2 - Digestion and Absorption of Carbs, Proteins, Lipids, Iron Flashcards
Objectives: Describe how carbohydrates are digested and absorbed
Digestion
Starches
Sucrose
Lactose
Trehalose
Cellulose
- Carbohydrates must be digested into monosaccharides before being absorbed
- Starches (Amylopectin / Amylose)
- Converted by Salivary and Pacreatic a-amylase to maltose, maltotriose, and a-limit dextrins
- Hydrolyzed to glucose by glucoamylase, isomaltase, and maltase in brush border membrane
- Sucrose
- Cleaved by sucrase in brush border to glucose/fructose
- Lactose
- Cleaved by lactase in brush border to glucose/galactose
- Trehalose (glucose dimer)
- Cleaved by trehalase in brush border to glucose
- Cellulose
- Not digested by human enzymes
Objectives: Describe how carbohydrates are digested and absorbed
Absorption
Glucose / Galactose
Fructose
- Intestine will only absorb monosaccharides (glucose, galactose, fructose)
-
Glucose / Galactose:
- Absorbed into enterocyte by common Na+-dependent active transport system (SGLT-1)
- Almost all absorbed in intestine
- Cell to blood via facilitated diffusion
-
Fructose:
- Cannot be absorbed against concentration gradient
Objectives: Describe how carbohydrates are digested and absorbed
Clinical Disorders
Lactose Intolerance
Sucrase-Isomaltase Deficiency
Glucose / Galactose Malabsorption
- Lactose Intolerance
- Deficiency in lactase
- Lactose remains in GI tract; leads to diarrhea
- Colonic bacteria metabolize; leads to gas
- Sucrase-Isomaltase Deficiency
- Inherited disorder, cannot digest sucrose/isomaltase
- Glucose / Galactose Malabsorption
- Genetic defect in SGLT1
- Cannot absorb glucose / galactose
Objectives: Describe how proteins are disgested and absorbed
Overview
Sources
- Sources:
- Endogenous - Secretory proteins and cells shed into GI tract lumen
- Exogenous - Dietary proteins
- Proteins absorbed as amino acids, dipeptides, and tripeptides; larger peptides poorly absorbed
- All ingested protein assimilated
Objectives: Describe how proteins are disgested and absorbed
Digestion
Endopeptidases
Pepsin, Trypsin - Role of Enterokinase?
- Endopeptidases - Hydrolyze Interior Peptide Bonds
- Pepsin digests small amount of ingested protein
- Pancreatic Enymes secreted as Inactive precursors
- Trypsinogen converted to trypsin by enterokinase, secreted by brush border in small intestine
-
Trypsin autocatalyzes:
- Trypsinogen
- Chrymotrypsin
- Elastase
Objectives: Describe how proteins are disgested and absorbed
Exopeptidases
- Exopeptidase: Hydrolyze ONE amino acid at a time from C-terminus of proteins and peptides
- Carboxypeptidases A and B
- Secretec from pancrease as proenzymes
- Trypsin activates
- Peptidases in brush border cleave peptides produced by pancreatic proteases to oligopeptides and amino acids
Objectives: Describe how proteins are disgested and absorbed
Absorption; Transport Mechanism(s)+Method(s)?
Enterocytes: Di- and Tri- Peptide Absorption; Transporter
Food Allergies
- Most amino acid absorbed into enterocytes by Na-Depenent Co-transport
- Separate tx for Neutral, Acidic, Basic, and Imino Amino Acids
- Transported via facilitated diffusion
-
Enterocytes:
-
Transport of di- and tri- peptides faster, more efficient
- Uses H+ dependent co-transporter (PEPT1)
- Majority of protein absorbed in di- tri- peptides
- Peptides hydrolyzed to amino acids by cytoplasmic peptidases
-
Transport of di- and tri- peptides faster, more efficient
- Whole proteins can lead to food allergies
Objectives: Describe how proteins are disgested and absorbed
Abnormalities
-
Trypsin Deficiency
- Congenital lack of trypsin or pancreatic disease
-
Defect in Transport
- Cystinuria - Basic Amino Acids (Aginine, Lysine, Ornithine)
- Hartnup Diease - Neutral Amino Acids
- Familial Iminoglycinuria - Proline, Hydroxyproline
Objectives: Descrie how lipids are digested and absorbed
Digestion overview (stomach)
What is key hormone to ensure lipids are given time to digest?
- Lipids must be made water soluble before they can be absorbed
- Digestion:
- Lipids will separate out into oily phase
- Stomach mixing breaks into droplets (increase total surface area)
- CCK slows gastric emptying to allow sufficient time for digestion and absorption in duodenum
Objectives: Descrie how lipids are digested and absorbed
Small Intestine
Pancreatic Enzymes: Pancreatic Lipse, Cholestero ester Hydrolase, Phospholipase A2, Colipase
- Emulsification: Neurtal / Alkaline Environment
- Detergent action of bile salts, lecithin, and increased surface area
- Pancreatic Enzymes: Hydrolyze lipids
-
Pancreatic Lipase
- Cleaves fatty acids from 1 / 3 position in TAGs
- = 2x FAs , 2-monoglyceride
- Requires Colipase
-
Cholesterol Ester Hydrolase
- Cleaves fatty acid from cholesterol ester
- = Cholesterol, 1x FA
-
Phospholipase A2
- Releases FA from 2 position in phospholipids
- = 1x FA, lysophospholipid
-
Colipase (inactive) - Needed for Pancreatic Lipase
- Activated by trypsin
- Prevents inhibition of pancreatic lipase by bile salts
- Acts as anchor for lipase; binds 1:1 ratio
-
Pancreatic Lipase
Objectives: Descrie how lipids are digested and absorbed
Absorption
- Emulsified products of lipolysis form water-soluble mixed micelles with bile
- Bring into contact with microvilli on surface of enterocytes
- Diffuse across luminal membrane into cells
Objectives: Descrie how lipids are digested and absorbed
Intracellular Processing
Monoglyceride Acylation Pathway
Phosphatidic Acid Pathway
Lysophospholipid Combination
Cholesterol
- Enterocytes:
- Products (think TAG building blocks) reesterified to TAGS and Phospholipids
- Monoglyceride Acylation Pathway
- TAGs synthesized from 2-monoglycerides and CoA-activated FAs
- Reesterification in smooth ER
- ***Medium-Chain FAs absorbed directly into bloodstream w/out resynthesis into TAG***
- Phosphatidic Acid Pathway
- 2-Acyl CoA + a-Glycerophosphate to Phosphatidic Acid
- Phsphatidic Acid + Acyle CoA to TAG + P
- Lysophospholipid
- FA = Phospholipid
- Cholesterol
- Absorbed in free form
- Reesterified with FA in enterocytes
- Both free, reesterified TX’ed in chylomicron
Objectives: Describe how the products of lipid digestion are normally processed
Chylomicron Formation
Chylomicron Transportatio
-
Resynthesized TAGs, Cholesterol / Cholesterol Esters, and Phospholipids form Chylomicrons
- Core: TAGs, Esterefied Cholesterol, Fat-Soluble Vitamins
- Surface: Phospholipids, Apoproteins, Free Cholesterol
- Transportation
- Out of Cell: Exocytosis
- Lymphatic System: Enter via gaps between endothelial cells
- Larger Than Capillaries?
- Reach blood via thoracic duct
Objectives: Describe how the products of lipid digestion are normally processed
Lipid Malabsorption: Steatorrhea
Causes:
Failure of Digestion
Absence of Bile Salts
Defect in Absorbing Cells
Failure of Synthesis of Apoproteins
- Steatorrhea: Excretion of high fat in feces
- Failure of Digestion
- Pancreatic enzymes not secreted
- Pancreatitis, Pancreatic Cancer, Cystic Fibrosis
- Absence of Bile Salts
- Liver disease, obstruction of bile ducts (gallstones), bacterial obergrowth in small intestine, low diodenum pH
- Treated with feeding of medium chain FAs (absorbed directly into bloodstream)
- Defect in Absorbing Cells
- Tropical Sprue
- Gluten Enteropathy / Celiac Sprue
- Gluten Allergy
- Loss of villi lining small intestine
- Malabsorption of carbs, proteins
- Treated by gluten free diet
- Failure of Synthesis of Apoproteins
- ApoB protein component of chylomicron not synthesized
- Chylomicrons do not form, or cannot be transported
- Fats, Cholesterol, Fat-soluble vitamins not absorbed
Objectives: Describe how iron stores are regulated
Background
Heme Iron vs Free Iron (two forms)
Digestion, Transport
- Background
- Amount absorbed = Amount lost
- Absorbed as heme (bound to hemoglobin or myoglobin from meat) or as free iron
- Heme Iron
- Most easily absorbed
- Taken up by endocytosis or TX protein (HCP1)
- Heme broken down by heme-oxygenase to release free iron
- Free Iron
- Forms:
- Ferrous (Fe2+) - easier absorption
- Ferric (Fe3+) - insoluble; ppt easily; most dietary iron
- Digestion:
- Gastric acid dissolves iron; complexes with Ascorbic Acid (Vitamin C)
- Vit C reduces Ferric to Ferrous
- Dcytb (Duodenal Cytochrome b) reduces ferric to ferrous
- Gastric acid dissolves iron; complexes with Ascorbic Acid (Vitamin C)
- Transportation:
- Ferrous Iron - DMT1 Transporter
- Binds to Apoferritin as Ferritin for storage (prevents toxicity)
- May be transported out by ferroportin
- Converted to ferric iron to bind to plasma protein for TX to other tissues
- Forms: