Amino Acids & Protein Digestion/Absorption

Amino acids are the building blocks of proteins and play a central role in numerous metabolic pathways. Unlike carbohydrates and fats, the body has no dedicated storage form for amino acids. Instead, there's a dynamic "amino acid pool" that constantly receives and donates amino acids for various purposes.

The General Fates of Amino Acids

Once available in the body (either from diet, protein turnover, or de novo synthesis), amino acids follow several major metabolic pathways:

1. Protein Synthesis (Anabolism): This is the primary and most vital role of amino acids. They are precisely assembled into new proteins (structural, enzymatic, hormonal, transport, etc.) within cells according to genetic instructions. This process is continuous, as proteins have finite lifespans and are constantly being synthesized and degraded (protein turnover).
2. Synthesis of Non-Protein Nitrogenous Compounds: Amino acids are precursors for a vast array of other essential nitrogen-containing molecules that are not proteins. These include:
   • Neurotransmitters: e.g., dopamine, serotonin, GABA
   • Hormones: e.g., thyroid hormones, adrenaline (epinephrine)
   • Nucleotides: Components of DNA and RNA
   • Heme: The iron-containing component of hemoglobin
   • Creatine: Involved in energy storage in muscles
   • Polyamines: Involved in cell growth and differentiation
3. Catabolism (Breakdown for Energy or Other Metabolites): When amino acids are in excess, or when energy stores (carbohydrates and fats) are insufficient, amino acids can be catabolized. This involves:
   • Removal of the Amino Group: The nitrogen-containing amino group is removed (primarily as ammonia), which is then typically converted to urea for excretion.
   • Metabolism of the Carbon Skeleton: The remaining carbon skeleton (α-keto acid) can be:
     • Oxidized directly for energy (e.g., to Acetyl-CoA, TCA cycle intermediates).
     • Converted into glucose (via gluconeogenesis).
     • Converted into ketone bodies (via ketogenesis).
     • Converted into fatty acids for storage.

Protein Digestion and Absorption

The body acquires amino acids primarily from the diet through the breakdown of ingested proteins. This process occurs in several stages:

In the Stomach:

• Denaturation: Dietary proteins first encounter the highly acidic environment of the stomach (pH 1.5-3.5) due to hydrochloric acid (HCl) secreted by parietal cells. This low pH causes proteins to denature, unfolding their complex three-dimensional structures and making them more accessible to enzymatic degradation.
• Pepsin Activity: Chief cells in the stomach secrete pepsinogen, a zymogen (inactive enzyme precursor). HCl cleaves pepsinogen to its active form, pepsin. Pepsin is an endopeptidase, meaning it hydrolyzes peptide bonds within the protein chain, preferentially cleaving bonds involving aromatic amino acids. This produces a mixture of smaller polypeptides and some oligopeptides.

In the Small Intestine (Duodenum):

• Neutralization: As the acidic chyme (partially digested food) moves from the stomach into the duodenum, its acidity stimulates the release of secretin and cholecystokinin (CCK). Secretin stimulates the pancreas to release bicarbonate, which neutralizes the stomach acid, raising the pH to around 7. This optimal pH is crucial for the activity of pancreatic proteases.
• Pancreatic Proteases: The pancreas secretes a cocktail of zymogens, including:
  • Trypsinogen: Activated by enteropeptidase (also called enterokinase), an enzyme on the intestinal brush border, to form trypsin. Trypsin is a key enzyme because it then activates all other pancreatic zymogens.
  • Chymotrypsinogen: Activated by trypsin to form chymotrypsin.
  • Proelastase: Activated by trypsin to form elastase.
  • Procarboxypeptidases A and B: Activated by trypsin to form carboxypeptidases A and B.
• Endopeptidases (Trypsin, Chymotrypsin, Elastase): These enzymes continue to hydrolyze internal peptide bonds within the polypeptides, breaking them down into smaller oligopeptides and tri- and di-peptides. Trypsin preferentially cleaves at basic amino acids (lysine, arginine), while chymotrypsin prefers aromatic amino acids (phenylalanine, tyrosine, tryptophan).
• Exopeptidases (Carboxypeptidases A and B): These enzymes remove amino acids one by one from the carboxyl (C-terminal) end of the polypeptide chains, producing free amino acids.

At the Intestinal Brush Border and Within Enterocytes:

• Brush Border Peptidases: The surface of the enterocytes (intestinal absorptive cells) contains various aminopeptidases and dipeptidases. Aminopeptidases cleave amino acids from the amino (N-terminal) end of oligopeptides. Dipeptidases and tripeptidases hydrolyze di- and tripeptides into free amino acids.
• Absorption into Enterocytes:
  • Free Amino Acids: Absorbed by specific Na⁺-dependent co-transporters on the apical membrane (lumen side) of enterocytes. Different transporters exist for different classes of amino acids (e.g., neutral, basic, acidic).
  • Di- and Tri-peptides: A significant portion of di- and tri-peptides are absorbed intact into the enterocytes via a separate proton-dependent cotransporter (PepT1).
• Intracellular Hydrolysis: Once inside the enterocyte, most absorbed di- and tri-peptides are further hydrolyzed into free amino acids by intracellular peptidases.
• Exit into Bloodstream: The free amino acids are then transported across the basolateral membrane (facing the bloodstream) into the portal circulation, primarily via facilitated diffusion and other transporters, and delivered to the liver.

Cholesterol Metabolism

Cholesterol often gets a bad rap due to its association with heart disease, but it's crucial to understand that it is an essential molecule for life. Our bodies need cholesterol to function properly. The problem arises when its levels are imbalanced or when it's handled improperly within the body.

Importance and Physiological Functions of Cholesterol

Cholesterol is a lipid belonging to the steroid family. Its unique amphipathic structure (a polar hydroxyl group and a nonpolar steroid ring system and hydrocarbon tail) allows it to insert into cell membranes, giving it critical structural and signaling roles.

• Essential Component of Cell Membranes:
  • Cholesterol is a major constituent of virtually all animal cell membranes.
  • It modulates membrane fluidity, permeability, and stability, acting as a "buffer": at high temperatures, it stiffens the membrane, while at low temperatures, it prevents rigidity.
  • It is particularly abundant in myelin sheaths, enhancing nerve signal transmission.
• Precursor for Steroid Hormones:
  • Cholesterol is the obligate precursor for all five major classes of steroid hormones: Glucocorticoids (e.g., Cortisol), Mineralocorticoids (e.g., Aldosterone), Androgens (e.g., Testosterone), Estrogens (e.g., Estradiol), and Progestogens (e.g., Progesterone).
• Precursor for Bile Acids (and Bile Salts):
  • In the liver, cholesterol is converted into primary bile acids.
  • Bile acids emulsify dietary fats in the small intestine, facilitating their absorption. This is the primary way the body eliminates excess cholesterol.
• Precursor for Vitamin D Synthesis:
  • 7-Dehydrocholesterol, a precursor in the cholesterol synthesis pathway, is converted to pre-vitamin D3 in the skin upon exposure to UV light.
  • This is then converted to the active hormone, calcitriol, essential for calcium homeostasis.

Sources of Cholesterol

The body acquires cholesterol from two main sources:

• Endogenous Synthesis (De Novo Synthesis):
  • The vast majority of cholesterol (about 80%) is synthesized internally, primarily in the liver (~50% of total synthesis), but also in the intestine, adrenal cortex, and reproductive organs.
• Dietary Intake (Exogenous Cholesterol):
  • Cholesterol is consumed in the diet, found exclusively in animal products (meat, eggs, dairy). Plant foods do not contain cholesterol.
  • The amount absorbed can vary significantly among individuals.

Absorption of Dietary Cholesterol

The process of dietary cholesterol absorption primarily occurs in the small intestine:

• Emulsification: Dietary cholesterol esters are emulsified by bile salts into smaller micelles.
• Hydrolysis: Cholesterol Esters (CE) are hydrolyzed into free cholesterol (FC) by pancreatic cholesterol esterase. Only free cholesterol can be absorbed.
• Micelle Formation: Free cholesterol and other digested lipids become incorporated into mixed micelles with bile salts.
• Uptake by Enterocytes:
  • Mixed micelles diffuse to the brush border of the intestinal enterocytes.
  • The primary transporter responsible for cholesterol uptake is the Niemann-Pick C1-Like 1 (NPC1L1) protein. This transporter is the target of the drug ezetimibe.
• Intracellular Processing and Re-esterification:
  • Once inside the enterocyte, free cholesterol can be either effluxed back into the lumen via ABC G5/G8 transporters or re-esterified to cholesterol esters by the enzyme Acyl-CoA Cholesterol Acyltransferase 2 (ACAT2).
• Chylomicron Assembly and Secretion:
  • The newly formed cholesterol esters and re-formed triacylglycerols are packaged with apolipoproteins (primarily apoB-48) into large lipoprotein particles called chylomicrons.
  • Chylomicrons are then released into the lymphatic system, which eventually drains into the bloodstream.

Integrated Metabolism and Fuel Homeostasis

Fuel Homeostasis refers to the dynamic equilibrium and finely tuned regulation of energy substrates (glucose, fatty acids, ketone bodies, amino acids) in the body. Its primary goal is to ensure a continuous and adequate supply of fuel to all tissues, particularly the brain, under varying physiological conditions.

It is crucial for survival, allowing the body to adapt to fluctuations in nutrient availability and energy demand. Disruptions lead to metabolic diseases like diabetes, obesity, and metabolic syndrome.

Key Metabolic Organs and Their Specialized Roles

The human body is a highly integrated system where different organs specialize in fuel storage, production, and utilization.

Liver (Hepatocytes): The Metabolic Hub

• Glucose Homeostasis: Central to maintaining blood glucose levels.
  • Fed State: Takes up excess glucose, converting it to glycogen (glycogenesis) or fatty acids (lipogenesis).
  • Fasting State: Releases glucose into the blood via glycogenolysis and gluconeogenesis.
• Lipid Metabolism: Site of de novo fatty acid synthesis, cholesterol synthesis, and VLDL assembly. It is also the primary site for ketogenesis during prolonged fasting.
• Amino Acid Metabolism: Site for amino acid uptake, protein synthesis, deamination, and the urea cycle.
• Lack of Ketone Body Utilization: Cannot use ketone bodies as fuel due to the absence of thiophorase.

Adipose Tissue (Adipocytes): The Energy Storehouse

• Storage: Primary site for the long-term storage of energy as triacylglycerols (TAGs).
• Mobilization: Releases free fatty acids and glycerol via lipolysis during fasting.
• Synthesis: Can synthesize TAGs from fatty acids and glycerol-3-phosphate.
• Endocrine Organ: Produces adipokines (e.g., leptin, adiponectin).

Skeletal Muscle (Myocytes): The Major Energy Consumer

• Fuel Utilization: Highly versatile; can use glucose, fatty acids, and ketone bodies.
• Glycogen Storage: Stores significant amounts of glycogen, but only for its own use (lacks glucose-6-phosphatase).
• Fatty Acid Oxidation: Major site for fatty acid oxidation, particularly during exercise and fasting.
• Protein Reservoir: A significant protein reserve that can be catabolized during prolonged fasting.

Brain (Neurons and Glial Cells): Obligate Glucose User, Adaptable in Fasting

• Primary Fuel: Under normal conditions, relies almost exclusively on glucose.
• Adaptation in Fasting: During prolonged fasting, the brain can adapt to utilize ketone bodies as a significant alternative fuel, sparing muscle protein.
• Cannot use Fatty Acids: Fatty acids cannot cross the blood-brain barrier.

Pancreas (Islets of Langerhans): The Endocrine Regulator

• Insulin (Beta Cells): Released in response to high blood glucose (fed state). Promotes fuel storage.
• Glucagon (Alpha Cells): Released in response to low blood glucose (fasting state). Promotes fuel mobilization.
• Somatostatin (Delta Cells): Inhibits secretion of both insulin and glucagon.

Major Hormones Orchestrating Fuel Homeostasis

These hormones act synergistically and antagonistically to maintain metabolic balance.

Insulin (Anabolic Hormone)

• Source: Pancreatic β-cells.
• Stimulus: High blood glucose, amino acids.
• Overall Effect: Promotes fuel storage; lowers blood glucose.
• Actions:
  • Liver: Increases glycogenesis, lipogenesis; inhibits glycogenolysis, gluconeogenesis.
  • Muscle: Increases glucose uptake (via GLUT4), glycogenesis, protein synthesis.
  • Adipose: Increases glucose uptake (via GLUT4), TAG synthesis; inhibits lipolysis (inhibits HSL).

Glucagon (Catabolic Hormone)

• Source: Pancreatic α-cells.
• Stimulus: Low blood glucose.
• Overall Effect: Promotes fuel mobilization; raises blood glucose.
• Actions (primarily liver): Increases glycogenolysis, gluconeogenesis, ketogenesis; inhibits glycogenesis, lipogenesis.

Catecholamines (Epinephrine, Norepinephrine - Stress Hormones)

• Source: Adrenal medulla, sympathetic nervous system.
• Stimulus: Stress, exercise, hypoglycemia.
• Overall Effect: "Fight or flight"; rapid mobilization of energy stores.
• Actions:
  • Liver & Muscle: Increases glycogenolysis.
  • Adipose: Potent activator of HSL, promoting lipolysis.

Cortisol (Glucocorticoid - Stress Hormone)

• Source: Adrenal cortex.
• Stimulus: Stress (chronic), low blood glucose.
• Overall Effect: Sustained glucose production; catabolic.
• Actions:
  • Liver: Increases gluconeogenesis (by increasing enzyme synthesis).
  • Muscle: Increases protein breakdown.
  • Adipose: Increases lipolysis.
  • Decreases peripheral glucose utilization.

Fatty Acid Metabolism

Fatty acids are fundamental molecules in biology, playing roles in energy, structure, and signaling. Their metabolism is highly regulated and central to energy homeostasis in the body. For more details on Fatty Acids, Click Here.

Briefly, What are Fatty Acids?

Fatty acids are long hydrocarbon chains with a carboxyl group (-COOH) at one end. This makes them amphipathic molecules, meaning they have both hydrophobic (the hydrocarbon chain) and hydrophilic (the carboxyl group) regions. They are found esterified to glycerol in triacylglycerols (TAGs) or as components of phospholipids and sphingolipids. In their free form, they are called free fatty acids (FFAs).

Classification of Fatty Acids

A. Based on Saturation:

• Saturated Fatty Acids (SFAs): Contain no carbon-carbon double bonds. Examples: Palmitic acid (16:0), Stearic acid (18:0). Tend to be solid at room temperature.
• Unsaturated Fatty Acids (UFAs): Contain one or more carbon-carbon double bonds.
  • Monounsaturated (MUFAs): Have one double bond (e.g., Oleic acid).
  • Polyunsaturated (PUFAs): Have two or more double bonds (e.g., Linoleic acid).
  Properties: Tend to be liquid at room temperature. The double bonds usually have a cis configuration, causing kinks in the chain.

B. Based on Chain Length:

• Short-Chain (SCFAs): 2 to 4 carbons.
• Medium-Chain (MCFAs): 6 to 12 carbons.
• Long-Chain (LCFAs): 14 to 20 carbons (most common).
• Very Long-Chain (VLCFAs): >20 carbons.

C. Based on Essentiality:

• Non-Essential Fatty Acids: Can be synthesized by the body.
• Essential Fatty Acids (EFAs): Cannot be synthesized and must be obtained from the diet.
  • Linoleic Acid (Omega-6): Precursor to arachidonic acid.
  • α-Linolenic Acid (Omega-3): Precursor to EPA and DHA.

Major Pathways Involved in Fatty Acid Metabolism

• Fatty Acid Synthesis (Lipogenesis): The process of building fatty acids from Acetyl-CoA. Occurs primarily in the cytosol.
• Fatty Acid Oxidation (Beta-Oxidation): The pathway that breaks down fatty acids into Acetyl-CoA to generate energy. Occurs primarily in the mitochondrial matrix.
• Triacylglycerol (TAG) Synthesis and Degradation: The processes of storing (esterification) and mobilizing (lipolysis) fatty acids.
• Ketone Body Metabolism:
  • Ketogenesis: The liver converts excess Acetyl-CoA into ketone bodies during prolonged fasting.
  • Ketolysis: Other tissues use ketone bodies as an alternative fuel source.

Pentose Phosphate Pathway (PPP)

The Pentose Phosphate Pathway (PPP), also known as the Hexose Monophosphate Shunt (HMP Shunt), is an alternative metabolic route for glucose metabolism that runs parallel to glycolysis. The HMP pathway is also known as the Warburg-Dickens pathway. About 10% of glucose entering this pathway per day. The liver & RBCs metabolise about 30% of glucose by this pathway.

Unlike glycolysis, its primary purpose is not to generate ATP. Instead, its main functions are:

• Production of NADPH: Essential for reductive biosynthetic reactions and for protecting cells from oxidative stress.
• Production of Ribose-5-Phosphate: A vital precursor for the synthesis of nucleotides (DNA, RNA) and coenzymes.

Think of the PPP as a "shunt" because it diverts glucose-6-phosphate away from glycolysis to serve these distinct purposes, and can then feed intermediates back into glycolysis. It primarily occurs in the cytosol of cells.

Two Major Phases of the PPP

The Pentose Phosphate Pathway is divided into two distinct phases:

a) The Oxidative (Irreversible) Phase:

• Function: This phase is responsible for the generation of NADPH and the production of ribulose-5-phosphate (which is then converted to ribose-5-phosphate).
• Nature: It is largely irreversible.
• Key Reactions: Involves oxidative decarboxylation reactions where glucose-6-phosphate is oxidized, releasing CO₂, and reducing NADP⁺ to NADPH.

b) The Non-Oxidative (Reversible) Phase:

• Function: This phase interconverts various sugar phosphates, primarily transforming pentose phosphates into glycolytic intermediates (fructose-6-phosphate and glyceraldehyde-3-phosphate). This allows carbon skeletons to be recycled back into glycolysis or used for gluconeogenesis.
• Nature: This phase is entirely reversible.
• Key Enzymes: Involves transketolase and transaldolase enzymes, which facilitate the transfer of two-carbon and three-carbon units, respectively.

Products of the PPP

The PPP is critically important because it provides two essential molecules:

a) NADPH (Nicotinamide Adenine Dinucleotide Phosphate, reduced form)

• Structure: Similar to NADH, but with an additional phosphate group.
• Function: Unlike NADH (used in catabolism for ATP), NADPH is predominantly used in anabolic (biosynthetic) processes and as a reductant in antioxidant defense.
  • Reductive Biosynthesis: Providing reducing power for the synthesis of fatty acids, cholesterol, and steroid hormones. Tissues actively involved in these syntheses (e.g., liver, adipose tissue, adrenal cortex) have a highly active PPP.
  • Antioxidant Defense: Protecting cells from damage by reactive oxygen species (ROS) by maintaining the reduced state of glutathione.

b) Ribose-5-Phosphate

• Structure: A five-carbon sugar phosphate.
• Function: This molecule is the direct precursor for the synthesis of:
  • Nucleotides: The building blocks of DNA and RNA.
  • Coenzymes: Such as ATP, NADH, FADH₂, and Coenzyme A.
• Demand: Cells that are rapidly dividing (e.g., bone marrow, skin, cancer cells) will have a high demand for ribose-5-phosphate.

Location of the pathway

• The enzymes are located in the cytosol.
• The tissues such as liver, adipose tissue, adrenal gland, erythrocytes, testes & lactating mammary gland, are highly active in the HMP shunt.
• Most of these tissues are involved in the biosynthesis of fatty acids and steroids, which are dependent on the supply of NADPH.