F. Enzymes digestion and absorption Flashcards
Name the components of proteins
amino acid monomers form polypeptide chains which form proteins
Draw the structure of an amino acid
……………...R ……………….| H2N-------C-------COOH …………...….| …………..….H
How are dipeptides made
The condensation of two amino acids
Name the bond that forms between two amino acids
peptide
How many polypeptides does a functional protein contain
one or more
Name all 4 bonds present in a protein
peptide (between amino acids)
hydrogen (hold together proteins secondary and tertiary shape)
disulfide bridges (hold together proteins tertiary structure)
ionic bonds (hold together proteins tertiary structure)
Describe the primary structure of a protein
A polypeptide chain of amino acids joined by peptide bonds formed in a condensation reaction.
Why is a proteins primary structure important
- the primary structure that determines a proteins shape and therefore function
- single change in the polypeptide chain causes bonds formed in different places
- changing the proteins tertiary shape which may
- cause it to function less well or differently
- as a proteins shape is very specific to its function.
Describe the secondary structure of a protein
When amino acids are joined by peptide bonds they become polar. This allows hydrogen bonds to form (between the H d+ of -NH and O d- of C=O) between amino acids in the polypeptide chain causing it to form alpha helixes or beta plated sheets.
Describe the tertiary structure of a protein
Beta plated sheets and alpha helixes form a more specific shape which is known as the proteins tertiary structure. This is stabilized by disulfide bridges, ionic bonds and hydrogen bonds.
Describe the quaternary of a protein
complex molecules containing a number of polypeptide chains and even groups not associated with proteins such as the iron group in haemoglobin (prosthetic group). This is held together by hydrogen ionic bond + disulfide bridges
Describe a test for protein
Add biuret reagent to sample
Describe a positive and negative result for the protein test
Positive: solution turns purple
Negative: solution remains blue
Name two types of basic protein shapes and their functions
1) Fibrous proteins (eg. collagen)- structural functions
2) Globular proteins (eg. enzymes and haemoglobin)- carry out metabolic reactions
Definition: enzyme
globular proteins that act as a biological catalyst
Definition: catalyst
catalysts are substances that alter the rate of reaction without altering themselves by lowing the activation energy of the reaction
Describe the induced-fit model of enzyme action.
the induced fit model proposes the proximity of the substrate leads to a change in the enzyme that forms the functional active site so that it becomes complementary to the substrate and can bind
Name the earlier outdated enzyme action model
The lock and key model
Why was the lock and key model replaced with the induced fit model
The lock and key model suggests enzymes are rigid structures.
However, scientists have observed that other molecules could bind to enzymes at sites other than the active site,
which changed the enzymes activity.
This suggests the enzyme shape is being altered and that enzymes are a flexible structure (not as the lock and key suggests)
which caused the lock and key model to be replaced by the induced fit model
Describe and explain the effect of temperature on enzyme action
At first a rise in temperature increases rate of reaction,
as molecules have more kinetic energy.
This means more enzyme substrate complexes per unit time are formed,
as enzymes successfully collide more frequently with substrate molecules.
However, the rate begins to decrease,
as the temperature causes bonds (eg. hydrogen bonds) to break
resulting in the enzymes active site to change.
Eventually the enzyme denatures (permanently) and the reaction ceases.
Describe and explain the effect of pH on enzyme action
A change in pH (from the enzymes optimum pH) alters the charges on the amino acids
this breaks the bonds stabilizing the enzymes tertiary structure causing the active site to change shape.
This means less enzyme substrate complexes are formed per unit time resulting in a decreased rate of reaction.
Describe and explain the effect of enzyme concentration on the rate of reaction.
When there is excess substrate, an increase of enzyme concentration is proportionate to the rate of reaction.
This is because more substrate molecules are now being acted on at once.
However if there is limiting substrate increasing enzyme concentration wont effect the rate of reaction.
This is because there are enough active sites to accommodate all remaining substrate.
Describe and explain the effect of substrate concentration on the rate of reaction.
When enzyme concentration is fixed, an increase in substrate is proportionate to the rate of reaction.
This is because at low concentrations there are excess enzymes not acting on a substrate and an increase in substrate would mean more substrate is being acted on at once.
However at a certain point substrate concentration wont have an effect on reaction rate as there is a limited amount of enzyme and all the active sites are being occupied at one time.
Definition: competitive inhibitor
a molecule that binds to the active site of an enzyme
Definition: non-competitive inhibitor
a molecule that binds to an enzyme at a position other than the active site (allosteric site)
Describe and explain the effect of substrate concentration (with a fixed concentration of competitive indicator) on rate of reaction.
An increase in substrate increases the rate of reaction as competitive inhibitors reduce the rate of reaction
by occupying the enzymes active site so adding substrate would
reduce the inhibitors concentration and therefore effect.
Describe and explain the effect of substrate concentration (with a fixed concentration of non-competitive indicator) on rate of reaction.
An increase in substrate concentration does not decrease the effect of the inhibitor as they are not competing for the same active site.
However it does reduce the maximum rate of reaction as inhibitor molecules can permanently change the shape of an enzyme, reducing the number of enzymes that can catalyse the reaction.
Name the major parts of the digestive system
- oesophagus
- stomach
- ileum
- large intestine
- rectum
- salivary glands
- pancreas
What is the ileum
- a long muscular wall
- inner walls are folded into villi (large SA)
- micro villi on epithelial cells of each villus
- function is too absorb the products of digestion into the bloodstream
What is the pancreas
Large gland that secrets pancreatic juice. The secretion contains proteases, lipase and amylase
What are the two stages of digestion
- physical breakdown
- chemical digestion
What is physical breakdown
Food is broken down into smaller pieces by structures such as the teeth. Provides a larger surface area for chemical digestion
Name three important digestive enzymes
- carbohydrase
- lipases
- proteases
Where is amylase produced
Mouth and pancreas
What does amylase do
Hydrolyses the alternate glyosidic bonds of the starch molecule to produce disaccharide maltose
Where is maltase produced
Lining of the ileum
What does maltase do
Hydrolyses glyosidic bonds in maltose, producing the monosaccharide alpha glucose
Describe the steps of starch hydrolysis (mouth)
- food enters mouth and is mixed with saliva
- saliva contains salivary amylase and salts to keep pH around 7 (optimal pH for salivary amylase)
Describe the steps of starch hydrolysis (stomach)
- food enters stomach
- acid denatures the amylase stopping hydrolysis of starch
Describe the steps of starch hydrolysis (small intestine)
- food passes into small intestine where it mixes with pancreatic juice
- pancreatic juice contains pancreatic amylase which hydrolyses the remaining starch to maltose
- alkaline salts maintain the pH so amylase can function (~pH 7)
What are alkaline salts produced by
- the pancreas
- intestinal wall
Describe the steps of starch hydrolysis (ileum)
- muscle in intestine wall push food along the ileum
- epithelial wall produces disaccharidase maltase
- maltase is not released into the lumen of the ileum but is part of the cell surface membrane (membrane bound disaccharidase
- maltase hydrolyses the maltose into alpha glucose
What is different about the disaccharidase in the ileum compared to amylase
- not released into the ileum
- is part of the cell-surface membrane
- therefore known as membrane-bound disaccharidase
Describe how sucrose and lactose are hydrolysed
- sucrase or lactase hydrolyses the single glyosidic bond in the sucrose or lactose
- produces the monosaccharides glucose + fructose and glucose + galactose
Where are lipases produced and found
- produced in the pancreas
- act in the stomach
What do lipases do
Hydrolyse the ester bond in triglycerides to form fatty acids and monoglycerides
What are micelles
Tiny droplets of bile salts associated with monoglycerides and fatty acids
Describe emulsification
Where lipids are split into micelles by bile salts which increases the SA of lipids so the action of lipases speed up
Name 3 types of peptidases
- endopeptidases
- exopeptidases
- dipeptidases
What do endopeptidases do
Hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules
What do exopeptidases do
Hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. This releases dipeptides and single amino acids
What do dipeptidases do
Hydrolyse the peptide bond between the two amino acids of a dipeptide. These are membrane-bound, part of the epithelial cells lining the ileum
Name enzymes produced by the epithelium
- maltase
- lactase
- sucrase
- dipeptidases
Where do endopeptidases work
Stomach
Where do exopeptidases work
Small intestine (duodenum)
What is the function of the ileum
Absorb the products of digestion
Describe how villi increase the rate of absorption
- increase the surface area for diffusion
- they contain muscle so are able to move. The movement mixes the contents of the ileum which maintains concentration gradient
- thin wall, reduces diffusion distance
- well supplied with blood to maintain concentration gradient
- posses microvilli to further increase SA for diffusion
By what mechanisms are amino acids and monosaccharides absorbed
Co-transport
Describe the role of micelles in the absorption of triglycerides
- monoglycerides and fatty acids remain associated with bile salts, these are known as micelles
- they come in contact with epithelial cells lining the villi of the ileum
- this causes them to break down releasing the monoglycerides and fatty acids
- here they passively diffuse across the cell surface membrane of the epithelial cells
By what mechanism are triglycerides absorbed
Passive diffusion as they are non-polar molecules (so lipid soluble
What happens when monoglycerides and fatty acids are absorbed by the epithelial cells of the ileum
- they are transported to the endoplasmic reticulum
- recombined to form triglycerides
- they then are transported to the Golgi apparatus where they associate with cholesterol and lipoproteins to form chylomicrons
- chylomicrons move out the epithelial cells by exocytosis where they enter lymphatic capillaries (lacteals) found at the centre of each villus
- here they pass into the blood system
- triglyceride in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries from where they diffuse into cells
