Lecture 23 - Digestion of food molecules Flashcards
What are the main macronutrients that contribute to energy production in the body?
Carbohydrates protein and fats
These are all polymers and so they must be broken down into smaller building block structures in order to be utilised
Carbohydrate broken down into…
Monosaccharides
Protein broken down into…
Amino acids
Nucelic acids broken down into…
Nucleotides
Fat broken down into…
Monoacylglyceride, free fatty acids and cholesterol
How do we get macronutrients?
Macronutrients are consumed in the diet and pass through the gastrointestinal tract
Salivary glands
Saliva (neutral pH) contains mucus and amylase which starts the digestion of carbohydrates
Stomach
Storage and mixing of food with gastric juices, slowly releases chyme into the intestine
Secretes acid (0.1 M HCl) = denaturing
Secreted pepsinogen which is converted to the active form of pepsin = protein digestion
Secretes mucus layer (protective)
Pancreas
Slightly alkaline pH 7
Secretes most of the digestive enzymes including amylase, lipase and several other proteases
Liver
Synthesis of bile salts/acids (stored in the gallbladder) important for fat digestion
Small intestine
Final phase of digestion and absorption
Two main phases of digestion
1- Hydrolysis of bonds connecting monomer units in food macromolecules
Carbohydrate: Glycosidic bonds leads starch to turn into disaccharides
Proteins: peptide bonds
Fat: Triacylglycerol ester bonds
2- Absorption of products from the gastrointestinal tract into the body
Breaking of bonds enables the absorption of them which allows for utilisation
Digestion of dietary carbohydrates
Provides 40-50% of energy intake
Starch i.e.alpha amylose, amylopectin
Simple sugars i.e. sucrose, lactose, fructose, glucose
Fibre - such as cellulose (undigestible by most mammals (because the glycosidic bonding cannot be processed as it is beta rather than alpha))
Amylopectin is the main component of plant starch - polymers of up to 1 million glucose units
Maltose is present in honey
Cellobiose and lactose
Cellulose and lactose are stereoisomers of one another
The functional groups in monosaccharides can be in either of two orientations. There is a convention of numbering the carbons (alpha and beta)
Cellubiose is a repeating disaccharide unit in cellulose
Mammals do not have an enzyme that can hydrolyse the beta 1,4 glycosidic bonds in cellulose
Lactose is present in milk (different orientation of hydroxyl functional groups)
Some people do not have the lactase enzyme and are unable to hydrolyse lactose
Sucrose hydrolysed to
Glucose and fructose
Salivary amylase
Source is the salivary glands
Acts on starch
Sit of action is in the mouth
Pancreatic amylase
Made in the pancreas
Acts on starch
Site of action is in the small intestine - more active on starch in the small intestine than in the mouth, it is another round of starch digestion
Maltase
Made in the small intestine
Acts on maltose as the substrate
Site of action is in the small intestine
Lactase
Made in the small intestine
Acts on lactose
Site of action is in the small intestine
Sucrase
Made in the small intestine
Acts on sucrose
Site of action is in the small intestine
Isomaltase
Made in the small intestine
Acts on isomaltose
Site of action is in the small intestine
Hydrolysis of starch and glycogen
Starch from plants consists mainly of: Amylose (a linear polymer of alpha 1,4 linked glucose units), amylopectin (a branched polymer of alpha 1,4 and alpha 1,6 linked glucose units)
Glycogen has a similar branched structure to amylopectin
Glycogen can be present in consumed foods such as liver and muscle
Glycogen is synthesised in animals from glucose and stored in liver and muscle, and then broken down to glucose when required by the body
The breakdown of glycogen stored in liver and muscle cells to glucose requires a ‘debranching enzyme’
Hydrolysis of starch
Starch
Amylose (plants)
Amylopectin (plants_
Starch digestion: the enzyme amylase hydrolyses alpha 1,4 glycosidic linkages
Repeated internal attack yielding smaller and smaller oligosaccharides producing maltose/isomaltose disaccharides as end products
Final digestion of carbohydrates at ‘brush border’ after amylase digestion
Intestinal epithelial cells secrete (examples):
Maltase/isomaltase turns maltose/isomaltase into 2 glucose molecules
Sucrase turns sucrose into fructose and glucose
Lactase turns lactose into galactose and glucose
The monosaccharides are absorbed into the body
Lactose intolerance
Lactase enzyme deficiency (genetic basis)
Causes bloating, flatulence and diarrhoea due to fermentation of lactose by intestinal bacteria
Need to avoid lactose in their diet because they cannot process it
No lactase - accumulation of lactose which is fermented to acids, H+ and CO2 - bloating and flatulence
Acids and lactose drive H2O into the intestine which results in diarrhoea
Digestion of dietary protein
Supplies amino acids to make body proteins
Supplies essential amino acids
Source of nitrogen for purines, pyrimidines, ham
Carbon skeletons can be used as fuel (N converted to urea and excreted in urine)
Essential amino acids
Leucine Lysine Threonine Tryptophan Isoleucine Methionine Phenylalanine Valine
Humans cannot synthesise all the amino acids and these are the ones that we cannot make and must instead receive them from an outside source
Kwashiorkor
Result of a deficiency of dietary protein which causes an osmotic imbalance in the GI system, causing the abdomen to swell (oedema) due to retention of water
In addition, the level of albumin in the blood is low affecting colloidal osmotic (oncotic) pressure and also transport of molecule e.g. hormones and drugs
Protein digestion
Involves hydrolyses of specific peptide bonds, performed by several different proteases
All proteases secreted as inactive forms (zymogens or proenzymes) - evolved because it is important to keep proteases under control in the body so that they do not damage it
All proteases are activated by the cleavage of peptides from their structure - most are activated by the cleavage of part of the protease protein polymer from the structure which allows refolding of the protease structure into its active and functional form
Protease
Protease specificity is determined by adjacent amino acid side chains (determines specificity of protease binding protein substrate)
Pepsin = aromatic i.e. Phe, Tyr
Trypsin = positively charged i.e. Lys, Arg
Chymotrypsin = aromatic i.e. Phe, Tyr
Two stages of protein digestion
Endopeptodases attack peptide bonds within the protein (peptide) polymer
Exopeptidases attack peptide bonds at the end of protein (peptide) polymer
Endopeptidases
Endopeptodases attack peptide bonds within the protein (peptide) polymer
Examples - pepsin, trypsin and chymotrypsin
Exopeptidases
Exopeptidases attack peptide bonds at the end of protein (peptide) polymer
Examples -amino peptidases, carboxypeptidases
Aminopeptidases
Hydrolyse peptides from the N terminal end
Source is the small intestine
Substrate is polypeptides
Site of action is in the small intestine
Carboxypeptidases
Hydrolyse peptides from the C terminal end
Pepsin
Stomach mucosa is the source
Substrate is proteins, pepsinogen
Site of action is the stomach
Trypsin
Source is the pancreases
Substrate is polypeptides and chymotrypsinogen
Site of action is in the small intestine
Chymotrypsin
Source is the pancreas
Substrate is polypeptides
Site of action is in the small intestine
Carboxypeptidase
Source is the pancreas
Substrate is polypeptides
Site of action is in the small intestine
Dipeptidase
Source is the small intestine
Substrate is dipeptides
Site of action is the small intestine
Pepsinogen activation to pepsin
Pepsinogen is the inactive zymogen (proenzyme) is activated to pepsin following exposure of pepsinogen to HCl in the stomach
Once there is active pepsin in the environment, it can hydrolytic ally activate other pepsinogen proenzyme molecules
In the stomach acidic environment, part of the pepsinogen protein unfolds, which activates the pepsin protease and results in hydrolysis of part of the pepsinogen protein sequence to generate stably activate pepsin protease
Proteases are initially produced as ….
Zymogen (proenzyme) inactive forms
Sequential hydrolysis by proteases
Pepsin in the stomach - trypsin in the small intestine - chymotrypsin in the small intestine - carboxypeptidases in the small intestine - amino peptidases in the small intestine
Dietary protein hydrolysis gives the end products of amino acids, di- and tri- peptides
