Protein and Amino Acid Metabolism
Protein Digestion and Absorption
Dietary Proteins
Proteins undergo digestion in the gastrointestinal tract, starting from the mouth through to the colon.
Different Proteins
Structural
Muscle - Responsible for movement and strength, composed primarily of actin and myosin filaments.
Connective Tissue - Provides support and structure to various body parts, including tendons, ligaments, and cartilage, and is composed of collagen and elastin fibers.
Enzymes - Catalysts that accelerate biochemical reactions, made up of amino acids and play a crucial role in metabolic pathways.
Hormones - Chemical messengers that regulate physiological processes, including growth, metabolism, and reproduction, composed of peptide or steroid structures.
Receptors - Proteins located on cell surfaces or within cells that bind to specific molecules (ligands), initiating a cellular response and facilitating communication between cells.
Transport - Molecules that facilitate the movement of substances across cell membranes, including nutrients, ions, and waste products, often utilizing specific transporter proteins or channels.
Immunoproteins - Specialized proteins that play critical roles in the immune system, including the identification and neutralization of pathogens such as bacteria and viruses.
Albumins - Mostly found in seeds. A class of proteins found in the blood plasma that help maintain osmotic pressure and transport hormones, vitamins, and drugs, as well as providing a reservoir of amino acids for metabolic processes.
Globulins - Mostly found in seeds. A group of proteins in the blood plasma that are essential for immune function, transport, and regulation; they include antibodies, enzymes, and carriers for lipids and fat-soluble vitamins. High in beans. Low in sulfur-containing amino acids (Cysteine, methionine)
Prolamins - A class of storage proteins primarily found in cereal grains, such as wheat and maize, which serve as a source of amino acids and energy for germinating seeds. High in cereal grains. High in proline, low in lysine.
Glutelins - A type of storage protein found in cereal grains, particularly rice and wheat, that provides a reservoir of amino acids and serves as a source of energy for the growth of seedlings.
Stages of Protein Digestion
Mouth
Initial mechanical digestion.
Stomach
Gastric juices convert proteins into large peptides, primarily by pepsins.
Small Intestine
Enterocytes play a crucial role in further digestion.
Duodenum releases entero-peptidase, which activates pancreatic proteases that continue breaking down peptides into smaller amino acids and peptides for absorption.
Pancreatic Secretions
Pancreatic trypsinogen is activated by enterokinase on the enterocyte apical membrane.
Activated trypsin can then activate further enzymes:
Trypsinogen → Trypsin
Chymotrypsinogen → Chymotrypsin
Proelastases → Elastases
Procarboxypeptidases A and B→ Carboxypeptidases A and B
Pancreatic Endoproteases
Trypsins - Endopeptidases that cleave peptide bonds at the carboxyl side of lysine and arginine residues, essential for protein digestion in the small intestine.
Chymotrypsins - Endopeptidases that preferentially hydrolyze peptide bonds involving aromatic amino acids such as phenylalanine, tryptophan, and tyrosine, playing a crucial role in further protein digestion and the release of smaller peptides.
Elastases - Endopeptidases that specifically target peptide bonds adjacent to small, hydrophobic amino acids like alanine, glycine, and valine, facilitating the degradation of elastin and other protein substrates, thus aiding in protein digestion.
Digestion Process in Small Intestine
Proteins and large peptides are broken down into oligopeptides by pancreatic endoproteases.
Pancreatic Exoproteases:
Carboxypeptidases convert peptides into individual amino acids.
Final products include smaller peptides and amino acids.
Enterocyte Brush-Border Peptidases
Aminopeptidases and Dipeptidase further reduce oligopeptides into tri- and dipeptides, and free amino acids for absorption.
Colon
Bacterial enzymes further act on any undigested/unabsorbed peptides and amino acids, producing nitrogenous products such as:
Ammonia
Amines
Other nitrogenous compounds
Synthesis of amino acids and other compounds for bacterial growth.
Urea Cycle
Urea produced from ammonia in the liver can enter the feces as nitrogenous waste.
What are the steps of the urea cycle in depth?
The urea cycle consists of a series of enzymatic reactions that convert ammonia into urea, which can then be excreted. The steps are as follows:
Formation of Carbamoyl Phosphate: Ammonia combines with bicarbonate and ATP in the mitochondria, catalyzed by the enzyme carbamoyl phosphate synthetase I, forming carbamoyl phosphate.
Citrulline Formation: Carbamoyl phosphate then reacts with ornithine to produce citrulline, with the help of the enzyme ornithine transcarbamylase.
Argininosuccinate Formation: Citrulline is transported out of the mitochondria into the cytoplasm, where it combines with aspartate to form argininosuccinate, catalyzed by argininosuccinate synthetase.
Arginine and Fumarate Production: Argininosuccinate is then cleaved by argininosuccinate lyase to yield arginine and fumarate.
Urea Generation: Finally, arginine is hydrolyzed by arginase to produce urea and regenerate ornithine, allowing the cycle to continue.
Overall, the urea cycle is crucial for detoxifying ammonia, with each step facilitated by specific enzymes.
The nitrogens in the urea cycle primarily come from two sources: the deamination of amino acids and the hydrolysis of ammonia. During deamination, amino groups are removed from amino acids, producing ammonia, which is subsequently converted into urea.
Ornithine serves as a key intermediate in the urea cycle, acting as a carrier for nitrogen and facilitating the conversion of ammonia into urea through a series of enzymatic reactions.
Allosteric regulation
N-acetylglutamate synthetase
is an essential enzyme in the urea cycle, as it catalyzes the formation of N-acetylglutamate, which functions as an allosteric activator of carbamoyl phosphate synthetase I. This regulation ensures that the urea cycle is activated when amino acid catabolism increases, thereby enhancing the conversion of excess nitrogen into urea for excretion.
More synthesized increases activity of carbamoyl phosphate synthetase 1, more carbomyl phosphate, more urea cycle
High concentration of arginine also means more urea cycle activity
Transport of Amino Acids
Amino acids primarily enter portal blood and move to various body tissues for metabolism.
Some metabolism of amino acids occurs in enterocytes, leading to nitrogen products like ammonia.
Regulation of Protein Digestion
Cholecystokinin (CCK)
Stimulates intestinal pancreas acinar cells to release proenzymes into the intestine (duodenum).
Absorption of Amino Acids
The absorption of amino acids is conceptually identical to monosaccharides.
Transport Mechanisms
At least four sodium-dependent carriers are involved.
Dependent on the electrochemical gradient of Na+.
At the basolateral membrane, transporters exist that do not depend on the Na+ gradient.
Tri- and dipeptides can be absorbed by co-transport with H+ via PEPT1.
Intact proteins cannot be absorbed intact.
BCAA + methionine usually prioritized to be absorbed before essential amino acids, then non-essential amino acids, then aspartate and glutamate
Hartnup Disease is a genetic disorder that affects the absorption of certain amino acids, particularly neutral amino acids like tryptophan, due to a defect in the transport system in the intestines, leading to various clinical manifestations including nutritional deficiencies.
Niacin is synthesized from tryptophan, highlighting the importance of this amino acid in maintaining adequate levels of vitamin B3, which is crucial for metabolic processes.
Fates of Amino Acids in Enterocytes
Pass unchanged through cytosol in and out of cell
Used by enterocytes for protein synthesis
Partially or completely oxidized for E
Used for synthesis of other N-cont. compound
Endogenous Proteins - Proteins synthesized within the body that play crucial roles in various physiological processes, including enzymatic reactions and cellular structure.
Amino Acid Dynamics in Various Body Tissues
Fed State
Dietary Protein + Endogenous Proteins digested
Small Intestine
Absorption of amino acids
Utilization of Glutamine - Glutamine serves multiple roles in the body, including acting as a primary energy source for rapidly dividing cells, maintaining gut integrity, and regulating acid-base balance.
Arterial Blood: Majority of amino acids (especially Alanine, Serine) are derived from dietary and endogenous proteins.
Liver: Responsible for protein synthesis and partial oxidation of amino acids.
50% of amino acids broken down by liver
BCAA can’t be broken down by liver
The liver makes urea as a means of removing excess nitrogen that is generated during the breakdown of amino acids, thus preventing toxic accumulation in the bloodstream.
Leaving liver are amino acids and net release of glutamate
Muscle and Kidney: Also participates in protein synthesis, ammoniagenesis, and partial oxidation of amino acids.
Ammonia that kidneys excrete is made from the deamination of amino acids, particularly during the metabolism of glutamine and other amino acids, which generate nitrogen waste products that must be removed from the body.
Fasted State
Increased reliance on amino acid pool from muscle and other tissues for energy production during prolonged fasting.
In the kidney, there is an increase in ammoniagenesis and excretion.
The kidney utilizes specific amino acids, particularly alanine and glutamine, which are deaminated to yield carbon skeletons that can enter gluconeogenic pathways, ultimately contributing to the production of glucose that is released into the bloodstream.
Metabolism of Amino Acids in the Liver
General Functionality
The liver plays a crucial role in the metabolism of amino acids.
Synthesis of nonprotein nitrogenous compounds occurs.
Example reactions involve transamination and deamination, influencing the carbon skeletons of amino acids.
Urea Cycle
The urea cycle is sensitive to dietary changes, can increase enzyme concentrations by over 20-fold during prolonged protein intake or starvation.
Key Regulatory Enzymes:
CPS-I: Allosterically activated by N-acetylglutamate.
Arginine acts as a positive effector for N-acetylglutamate synthetase, regulating urea cycle activity effectively.
Summary of Key Amino Acid Concentrations in Human Plasma
Key amino acid concentrations measured in micromoles (µM):
Glutamine (Gln): 656 (range 600-800)
Alanine (Ala): 360
Other significant amino acids include: Val, Leu, Ile, Gly, Glu, Met, Asp, with varying levels recorded across different states.
Essential Amino Acids
Histidine, Leucine, Isoleucine, Valine, Threonine, Methionine, Phenylalanine, Tryptophan, Lysine
Need to get these from food
Synthesis of Nonessential Amino Acids from the Carbon Skeletons of Amphibolic Intermediates
Glutamine: Formed from glutamate and ammonia
The synthesis involves the enzyme glutamine synthetase, which catalyzes the reaction where glutamate reacts with ammonia in the presence of ATP to produce glutamine.
Alanine: Synthesized from pyruvate
The enzyme alanine transaminase (ALT) facilitates this process by transferring an amino group from glutamate to pyruvate, resulting in the production of alanine and alpha-ketoglutarate.
Aspartate: Derived from oxaloacetate
This process involves the transamination reactions where amino groups are transferred between amino acids and keto acids.
Serine: Generated from 3-phosphoglycerate
The conversion of 3-phosphoglycerate to serine involves several enzymatic steps, including the reduction of 3-phosphoglycerate to 3-phosphohydroxypyruvate, followed by the transamination with glutamate or other amino donors to yield serine.
Glycine: Synthetically derived from serine via the enzyme serine hydroxymethyltransferase, which transfers a hydroxymethyl group from tetrahydrofolate to serine, producing glycine and tetrahydrofolate.
Arginine: Can be produced via ornithine and citrulline pathways
The ornithine pathway involves the conversion of ornithine and carbamoyl phosphate through a series of enzymatic reactions leading to arginine, while the citrulline pathway utilizes argininosuccinate synthase and argininosuccinate lyase enzymes to facilitate the conversion of citrulline and aspartate into arginine.
Proline: Synthesized from glutamate through an intermediate cycle.
This process involves the reduction of the intermediate, 1-pyrroline-5-carboxylate, back to proline, allowing for the regeneration of glutamate and completion of the metabolic cycle.
Transamination Reaction
The transamination reaction is a critical step in amino acid metabolism, where an amino group is transferred from an amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. This process is facilitated by transaminase enzymes and plays a vital role in the synthesis and degradation of amino acids, including the interconversion of different amino acids based on cellular needs.
Aminotransferase, also known as transaminase, catalyzes the transfer of an amino group from one amino acid to a keto acid, effectively facilitating the synthesis of new amino acids while simultaneously regenerating keto acids that can enter metabolic pathways.
Pyridoxal phosphate (PLP), a coenzyme derived from vitamin B6, is essential for the action of aminotransferases as it forms a temporary schiff base with the amino acid substrate, thereby facilitating the transfer of the amino group during transamination reactions.
Enzymes and Their Reactions Related to Amino Acid Metabolism
Glutaminase and Asparaginase are vital for the hydrolysis of nitrogenous compounds from glutamine and asparagine, respectively, leading to the production of ammonia and alpha-keto acids.
Glutamate Dehydrogenase (GLUD) catalyzes the conversion of glutamate to α-ketoglutarate, highlighting its role in nitrogen balance.
Carbamoyl Phosphate Synthetase I (CPS1) is a key enzyme in the urea cycle, linking ammonia and bicarbonate to form carbamoyl phosphate within the mitochondria.
Key regulator step in the urea cycle
Reflection of Metabolic Processes
Interconnected pathways of protein synthesis, oxidative deamination, gluconeogenesis, and ureagenesis illustrate the complexity of amino acid metabolism, particularly in response to physiological states such as fasting or feeding.
Understanding these processes opens possibilities for clinical applications in metabolic disorders and nutrition therapy.