Digestion of Carbohydrates and Proteins

Protein digestion

in the lumen

GLUT5

specific for getting frutose from the lumen into the enterocyte. *GLUT2* then gets it from the enterocyte to the interstitial space.

Small intestine absorbs

-80 % ingested water -Electrolytes -Vitamins -Minerals -Carbonates - - active/facilitated transport - - monosaccharides Proteins - - di-/tripeptides - - amino acids Lipids - - monoglycerides - - fatty acids - - micelles - - chylomicrons

Pancreatic lipase and Colipase

-Hydrolyze triglycerides to *free fatty acids* and *monoglycerides*. These then associate with bile salts to form micelles whic "ferry" them to the intestinal mucosa. -Note: The colipase coats the emulsification droplets and anchors the lipase enzyme to them.

Small intestine Secretion

-Juice secreted by small intestine does not contain any digestive enzymes. -Synthesized enzymes act within brush-border membrane of epithelial cells: --Enterokinase --Disaccharidases --Aminopeptidases

Absorption of Carbohydrates & Proteins What drives the entire absorption process?

-Na+/K+ ATPase -Fructose is the only one not dependent on ATPase. It is not co-transported and has its own channel. -Amino acids, dipeptides, tripeptides, glucose, galactose all needs a Na+ cotransport -Glucose absorption is not directly dependent on energy. It is secondary to active transport. *-Insulin does not affect secondary active transport!* -Insulin does not affect glucose transport in: -stomach, kidney, -Insulin mainly affects glucose transport in adipose tissue and resting muscles. -Monosaccharides, di and tripeptides can be absorbed alone.

Brush border enzymes

-activate zymogens --trypsinogen will encouner enterokinase here to become trypsin -complete digestion of carbohydrates and proteins -peptidases (most) are linked on the brushborder enzymes.

Hartnup disease

-an autosomal recessive disorder caused by impaired neutral (ie, mono-amino-monocarboxylic) amino acid transport in the apical brush border membrane of small intestine and proximal tubule of kidney. *Impairment of PEPT1* -present with *pellagra like skin eruptions, cerebellar ataxia, and gross aminoaciduria.* -manifests during infancy with variable clinical presentation: failure to thrive, photosensitivity, intermittent ataxia, nystagmus and tremor -*A high-protein diet can overcome the deficient transport of neutral amino acids in most patients.* -Poor nutrition leads to more frequent and more severe attacks of the disease, which is otherwise asymptomatic.

2 steps of carbohydrate digestion

1. Intraluminal hydrolysis of starch to oligosaccharides by amylases (salivary & pancreatic). 2. Membrane digestion of oligo to mono by brush border disaccharidases. --There's 4 brush border oligosaccharidases: ----lactase, glucoamylase (maltase), sucrase, isomaltase

2 steps of carbohydrate absorption

1. Uptake across apical membrane into epithelial cell and, 2. Coordinated exit across basolateral membrane.

Small intestine

All chenmical digestion & nutrient absorption occurs here. -Circular folds of mucosa, villi, and microvilli - enhance surface area for absorption. -Largest folds -circular folds-mucosa & submucosa-plicae circularis.

Lipoprotein lipase

In blood, lipoprotein lipase hydrolyzes triglycerides to free fatty acids and glycerol for use in cells. The remnants containing cholesterol are taken to the liver. They form VLDLs which take glycerides to cells. Once glycerides are removed, VLDLs are converted to LDLs. LDLs transport cholesterol to organs and blood vessels. -HDLs transport excess cholesterol back to the liver.

PepT1

Moves dipeptides, tripeptides and tetrapeptides into enterocytes across apical membrane. -*Peptidases* then hydrolyze the oligopeptides into AA. The AAs exit across basolateral membrane thru Na+ independent AA transporters. -Because of this, PEPT1 also plays a major role in the nitrogen supply to the body.

Enzymes secreted into or of the small intestine

Peptidase Sucrase, maltase, lactase Lipase Enterokinase Somatostatin Cholescystokinin Secretin

pancreatic amylase

Polysaccharides broken into disaccharides by *salivary* and *pancreatic amylase.*

M cells

and enterocytes Can absorb whole proteins. Lysosomal proteases in enterocytes degrade proteins in these cases. -H+/oligopeptide cotransporter PepT1: Moves dipeptides, tripeptides and tetrapeptides into enterocytes across apical membrane. So if we don't have PepT1, we'll be deficient in those proteins.

Starch and glycogen

are broken down to maltose by salivary amylase and pancreatic amylase

Sucrase, maltase, lactase

break down disaccharides into monosaccharides

Lipase

breaks down fats into fatty acids and glycerol

Peptidase

breaks down peptides into amino acids

TAG digestion

broken down into glycerol and fatty acids before absorption. Cell then resynthesizes them back into TAGs. -These triglycerols will be called chylomicrons. Remember, the dietary fat that we absorb is ALWAYS called CHYLOMICRONS. remember that from MCM??. These chylomicrons will be taken up by the lymphatics. Not in the blood stream as is done for glucose, aa, and fructose. By the lacteals (lymphatic system), the chylomicrons are by passing the liver, go to the thoracic duct, the right subclavian vein and then the right atrium of the heart.

Sucrose digestion

by brush border enzymes before absorption

Enterokinase

converts trypsinogen to trypsin it is from the intestinal glands

Secretin

stimulates pancreas to release bicarbonate ions in pancreatic juice

Na-K Pump

stores energy that drives glucose (and galactose) uptake by creating a steep concentration gradient for Na+ entry into intestinal cells. -Downhill movement of Na into the cell through *SGLT* brings glucose (and galactose) with it in a co-transport way. -frucose doesn't need this. it enters by facilitated diffusion. -All 3 monosaccharides (glucose, galactose, fructose) exit the cell across the basolateral membrane via facilitated diffusion on the *GLUT2* sugar transporter.

Maltase

takes Maltose to glucose and glucose a brushborder enzyme in small intestine. absorbed into blood of villus. Think: Malts are chocolate icecream which have a lot of sugar. 2 sugars actually.

Lactase

takes lactose to glucose and galactose. a brushborder enzyme in small intestine. absorbed into blood of villus Think of the song.

Sucrase

takes sucrose to glucose and fructose a brushborder enzyme in small intestine. absorbed into blood of villus

amino acid carrier

The channel thing on the enterocyte that takes amino acids into the blood.

Monosaccharides Disaccharides

Galactose, Glucose, Fructose Lactose, Maltose, Sucrose

Main functions of the large intestine

*Completion of fluid absorption* -most water absorption occurs in the right colon (the cecum, the ascending colon and the first half of the transverse colon) *Active secretion* -Active secretion of K+ and HCO3- into colonic lumen *Absorption* -Na+ actively absorbed -Bile salts -Vitamins produced by bacteria-organic molecules -Cofactors or coenzymes *Storage and elimination of fecal waste* -Prior to defecation, fecal waste is stored in *left* colon (distal half of the transverse colon, descending colon and sigmoid colon) -By the time fecal material reaches the rectum, it consists of a small volume of K+ rich fluid containing undigested plant fibers, bacteria, and inorganic material.

major digestive enzymes of: mouth stomach pancreas gallbladder small intestine

*Mouth* -lingual lipase, -salivary a amylase *Stomach* -Pepsin -HCl, IF -Bicarbonate -Gastric lipase *Pancreas* -amylases -peptidases -Lipase -Nucleases -Bicarbonate -Insulin, Glucagon, somatostatin, etc *Gallbladder* -Bile *Small Intestine* -Disaccharidases -Trypsin -Other protein digesting enzymes -Colipase

Protein: Digestion and Absorption

Begins in the stomach when *pepsin* digests proteins to form polypeptides. -In the duodenum and jejunum, *endopeptidases like trypsin, chymotrypsin, and elastase,* cleave peptide bonds in the interior of the polypeptide (making smaller peptides). Then *exopeptidases like carboxpeptidases and aminopeptidases* cleave peptide bonds from the ends of the polypeptide (releasing amino acids). -Protein digestion is completed in the small intestine by brush border enzymes, carboxypeptidase, aminopeptidase, and dipeptidase. -Amino acids (and remaining polypeptides) then are absorbed by active transport with sodium and then enter the capillary blood in the villi across the basolateral membrane by facilitated or simple diffusion and are transported to the liver via the hepatic portal vein. as a side note, the proteins much be broken down to monopeptides or amino acids in order to get into the capillary blood. They can come into the enterocyte as di or try, but to get to the blood, they must be singular.

Trypsin Chymotrypsin Carboxypeptidase

Break down polypeptides into small peptide chains

Aminopeptidase Peptidase

Break down small peptides chains into tripeptides, dipeptides, and amino acids.

Complex foodstuffs and their absorbable units

Carbohydrates → monosaccharides Proteins → amino acids Fats → glycerol and fatty acids

Trypsin

The main protease secreted by the pancreas; trypsin is activated (from trypsinogen) by enterokinase, and subsequently activates: -chymotrypsinogen to chymotrypsin and -procarboxypeptidase to carboxypeptidase

Oligopeeptide digestion

directly absorbed by cell, then broken down inside cell.

Glucose digestion

does not require digestion

Somatostatin

hormone that inhibits acid secretion by stomach

Cholescystokinin

hormone that inhibits gastric glands, stimulates pancreas to release enzymes in pancreatic juice, stimulates gallbladder to release bile

disaccharidase

in brush borders of S.I. epithelial cells (maltase, lactase and sucrase). -They break disaccharides into monosaccarides. -(sucrose, lactose, and maltose and disaccharides)

Bile salts micelles

secreted into duodenum to break up fat droplets. They emulsify hem to make smaller droplets

Arrival of lipids in the duodenum

serves as a stimulus for secretion of bile