lecture 23 - digestion of food molecules Flashcards
What are the 4 key macronutrients found in food?
Carbohydrate, protein, nucleic acids, fat
What are the 3 main macronutrients that contribute to energy production in the body?
Carbohydrate, protein and fat
In what form do carbohydrates contribute to energy production in the body?
Monosaccharides
In what form do proteins contribute to energy production in the body?
Amino acids
In what form do nucleic acids contribute to energy production in the body?
nucleotides
In what form do fats contribute to energy production in the body?
Free fatty acids (FFA), monoacylglycerol (MAG), cholesterol
What enzyme is used in the stomach for protein digestion?
Pepsin
What type of enzyme is pepsin?
Protease
What is the inactive precursor of the protease pepsin?
Pepsinogen
What is the concentration of stomach acid secreted into the stomach?
0.1mol/L
What is the function of stomach acid in terms of protein digestion?
Helps to denature proteins
What are the 2 main phases of biochemical digestion?
Hydrolysis of bonds connecting monomer units in macromolecules, then the absorption of these hydrolysis products via the GI tract
What bonds are broken in carbohydrate digestion?
Glycosidic bonds
What bonds are broken in protein digestion?
Peptide bonds
What bonds are broken in fat digestion?
Triacylglycerol ester bonds
What are the 2 types of starch used by the body?
alpha-amylose, amylopectin
What are the 3 types of carbohydrate taken in by the body for digestion?
Starch, simple sugars, fibre (cellulose)
What are the main components of plant starch?
Amylopectin and amylose
What is cellobiose?
A repeating disaccharide unit in cellulose that has Beta-1,4 bonds
What is the stereoisomer of cellobiose?
Lactose
What is the stereoisomer of lactose?
Cellobiose
Why can mammals not hydrolyse the bonds in cellulose?
The bonds in cellobiose, which are the disaccharides that make up cellulose, have Beta 1,4 bonds, and mammals have no enzymes that hydrolyse this specific bond.
What enzyme hydrolyses the bonds in lactose?
Lactase
What is sucrose hydrolysed to?
Glucose and fructose
What are the 2 enzymes that break down starch?
Salivary amylase and pancreatic amylase
What is the structure of amylose?
A linear polymer of alpha-1,4 linked glucose units
What is the structure of amylopectin?
A branched polymer of alpha-1,4 AND alpha-1,6 linked glucose units
What foods typically contain glycogen as a starch source?
Liver and muscle
How does hydrolysis of starch progress?
Repeated internal attack of alpha-1,4 glycosidic bonds all along the chain by amylase, yielding smaller and smaller oligosaccharides.
What are the end products of the hydrolysis of starch?
Maltose and isomaltose - disaccharides
Where does the final digestion of the products of amylase digestion occur?
At the brush border within the small intestine
What enzymes are used at the brush border of the small intestine to digest disaccharides into monosaccharides?
Maltase, isomaltase, sucrase, lactase
What is the key purpose of the digestion of dietary protein?
Supplying the body with amino acids, especially essential ones, to make body proteins
What molecules does the digestion of dietary protein supply a source of nitrogen for?
Bases - purines and pyrimidines, and haem
How are carbon skeletons from the digestion of dietary protein used as fuel?
Carbon skeletons used, while Nitrogen is converted to urea and then excreted in urine
What are essential amino acids?
Amino acids that are essential for body function and protein synthesis, but cannot be directly synthesised by the body so must be obtained from the diet
What is ‘Kwashiorkor’?
A condition resulting for a deficiency in dietary protein that causes osmotic imbalances in the GI system
In what form are proteases secreted?
As inactive forms - zymogens/proenzymes
How are proteases activated?
By the cleavage of peptides from their structure
What is protease specificity determined by?
The adjacent amino acid side chains in the protein substrate impact on the specificity of binding and therefore the hydrolysis of the bond in the substrate.
What are the 2 stages of protein digestion?
Endopeptidases attack peptide bonds within the protein, Exopeptidases attack peptide bonds at the end of the peptides.
What are endopeptidases?
Enzymes that attack peptide bonds within a peptide during protein digestion
What are exopeptidases?
Enzymes that attack peptide bonds at the end of peptides
What are the 2 types of exopeptidase?
Aminopeptidase, Carboxypeptidase
What is the function of aminopeptidase?
Attack peptide bonds at the amino-terminus of peptides
What is the function of carboxypeptidase?
Attack peptide bonds at the carboxy-terminus of peptides
What are the substrates of trypsin?
Polypeptides and chymotrypsinogen (is activated)
What enzyme breaks the peptide bonds in dipeptides?
Dipeptidase
What is pepsinogen activated to pepsin by?
Exposure to HCl at low pH in the stomach
How is pepsinogen activated to pepsin in the stomach?
In the acidic environment, part of the protein unfolds, activating the pepsin protease and resulting in the hydrolysis of part of the protein sequence to generate stably activated pepsin.
What is the order of proteases that break down protein, from large polypeptides to single amino acids and dipeptides/tripeptides?
Pepsin (stomach), trypsin (small intestine), chymotrypsin (small intestine), carboxypeptidases (small intestine), aminopeptidases (small intestine)
What protein activates trypsinogen to trypsin?
Enterokinase (membrane bound in the small intestine)
What protein activates chymotrypsinogen to chymotrypsin?
Trypsin
