmic reticulum (SER) with specific enzymes and reactions.
• Enzymes such as 3-hydroxysteroid dehydrogenase play critical roles in the transformation of intermediate products to produce the various steroid hormones.

Synthesis of Vitamin D

• Vitamin D3 (cholecalciferol) is synthesized from 7- dehydrocholesterol in the skin in response to UV light.
• It undergoes two hydroxylation steps in the liver and kidney, converting it to the active form, 1,25-dihydroxycholecalc- iferol (calcitriol).
• Calcitriol is involved in calcium homeostasis and bone metabolism

Lipoproteins: Structure and Function

• Lipoproteins are complex particles with a core of triacylglycerols (TAGs) and cholesteryl esters (CEs), surrounded by a phospholipid and apolipoprotein monolayer.
• Lipoproteins are classified basedon density; including chylomicrons, VLDL (very low-density lipoproteins), IDL (intermediate-density lipoproteins), LDL (low density lipoproteins), and HDL (high-density lipoproteins).

Lipoprotein Metabolism: Chylomicrons

• Chylomicrons are the primary transporters of dietary lipids (TAGs, cholesterol esters, and fat-soluble vitamins) from the intestine to peripheral tissues.
• In the small intestine, chylomicrons are synthesized, packaged, and released into the lymphatic system.
• They are then incorporated into the bloodstream.
• Lipoprotein lipase (LPL) in capillary walls breaks down chylomicron TAGs.
• The remnants, containing cholesteryl esters (CE), are taken up by the liver.

Lipoprotein Metabolism: VLDLs

• VLDLs (very-low-density lipoproteins) are synthesized and secreted by the liver to transport TAGs and cholesterol from the liver to peripheral tissues.
• Through the action of LPL, VLDL lose TAGs to tissues;
• some VLDL remnants become intermediate-density lipoproteins (IDLs).
• IDLs lose more TAGs through LPL and become low- density lipoproteins (LDLs).
• The liver takes up some IDL.

Lipoprotein Metabolism: LDLs

• These lipoproteins are enriched with cholesteryl esters (CEs), primarily transporting cholesterol to peripheral tissues.
• LDL can be taken up by receptor-mediated endocytosis by specific tissues that need cholesterol.
• The excess cholesterol is used for building new membranes for hormones or is stored.
• LDL uptake by the liver regulates cholesterol levels in the body.

Lipoprotein Metabolism: HDLs

• HDLs (high-density lipoproteins) are involved in reverse cholesterol transport.
• HDLs pick up free cholesterol from peripheral tissues and other lipoproteins.
• It esterifies cholesterol to form cholesteryl esters (CEs) with the help of LCAT.
• The resulting CE-rich HDL transfers the CEs to the liver or steroidogenic tissues by a process called scavenger receptor class B (SR-B1)-mediated uptake.

Eicosanoids: Overview and Synthesis

• Eicosanoids are locally acting signaling molecules derived from the 20-carbon polyunsaturated fatty acid arachidonic acid.
• Their synthesis occurs in most cells but are not stored. The pathway involves the release of arachidonic acid from phospholipid membranes primarily via the activation of phospholipases A2 or C.
• After release, arachidonic acid can go through the synthesis of different types of eicosanoids depending on the enzymatic pathways, e.g. cyclooxygenase pathway, the lipoxygenase pathway, or cytochrome P450 enzyme/pathway.

Eicosanoid synthesis-Cyclooxygenase Pathway

• A group of hormones— including the prostaglandins, thromboxanes, and prostacyclins— are produced from arachidonic acid via a biochemical pathway involving cyclooxygenase enzymes.

Eicosanoid synthesis-Lipoxygenases pathway

• Involved in the synthesis of leukotrienes (LTs) and lipoxins.
• This pathway differs from cyclooxygenases in several aspects, including enzyme, substrates or reactions.
• A notable point is that the synthesis of LTC4, LTD4, and LTE4 involves glutathione (a tripeptide).

Eicosanoid synthesis-Cytochrome P450 Pathway

• The synthesis of HETEs, epoxides and di-HETEs, involves a different enzyme and a separate pathway.
• This process uses cytochrome P450 enzymes, molecular oxygen, and NADPH to produce different eicosanoids, like the HETEs, epoxides, and diHETEs.

Functions of Prostaglandins and Thromboxanes

• Prostaglandins and thromboxanes are a class of eicosanoid hormones; these molecules have diverse effects in regulating many biological functions.
• They play a major role in inflammation, blood pressure, uterine contractions, pain, and fever.
• These molecules are made from arachidonic acid.