Chapter 14 Flashcards
Digestion
Definition: Process of breaking down large molecules of food into smaller units by hydrolytic enzymes (creating cellular biomolecules)
Further Notes:
- Components of a meal include proteins, lipids, and polysaccharides. These must be degraded into smaller biomolecules for absorption and transport
- Small biomolecules are used as precursors for growth and repair
- Degradation is facilitated by a diverse set of hydrolytic enzymes
Example: Proteins are hydrolyzed into 20 different amino acids, polysaccharides are hydrolyzed into simple sugars like glucose, fats are hydrolyzed into fatty acids
Digestion: Stage one of energy production
Process:
1. Complex food components are degraded into basic units
- Basic units are than converted into acetyl-CoA (a 2-carbon molecule that is an activated carrier)
- Acetyl-CoA can than enter the citric acid cycle to make energy
Further Notes:
- Acetyl (COCH3) 2 carbons
- CoA (Coenzyme A) serves as a carrier
Example:
- (Complex foods) fats, polysaccharides, proteins (basic units) fatty acids and glycerol, monosaccharides (glucose), amino acids acetyl-CoA
Main components of digestive tract
- Mouth
- Esophagus
- Stomach
- Small intestine
- Large intestine
- Anus
Accessory organs in digestive tract
- Salivary glands
- Pancreas
- Liver/gall bladder
Proteolytic enzyme (proteases)
A class of enzymes that hydrolyze the peptide bonds between amino acids, thus digesting proteins. Also called protease.
What does it mean to hydrolyze something?
- Addition of water to breakdown chemical bonds between a molecule
- adding water which breaks down large molecule into smaller pieces
Amylase
breakdown carbohydrates (starchers)
Lipase
breaks down fats
H+ - K+ ATPase
- Specialized cells lining the stomach contain the membrane protein H+ - K+ ATPase (gastric proton pump) that pumps protons into the stomach in exchange for K+ at the expense of ATP hydrolysis
- This proton pump is similar to the Na+ - K+ ATPase
What is GERD? How does it occur? What are the treatments
- In some individuals, the gastric proton pump can be too active, resulting in gastroesophageal reflux disease (GERD), where stomach acid leaks back to esophagus
- In addition to being painful, it may result in esophageal cancer if left untreated
- Common treatment for GERD is to irreversibly inhibit the proton pump
- One such inhibitor, omeprazole, is converted into sulfenic acid by the stomach acid, which rearranges to yield sulfonamide. Sulfonamide irreversibility modifies a cysteine residue on the pump. Omeprazole and other proton pump inhibitors are the most commonly prescribed drugs
Hydrolases
- cleave (split) their substance by adding a water molecule
- all digestive enzymes are hydrolases
Proteolysis
- Breakdown of proteins into amino acids
- Hydrolysis of proteins
- Proteins are broken down into peptides or amino acids by proteolytic enzymes
Enteropeptidase
- A proteolytic enzyme
- Secreted by the epithelial cells of the small intestine to activate the pancreatic zymogen trypsinogen to form trypsin (and cascade continues)
- Also called enterokinase
Endopeptidase
- group of enzymes that hydrolyze peptide bonds within the long chains of proteins
- cleaves/hydrolyses internal peptide bonds of proteins
Zymogen or Proenzyme
general name for category of inactive precursor enzymes
- Inactive forms of enzymes are activated upon secretion and at site of activity (allow for synthesis of nonactive enzymes)
- It’s important that the enzymes are nonactive within the cell that synthesized them b/c an active enzyme inside cell will destroy cell protein
- Enzymes become activated at site of digestion where they are needed to be active – this allows for the control of location of digestion (ex. compartmentalization)
List the gastric/pancreatic zymogens, their site of synthesis and their active enzyme form
SITE ZYMOGEN ACTIVE ENZYME
Stomach - pepsinogen - pepsin
Pancreas - Chymotripsinogen - chymotripsin
Pancreas - Tripsingogen - tripsin
Pancreas - Procarboxypeptidase - carboxypeptidase
Pancreas - Proelastase - eleastase
Most digested enzymes are secreted as ________
inactive precursors
Major enzymes of carbohydrate digestion
- Salivary amylase
- Pancreatic amylase
- Intestinal enzymes
source where its active substance digested BD Product
salivary glands mouth polysaccharides diasaccarides
pancreas small in polysaccharides diasaccarides small in small in diasaccarides monosaccarides
Major enzymes of protein digestion
- Pepsin
- Trypsin
- Chymotripsin
- Carboxypeptidase
- Intestinal enzyme
source where its active substance digested BD Product
stomach stomach proteins peptides
pancreas small in proteins peptides
pancreas small in proteins peptides
pancreas small in peptides amino acids
small in small in peptides amino acids
Major enzymes of lipid digestion
- Lipase
source where its active substance digested BD Product
pancreas small in triglycerides free fatty acids, monoglycerides
Digestion pancreas
Pancreas secretes bicarbonate into small intestine, helps neutralize stomach acidity of the partially digested food items as they transition from stomach to intestine
Exocrine and endocrine roles of pancreas
Exocrine roles
- breakdowns biomolecules by producing and secreting pancreatic enzymes and secreting them as zymogens or proenzymes
Endocrine roles
- coordinates whole body tissue response to feeding
- production and secretion of two hormones: insulin – with feeding, and glucagon – when not fed
Steps to zymogen activation
- Enteropeptidase (proteolytic enzyme) is secreted by the epithelial cells of the small intestine
- Enteropeptidase first activates zymogen trypsinogen to form trypsin
- Trypsin then goes to activate chymotrypsinogen into chymotrypsin, proelastase into elastase, procarboxypeptidase into carboxypeptidase, and prolipase into lipase
Digestion small intestine
- Small intestine = site of action for pancreatic enzymes to facilitate digestion – further facilitated by brush border enzymes (enzymes anchored to intestinal epithelial cells)
- Absorption of nutrients occur w/ breakdown of large biomolecules into small components – optimal nutrient absorption is facilitated by massive surface area via villi and microvilli
Digestive hormones
- In stomach, formation of oligopeptides and initial products of digestion will stimulate intestine to release two key digestive hormones: secretin and cholecystokinin (CCK)
- Secretin causes release of sodium bicarbonate from pancreas, which helps neutralize stomach acidity as food transitions from stomach to small intestine
- CCK stimulates pancreas to release digestive enzymes secreted into small intestine to facilitate digestion. CCK will also stimulate gallbladder to secrete bile acids
Bile salts
make it easier for your body to absorb and digest the fats and fat-soluble vitamins that you’ve eaten
Peptidases
- catalytically active proteins (enzymes) that cleave peptide bonds in proteins and peptides by hydrolysis
- enzymes capable of cleaving (splitting), and thereby inactivating, small peptides
- enzyme that hydrolyzes peptides
Protein digestion steps
- Peptidases, on surface of intestinal cells, cleave (split) oligopeptides (proteins digested into small units) into amino acids, di- and tripeptides, which are transported into intestinal cell by transporter
- Transporters are located on apical or luminal side of cell. Further movement of amino acids, di- and tripeptides into blood will occur via antiporter on basolateral side of cell
Uniporter
one way, one molecule
Symporter
one way, two molecules moving in same direction
Antiporter
two ways, two molecules moving in opposite direction
Carbohydrate digestion
- a-amylase initiates digestion by splitting alpha 1,4 glycosidic bonds
- Sucrose (consisting of glucose and fructose) is digested by sucrase. Lactose (consisting of glucose and galactose) is digested by lactase
- Galactose and glucose are transported into intestinal epithelial cell via secondary active transport process, sodium glucose linked transporter (SGLT)
- Fructose diffuses into the cell via transporter called GLUT5
- Glucose, galactose, and fructose are then released into the blood via transporter GLUT2 located on basolateral side of cell
- Glucose, galactose, and fructose being released into the blood can further be transported to tissue cells and utilized as fuel
alpha-amylase (alpha 1,4-glycosidic bonds)
A pancreatic enzyme that digests starch and glycogen by cleaving the a-1,4 bonds of starch and glycogen but not the a-1,6 bonds. Products are the di- and trisaccharides maltose and maltotriose and material not digestible by the enzyme (limit dextrin)
SGLT (sodium glucose linked transporter)
Glucose is moved into some animal cells by the sodium-glucose linked transporter (SGLT), a symporter powered by the simultaneous entry of Na+
Glucose transporters
wide group of membrane proteins that facilitate the transport of glucose across the plasma membrane
Enzymes
- are proteins
- act as a protein catalyst for biochemical reactions (have catalytic properties)
- accelerate the rate of a reaction
- decrease activation energy for a reaction to proceed
- will not change the equilibrium of a reaction
- highly specific for substrates
- will facilitate conversion of a substrate (reactants) into products
- are not degraded or destroyed during a reaction
- activity is measured in umol/min/mg protein
- specificity of enzyme is due to the precise interaction of the substrate w/ the enzymes
For reactions to take place in physiological useful way, must occur at a rate that meets cell’s needs and must display specificity – a particular reactant should always yield a particular product
Enzyme inhibition
Enzyme inhibitors are molecules that interact with enzymes (temporary or permanent) in some way and reduce the rate of an enzyme-catalyzed reaction or prevent enzymes to work in a normal manner
Competitive enzyme inhibitors possess similar shape to substrate molecule and competes w/ substrate for active site of enzyme
Noncompetitive enzyme inhibitors bind to a site other than the active site of enzyme, called an allosteric site. Due to binding, it deforms the structure of enzyme so that it doesn’t form the ES complex at its normal rate, and it prevents formation of enzyme-product complexes, which leads to fewer product formations
Uncompetitive inhibitor cannot bind to free enzyme, but only to ES complex. The resulting ES complex is enzymatically inactive
Covalent Modifications
Enzyme-catalysed alterations of synthesised proteins and include addition or removal of chemical groups.
Modifications can target single type of amino acid or multiple amino acids and will change chemical properties of the site
Dipeptide
Tripeptide
Oligopeptides
contains 2 amino acids bonded to each other via 1 peptide bond
contains 3 amino acids bonded to each other via 2 peptide bonds
Monosaccaride
Disaccaride
Trisaccaride
Oligiosaccarides
1
2 monosaccaride units
3 monosaccaride units
2-10 monosaccaride units
Glucose
- simple sugar, monosaccaride
- less sweet then fructose and sucrose
- aldehyde group
- rice, bread, pasta, starchy vegetables, etc.
Fructose
- monosaccaride
- ketone group
- fruit, honey, etc.
- sweetest taste
Sucrose
- common table sugar
- made up of glucose and fructose
- obtained from sugar cane and beats
Lactose
- disaccharide of milk
- consists of glucose and galactose
Galactose
- monosaccaride
- almost identical to glucose, differs slightly in orientation of functional groups around fourth carbon
- Galactose has a higher melting point than glucose as a result of the structural differences
Amphipathic
molecule w/ hydrophobic and hydrophilic parts
Triacyclglycerols
- type of fat (lipid) found in your blood
- When you eat, your body converts any calories it doesn’t need to use right away into triglycerides. The triglycerides are stored in your fat cells
Micelles
- lipid molecules that arrange themselves in a spherical form in aqueous solutions
- spherical amphiphilic structures that have a hydrophobic core and a hydrophilic shell
- help body absorb lipid and fat soluble vitamins. Help small intestine absorb essential lipids and vitamins from liver and gall bladder. Carry complex lipids like lecithin and lipid soluble vitamins (A, D, E and K) to small intestine
Fatty acids
- building blocks of the fat in our bodies and in the food we eat
- During digestion, body breaks down fats into fatty acids, which can then be absorbed into the blood
- Fatty acid molecules are usually joined together in groups of three, forming a molecule called a triglyceride
Glycerol
- naturally occurring alcohol
- used as a solvent, sweetening agent, and also as medicine
- When glycerol is in intestines, it attracts water into the gut, softening stools and relieving constipation
- glycerol is secreted from cells along with some fatty acids. These are transported to liver where glycerol may be converted to glucose. Fatty acids may be converted to ketones or transported to other cells and burn to produce ATP
Chylomicrons
- Chylomicrons are lipoproteins that enter blood and facilitate transport of fatty acids
- Exterior consists of phospholipid monolayer w/ embedded apoproteins (ApoB-48) and cholesterol
- Interior is a hydrophobic core consisting of triacylglycerols and cholesterol esters
- In the size range of 200 nanometers
- Will predominantly be made up of triglycerides, and will include proteins and phospholipids that will be found on the surface of the entities
Steatorrhea
if the production of bile salts is inadequate due to liver disease, large amounts of fats (as much as per day) are excreted in the feces.
Emulsification of Lipids
Emulsification: process of dispersing two or more immiscible liquids together
Emulsion: A mixture of two immiscible (incable of mixing) substances
- Lipids are prepared for digestion in stomach. Grinding and mixing that takes place in stomach converts lipids into an emulsion, mixture of lipid droplets and water
- After the lipids leave stomach, emulsification is enhanced with aid of bile salts - amphipathic molecules synthesized (produced) from cholesterol in liver and secreted from gallbladder in response to cholecystokinin (CCK)
- Body temp and peristalsis further facilitate emulsification and digestive process
- Salivary and gastric lipases release fatty acids that act as surfactants
- Carboxy ends interact w/ water, while hydrocarbon tail interact w/ other lipids
(Bile salts from liver/gall bladder cause the breakdown of fat in the duodenum into tiny micelles. Body temperature can liquefy fats. Peristalsis - Mixes the emulsion and move it to small intestine)
Fat Digestion Facilitation by Bile Salt Process (in-depth)
- Triacyclglycerols from the diet form lipid droplets in the stomach
–> Fat digestion is facilitated by bile salts, which help for emulsification
- Bile salts (amphipathic molecules) are made from cholesterol in liver and secreted and stored in gallbladder before being released into small intestine, stimulated by hormone (cholecystokinin) CCK. They are inserted into the lipid droplets
- Breakdown of triacylglycerols into two fatty acids and one monoacylglycerol is facilitated by enzyme lipase
- Fatty acids and monoacylgercol that are formed can be transported into the intestinal cells (enterocyte) via micelle
- Within enterocytes, fatty acids reform as triacylglycerols and are packaged w/ other lipids and proteins creating chylomicrons, which can be transported throughout circulation
