Biochem of Digestion Flashcards
What are the 3 things that food is broken down into?
Carbs
Fats
Proteins
What are carbs broken down into?
disaccharides & monosaccharides
What are proteins broken down into?
peptides–>AA & di or tripeptides
What are the 3 things that fats can be broken down into?
Free Fatty Acids
MAG
DAG
What is MAG broken down into?
glycerol
Fat is ingested as ____ but is broken down to ___ & ____.
Ingested as TAG
Broken down to fatty acids & MAG
What are the 3 phases of digestion?
Mechanical Phase
Hydrolysis
Transport
Where does the mechanical phase of digestion take place?
Lubrication in the mouth.
Peristalsis in the stomach.
Where does the hydrolysis phase of digestion take place?
Begins in the mouth with salivary glands. Stomach Small Intestine (does the heavy lifting)
Where does the transport phase of digestion take place?
in the small intestine
What are the 2 components of the mechanical phase & what is the goal?
Mastication
Peristalsis
**homogenize the nutrients
What are the 3 functions of gastric acid?
kill microbes
denature protein
make the pH optimal for activation & enzymatic activity of the pepsin
What is the main function of the enzymes secreted by the salivary glands?
to keep the teeth clean
What are the 3 things that help w/ hydrolysis in the mouth?
alpha amylase
lingual lipase
lyzozymes
The alpha amylase released in the mouth performs optimally under which pH range?
6.5-7
The lyzozymes released in the mouth are good at breaking which types of bonds?
beta 1, 4 glycosidic bonds (found in bacterial cell wall)…breaks down the peptidoglycans in the cell wall
Where is protein digestion initiated?
in the stomach where pepsin is released
What are the 2 things released in the stomach that are a part of hydrolysis?
HCl
pepsin
What are the functions of HCl that is released in the stomach? How does this relate to pepsin?
HCl kills microorganisms, denatures proteins, & helps w/ pepsin
Creates pH @ 5 for pepsinogen to be cleaved
Creates pH @ 2 for pepsin to be activated
What allows the chyme to be neutralized once it hits the duodenum?
pancreatic secretions of bicarb
What does the alpha amylase released in the mouth break down?
carbohydrates
What is the fcn of lingual lipase?
it binds to fat globules & begins liberating fatty acids
What type of enzyme is pepsin & what is its fcn?
endopeptidase
cleaves proteins
Once you hit the small intestine what are 5 things that are involved in hydrolysis?
alpha amylase glycosidases proteases lipases bacteria
What is the relationship b/w the pancreatic alpha amylase & the salivary alpha amylase?
they are isoenzymes
Once the chyme hits the duodenum what is released from where?
zymogens from the pancreas
At the level of the small intestine what happens?
we transport the metabolites!!
thru intestinal epithelial cells
What do we absorb in the SI from carbs?
monosaccharides
What do we absorb in the SI from proteins?
Amino acids; di or tripeptides
What do we absorb in the SI from fat?
glycerol; free fatty acids; monoacylglycerol
What are the 4 main hormones secreted thru endocrine related to GI?
Cholecystokinin
Gastrin
Histamine
Secretin
What are the 3 things that CCK does when it is released?
gallbladder contraction
stimulates the liver to make bile salts
causes the pancreas to make & activate proenzymes
What is another name for CCK?
pancreozymin
What is guanylin?
a peptide that regulates fluid secretion @ the brush border b/c it acts on guanylyl cyclase
What does secretin do?
stimulates the watery phase (including bicarb) of pancreatic secretions.
Why is the pancreas protected when it secretes so many enzymes?
They’re released in an inactive form: called zymogens or proenzymes.
The cytoplasm of exocrine cells contain____ where ____ is made. They empty into _____ & ultimately into ______.
contain rough ER
synthesis of zymogens
collecting ducts–>pancreatic duct–>SI
Where are all of the zymogens cleaved?
small intestine
What is the zymogen that is produced in the stomach?
pepsinogen
What are the zymogens that are produced in the pancreas?
trypsinogen chymotrypsinogen proelastase procarboxypeptidase phospholipase
What cells make pepsinogen & where?
chief cells in the stomach.
Tell the story of chyme down to the activation of zymogens in the SI.
Chyme gets to the duodenum.
Intestinal Endocrine cells sense it & release CCK & Secretin.
CCK activates intestinal mucosal epithelial cells to release enteropeptidase.
CCK & Secretin activate pancreatic acinar cells to release trypsinogen.
Enteropeptidase turns trypsinogen to trypsin.
Trypsin turns more trypsinogen into trypsin.
Trypsin turns various zymogens into their active forms.
What are 3 zymogens that are activated by trypsin?
Chymotrypsinogen–>Chymotrypsin
Proelastase–>Elastase
Procarboxypeptidase–>Carboxypeptidase
In addition to trypsinogen, what is something else that the pancreas secretes & what is its purpose?
a pancreatic trypsin inhibitor.
Its purpose is to inhibit any trypsin if it shows up in the pancreas before it is supposed to b/c then it would damage the pancreas.
85% of CF patients have ____ insufficiency. What are the implications of this? How is this treated?
pancreatic
mucus secretions in the pancreatic duct are very thick & they block the release of pancreatic enzymes into the SI.
This inhibits the absorption of important nutrients & esp fat-soluble vitamins.
Patients are given enzymes to treat this.
How much energy do you get from carbs on a per gram basis?
Carbs: 4kcal/g
How much energy do you get from fat on a per gram basis?
Fat: 9.3 kcal/g
How much energy do you get from protein on a per gram basis?
Protein: 4-4.5 kcal/g
How much energy do you get from alcohol on a per gram basis?
Alcohol: 7kcal/g
What are the 2 main types of dietary carbs & what are they each found in?
Sucrose (found in sugary foods)
Starch (found in corn & breads etc)
Explain the structure of sucrose.
disaccharide. It is made up of glucose & fructose.
It has a beta 1,2 glycosidic bond
What are the 2 most common disaccharides?
Lactose & Sucrose
What are the 2 types of starches? What are the differences b/w the 2?
Amylose & Amylopectin
Amylose: alpha 1,4 glycosidic bonds linking glucose & it has a linear structure
Amylopectin: plant form of glycogen w/ branches & both alpha 1,4 glycosidic bonds & alpha 1,6 glycosidic bonds.
What makes up lactose?
Lactose is a disaccharide made up of galactose & glucose.
It has a beta 1,4 glycosidic bond.
Starch digestion is mainly accomplished by which enzyme?
pancreatic alpha amylase
Not really salivary alpha amylase.
What types of bonds can pancreatic alpha amylase cleave on a starch molecule?
only alpha 14 glycosidic bonds, not alpha 16 bonds.
Therefore, it can’t fully digest amylopectin on its own.
Also, it can only cleave things that are in a chain of glucose.
What type of enzyme is pancreatic alpha amylase?
an endoglycosidase
After hydrolytic cleavage of amylopectin by pancreatic alpha amylase, what products are left?
(dimers & trimers of glucose +alpha limit dextrin)
maltose
maltotriose
alpha-limit dextrins
What bonds are found in alpha limit dextrins?
Alpha 14 glycosidic bonds & alpha 16 glycosidic bonds
T/F the products of pancreatic alpha amylase on amylopectin cannot be absorbed in the SI.
True. They can’t.
Where are disaccharidases found & how are they attached there? What is another name for them?
brush border of the small intestine
attached to the microvilli of the brush border
some have transmembrane helices & others are anchored there by glycosyl phosphatidylinositol.
aka ectoenzymes
All disaccharidases are technically glycoproteins. What does this mean?
This means that mono or polysaccharides are attached to their extracellular domains.
What is an example of a disaccharidase that is anchored with glycosyl phosphatidylinositol?
trehelase
What are some examples of disaccharidases that have transmembrane helices?
sucrase-isomaltase
lactase-cerebrosidase
How is the sucrase-isomaltase complex derived?
derived from a single polypeptide that is cleaved by pancreatic proteases.
Which disaccharidases are inducible & what does this mean?
All of them except for lactase. This means that the more you eat of that product…the more enzymes you produce.
What are the 2 main symptoms of lactose intolerance & what cause them?
Flatulence: bacteria digest the extra lactose & it produces gas.
Diarrhea: osmotic situation…tons of lactose molecules draws out more water.
So…after pancreatic alpha amylase does its thing with amylopectin…what happens to the maltose & triose etc if they can’t be absorbed as they are by the SI?
These sugars travel to the mucosal surface of the SI & the disaccharidases attached at the villi break them down into monosaccharides that can be absorbed by the SI.
What are 3 important mucosal epithelial transporters & 1 important pump in the SI? Where are they found?
SI...in enterocytes. SGLT1: on luminal side GLUT5: luminal side GLUT2: capillary side Na+/K+ ATPase pump: capillary side
What does SGLT1 transport? How? Where?
glucose & galactose
on the luminal side of the enterocyte
it is a secondary active process (co-transporter) driven by the Na+ gradient made by the pump.
What does GLUT5 transport? How? Where?
fructose
on the luminal side of the enterocyte
passive process…facilitated diffusion
What does GLUT2 transport? How? Where?
Glucose Galactose Fructose on the capillary side of the enterocyte a passive process...uniporter
Patients sometimes have a defect in which SI transporter? What happens?
SGLT1
causes severe GI issues
hyperosmotic environment in the lumen of the SI
can’t absorb glucose & galactose easily.
What is the rate limiting step of carbohydrate metabolism?
the transport of monosaccharides into cells.
What is our protein load each day? How much do we lose each day? Is this an efficient process?
Eat: 70-100g/day
Endogenous (shed from our body & digest): 35-200g/day
Feces: 5-20 g/day
Very efficient process.
Explain the process of protein digestion in the stomach & duodenum.
Stomach: low pH b/c of secretion of HCl & presence of pepsin (from pepsinogen released from chief cells)
These denature & break down the proteins in the stomach.
Duodenum: low pH prompts the release of CCK & secretin…stimulates the release of bicarb & zymogens from the pancreas…
Activated enzymes cleave the peptone into amino acids & oligopeptides. The bicarb lowers the pH & gets what’s left of the protein ready for the rest of digestion…
Which steps of protein digestion are not essential?
neither the low pH of the stomach nor the pepsin are essential for protein digestion…
As a protease…what type of enzyme is pepsin?
Both an endo & exopeptidase.
What are 4 examples of endopeptidases…which of these are secreted from the pancreas?
Pepsin Trypsin Chymotrypsin Elastase **everything here but pepsin is secreted by the pancreas
What are 3 examples of exopeptidases…which of them are secreted by the pancreas?
Pepsin
Carboxypeptidase A & B
everything but pepsin is secreted by the pancreas
Pepsinogen A & B are released by the ____.
STOMACH
Aside from being a protease & an exopeptidase…what type of enzymes are carboxypeptidase A & B?
metalloproteinases
dependent on zinc
What is the catalytic mechanism of pepsin & what is its specificity?
Mechanism: carboxyl protease
Specificity: NH side of hydrophobic amino acids
What is the catalytic mechanism of trypsin & what is its specificity?
Mechanism: serine protease
Specificity: CO side of basic amino acids
What is the catalytic mechanism of chymotrypsin & what is its specificity?
Mechanism: serine protease
Specificity: CO side of hydrophobic amino acids
What is the catalytic mechanism of elastase & what is its specificity?
Mechanism: serine protease
Specificity: CO side of small amino acids
What is the catalytic mechanism of carboxypeptidase A? What is its specificity?
Mechanism: metalloproteinase (Zn2+ dependent)
Specificity: hydrophobic amino acids @ the C-terminus
What is the catalytic mechanism of carboxypeptidase B? What is its specificity?
Mechanism: metalloproteinase (Zn2+ dependent)
Specificity: basic amino acids @ C-terminus
Explain the process of releasing zymogen granules from the pancreas.
Zymogens are synthesized in the rough ER of exocrine cells.
Exocrine cells are stuffed with RER.
Zymogens are concentrated in transport particles called zymogen granules.
Exocrine cell stimulated.
Membranes fuse & the zymogens are exocytosed into the lumen of the collecting duct.
The collecting ducts drain into the pancreatic duct & then empty into the SI.
How are the protein bits finally digested & absorbed…start at the point where we have free amino acids & oligopeptides after the active zymogens went at the protein…
Free amino acids go straight to the luminal side transporter on the SI enterocyte. They are co-transported w/ Na+ (active process b/c there is a sodium potassium pump on the capillary side of the enterocyte)…
Oligopeptides go to the brush border enzymes to be broken down into FAA & di & tripeptides.
The FAA go to the previous route.
The Di & tripeptides go thru a luminal side transporter of the enterocyte that is a cotransport w/ H+.
Inside the enterocyte they are broken down into FAA by di & tripeptidases.
They then leave the enterocytes by what looks like a passive transporter on the capillary side…
What composes 90% of the fat we eat? What are 2 other names for it? What are its components? How much fat do we eat each day?
Triglycerides!
aka TAG or triacylglycerol
Glycerol backbone w/ 3 fatty acids
60-155 grams of fat per day
What is the enzyme that is secreted in our oral cavity to break down fat?
lingual lipase
What is the general process that fats must go thru to be broken down & absorbed?
The globules of fat must be emulsified by water soluble enzymes so that they can then be digested.
In response to the amount of fat that we eat per day…how much cholesterol is released by the liver & how much phosphatidlycholine is released by the liver?
1.2 grams of cholesterol are released by the liver each day
7-22 grams of phosphatidylcholine are released by the liver each day.
How do you begin the digestion of lipids in the stomach?
Core body temp melts them a little.
Peristaltic movements lead to the formation of a lipid emulsion.
Salivary & gastric lipases help.
Fatty acids & diglycerides etc. produced that act as emulsifiers?
When you are digesting lipids why do the lingual & gastric lipases operate pretty slowly?
b/c of the small lipid water interface
What is the structure of bile?
Planar fat emulsifiers have a hydrophobic & hydrophilic surface built on a cholesterol backbone modified to cholic acid conjugated with amino acids like glycine & taurine
What is the difference b/w bile acids & bile salts? What is the functional difference?
Bile acids are the protonated forms of bile salts.
Bile acids are functionally important b/c they are the ones that form micelles.
_____ grams of bile is dumped into the intestines each day…but ____ grams of that is stored b/c a LOT of it is recycled.
20-50 grams dumped
3-5 grams stored
What is the most common bile salt in humans?
glycocholate
What is fiber generally considered? What are 5 specific examples?
plant polymers
cellulose, inulin, pectin, lignin, suberin
How is fiber broken down & where is it generally absorbed?
anaerobically fermented in the lower ileum by bacterial flora
absorbed by the ileum of the SI
What are the products of fiber digestion by anaerobic fermentation?
Gas:
Hydrogen, methane, CO2
Organic Acids:
Acetate, Propionate, Byrutate, Lactate
Where are the majority of all nutrients absorbed?
the ileum of the SI
What is the general function of the large intestine?
Absorption of water & electrolytes
recirculation of bile acids to the liver
Approximately what amount of fluid passes through the GI tract each day?
7 liters/ day
What is the time frame of the effect of hormones vs. allosteric regulators?
Hormones regulate on the level of minutes to days
Allosteric effectors regulate on a moment by moment basis
What does it mean that metabolic enzymes have tissue specific isoforms?
In different tissues there will be a different timing of when stuff happens…
Glucokinase vs. Hexokinase
Where is excess nitrogen converted to urea?
the liver
What is one of the GI-like functions of the lymphatic system?
carries lipids from the intestine to the liver
Where in the cell does non-oxidative catabolism occur?
the cytoplasm
Where in the cell does the storage of energy in the form of fat & glycogen occur?
the cytoplasm
What is the general function of lysosomes?
hydrolyze macromolecules
What is the general function of peroxisomes?
oxidative reactions.
What is the idea behind isozymes?
the cells in different parts of the body have similar enzymes that are specialized to that particular organ.
What are 2 examples of carb metabolism synthesizing biosynthetic monosaccharide precursors?
Amino sugars–>Activated sugars–>Nucleotides
Glycolipids–>Glycoproteins–>Proteoglycans
What are 2 monosaccharides that can be channeled into different energy-getting pathways?
fructose & galactose
What is the ideal blood glucose level in mM & in mg/dL?
4-5.5 mM
70-100 mg/dL
Which cell type is completely dependent on energy from pure glucose?
RBCs
What is the brain’s primary energy source? How much of this does it consume per day? What is its backup?
Glucose
120g/day
Back up: ketones
How is blood glucose taken up by most tissues?
thru facilitated diffusion
Blood glucose levels of ____ mg/mL & below constitute hypoglycemia.
40mg/100mL
How does glucose travel in the blood?
attached to albumin
What is the only source of glucose during prolonged fasting?
the liver
What are the 3 sources of blood glucose?
food (lasts for a few hours)
glycogen
gluconeogenesis
When does fasting technically begin?
3 hours after eating…
How long does it take to deplete the liver glycogen content?
10-12 hours
like an overnight fast
When glucagon rises relative to insulin…what process happens first? What process follows it?
First: glycogenolysis
Then: gluconeogenesis
What usu happens to glucose?
It is made into glycogen.
The liver metabolizes what percentage of glucose?
20-30%
