Absorption and digestion of lipids, carbs and proteins

what is the problem of lipid/fat digestion and transport

• they’re insoluble in water

what is one way we make it easier for enzymes to act on lipids

• reduce the size of large fat droplets

what is the main dietary fat large fat droplets are made from?

• TAG - triacylglycerol

what is TAG formed from?

• condensation reaction b/w glycerol ester + 3 fatty acids

what starts large fat droplet reduction?

• bile acids/salts

what hormone stimulates bile acid/salt secretion?

• CCK

what are bile acids

• derivatives of cholesterol which are polar

how can we describe bile acids?

• amphipathic molecules

where is Phospholipase A2 secreted from and what is its function?

• pancreas

• transforms lecithin from bile → lysophospholipid, reduces size of fat droplets

what 2 enzymes act on lipids?

• lingual lipase

• pancreatic lipase

what do lingual lipase and pancreatic lipase do?

• catalyse hydrolysis of TAGS at positions 1+3 → two FFAs and one 2-monoaclglycerol

what protein aids lipase binding to lipid, why is it needed

• colipase

• due to the water-lipid interface

how does fat accumulate in the epithelial cells?

• by passive diffusion/via transport proteins

what aids passive absorption of fat into enterocytes?

• concentration gradient

what happens to fatty acids inside enterocytes?

• activated to fatty acyl-CoA → SER → lipid assembly

what enzyme activates fatty-acyl-CoA inside enterocytes?

• fatty acyl-CoA synthetase

what are the 2 TAG reformation pathways?

1. monoglyceride pathway - predominant

2. glycerol-3-phosphate pathway - backup/fasting

what steps are involved in the monoglyceride pathway?

1. absorption

2. activation

3. esterification

what is the esterification process of the monoglyceride pathway?

1. 2-monoacylglycerol + fatty acyl-CoA → diacylglycerol (DAG)

2. DAG + fatty acyl CoA → TAG

what happens to the newly synthesised TAG inside enterocytes?

• assembled into chylomicrons with cholesterol, phospholipid and apolipoprotein → secreted into lymph

what are the steps of the glycerol-3-phosphate pathway?

1. glycerol-3-phosphate formation from glycerol

2. sequential acylation

3. conversion

what is the sequential acylation of glycerol-3-phosphate?

1. glycerol-3-phosphate + fatty acyl-CoA → lysophposphatidic acid

2. ^ + fatty acyl-CoA → phosphatidic acid

what is the conversion of phosphatidic acid, what pathway is this?

1. phosphatidic acid → DAG

2. DAG + fatty acyl-CoA → TAG

this is the glycerol-3-phosphate pathway

what enzyme is involved in converting phosphatidic acid → DAG

• phosphatidic acid phosphatase

what are chylomicrons

• lipoproteins

• TAGs + cholesterol esters carried in a core surrounded by proteins + hydrophilic shell

what is the major protein in chylomicrons?

• apolipoprotein B

what enables chylomicrons to form a stable structure in the blood?

• apolipoproteins which form a hydrophilic shell around lipid layer

why do chylomicrons travel in lymph?

• too big to enter systemic circulation

what metabolism do chylomicrons avoid, how?

• hepatic metabolism

• due to draining into circulation via thoracic duct.

what enzyme does apolipoprotein activate?

• lipoprotein lipase

where do we find lipoprotein lipase?

• lumenal surface of small blood vessels

what does lipoprotein lipase do?

• catalyses hydrolytic cleavage of fatty acids from TAGS of chylomicrons

why are lipoprotein lipases important?

• enable release of energy from chylomicrons to be used by cells

  • energy released in the form of fatty acids and monoacylglycerols

in naming chylomicrons, how are they classified?

• based on protein : lipid

• or density

what are the different lipids that travel in circulation?

• chylomicrons

• VLDL

• IDL

• LDL

• HDL

what do each of these stand for?

• VLDL

• IDL

• LDL

• HDL

1. very low density lipoprotein

2. intermediate density lipoprotein

3. low density lipoprotein

4. high density lipoprotein

briefly outline the structure of starch

• polysaccharide

  • amylose - helical structure, alpha-1,4-glycosidic bonds b/w glucose molecules

  • sits within amylopectin - branched structure - alpha-1,6-glycosidic bonds

• storage molecule in plants

how does the structure of glycogen compare to the structure of starch?

1. glycogen is more branched

2. glycogen is less spiraled

what gives amylose its helical shape?

• H bonds that holds the polysaccharide chain in its spiral shape

how is glycogen stored?

• as granules in liver and skeletal muscle

how is glycogen digested?

• by amylase

which links in glycogen are more readily broken down?

• alpha-1,4-glycosidic bonds more easily than 1-6

Outline the structure of cellulose

• polysaccharide

• beta-1,4-glycosidic bonds

• linear, extended structure - strong hydrogen bonds → rigid, fibrous bundles } high tensile strength

what gives cellulose high tensile strenght

• strong hydrogen bonds b/w chains

what makes cellulose beta vs glycogen alpha?

• direction of OH group

  • glycogen - faces upwards

  • cellulose - faces downwards

what is hemicellulose?

• composed of xylose, glucose, mannose and arabinose

• found b/w cellulose fibrils which is partly digestible by certain bacteria

• more soluble than cellulose

what breaks down carbs in the mouth?

• salivary alpha-amylase

how are carbs digested in the small intestine?

• luminal phase: pancreatic amylase

  • still can’t be absorbed though

how do the SI villi prepare for carbohydrates?

• increase their SA for absorption

what happens to carbohydrates during the membranous phase?

• glucosidase enzymes: disaccharides → monosaccharides

what are glucosidase enzymes

• maltase

• sucrase

• lactase

how are monosaccharides transported across intestinal mucosa

1. glucose, maltose - limited by rate of epithelial transport

2. lactose - limited by rate of hydrolysis

what cells in the pancreas secrete digestine enzymes and in what form are these enzymes secreted?

1. acinar cells

2. zymogen granules

state 3 enzymes that act on proteins + peptides

• trypsin

• chymotrypsin

• (pro)-elastase

how do trypsin, chymotrypsin and (pro)elastase affect proteins + peptides

• cleave interior peptide bonds

what enzyme acts on the carboxyl terminal of peptides?

• pro-carboxypeptidase?

what does pro-carboxypeptidase have an effect on?

• released amino acids at carboxy terminal

what enzyme acts on triglycerides?

• lipase

what effect does lipase have?

• cleaves ester bond at postion 1+ 3 → yields free fatty acids and monoglycerides

what enzyme acts on phospholipids

• phospholipase

how does phospholipase affect phospholipids

• cleaves ester bonds at 2-position

what enzyme cleaves starch to maltos

• amylase

what does amylase do

• cleaves starch to maltose

what enzyme acts on RNA

• ribonuclease

what enzyme acts on DNA

• deoxyribonuclease

what effect does ribonuclease and deoxyribonuclease have?

• cleave RNA/DNA respectively to nucleotides

what is a zymogen

• inactive enzyme precursor

how are zymogens activatedc?

• by enteropeptidase in the duodenum- c

how can we describe the pathway of zymogen activation by enteropeptidase?

• cascade reaction → activated enzymes help activate further enzymes

what are the 6 groups of proteases?

1. serine proteases

2. threonine proteases

3. cysteine proteases

4. aspartic acid proteases

5. metalloproteases

6. glutamic acid proteases

what does a protease do?

• conducts proteolysis by hydrolysis of peptide bond b/w adjacent amino acids in a polypeptide chain

what is an exopeptidase?

• detaches terminal amino acid from polypeptide

what is an endopeptidase

• hydrolyses internal peptide bonds of a protein

3 example serine endopeptidases?

1. chymotrypsin

2. trypsin

3. elastase

what happens in the luminal phase of protein digestion?

• bond-specific proteases hydrolyse protein to short chain peptides

what happens in membranous phase of protein digestion?

• hydrolysed into di/tripeptides but some free amino acids

how are proteins transported across the gut wall?

• secondary active transport

what is the route of absorption for di/tripeptides?

1. peptide transporter with high affinity for these peptides - driven by an electrochemical gradient produced by Na+ pump

why is the peptide transporter the more dominant method of transport?

• very few single amino acids are absorbed directly

what is the route of absorption for single amino acids?

• active process involving Na+ dependent carrier-mediated cotransport system

  • similar to that of glucose

state 4 amino acids that selective carrier systems are present for?

1. neutral aas

2. acidic (dicarboxylic) aas

3. imino aas

4. basic aas