Metabolism Session 2 - Energy production (Carbohydrate 1 + 2) Flashcards
What are the three most commonly found forms of monosaccharides?
trioses, pentoses and hexoses
What feature of monosaccharides allows there identification by enzymes?
Whether they’re alpha or beta - refers to orientation of oxygen.
Give two important physiochemical properties of monosaccharides
Hydrophillic – water soluble, do not readily cross cell membranes
Partially oxidised – need less oxygen than fatty acids for complete oxidation.
How are disaccharides formed?
condensation of two monosaccharides with the elimination of water and formation of an O-glycosidic bond
Name three major dietary disaccharides
Maltose, sucrose and lactose
What are the two bonds which link monomers in glycogen?
Which gives it its branching structure?
alpha 1,4 and alpha 1,6
alpha 1,6 gives branches
Where and how is glycogen stored? (2)
Liver and skeletal muscles, in granules
What is cellulose?
Polymer of glucose found in plants?
How are the glucose units joined in cellulose?
By B 1,4 linkages to form long linear polymers
Why is cellulose important in the diet?
Provides healthy bowel function
Why can’t the human gut digest cellulose?
Enzymes unable to hydrolyse B-1,4 linkages,
Name two dietary polysaccharides, and describe the initial stages of their digestion.
Starch and glycogen
Hydrolysed by salivary amylase in the mouth and then pancreatic amylase in the duodenum.
Describe the structure of starch
contains amylose (a-1,4 linkages) and amylopectin (a-1,4 and a-1,6 linkages)
Where does the digestion of maltose, dextrins and dietary disaccharides lactose and sucrose take place?
the duodenum and jejunum
Where are glycosidase enzymes found?
On the brush border membranes of the epithelial cells.
Name three type of glycosidase enzymes
Lactase, glycoamylase and sucrase
What is lactose intolerance caused by?
Low activity of lactase enzyme
What causes the symptoms of lactose intolerance?
Fermentation of lactose by bacteria in the gut
How are monosaccharides transport in cells lining gut?
Via active transport
How does glucose get from cells lining gut to blood?
Via facillitated difussion, involving glucose transport proteins (GLUT 1 - GLUT 5)
What effect does insulin have on GLUT-4
Increases the number of GLUT-4 proteins in plasma membrane, increasing glucose upatke
Give four tissues which together have an absolute requirement for 40g glucose a day
Red blood cells, white blood cells, kindey medulla, lens of the eye
Name a tissue which has an absolute requirement for 140g glucose per day
CNS
What is the major site of fructose and galactose metabolism?
The liver
How many steps are involved in glycolysis? Which are rate limiting?
Ten.
1,3 and 10 are rate limiting.
Give four uses of glycolysis
ATP for cell function. (Only pathway to generate ATP anaerobically)
NADH from NAD+
Building block molecules for anabolism
Useful intermediates for specific cell functions (C3)
Why is glycolysis considered a redox reaction?
Glucose is oxidised to pyruvate and NAD+ is reduced to NADH
What is the first step of glycolysis ? What enzyme catalyses this step?
Glucose –> Glucose-6-phosphate
Hexokinase and Glucokinase
Why is it important to phosphorylate glucose in step 1 of glycolysis?
Makes the sugar anionic (-‘ve) so cannot cross the plasma membrane
- Increases reactivity of sugar so that it can be metabolised by several different pathways (glycolysis, glycogen formation, pentose phosphate)
- Allows formation of compounds with high phosphoryle-group transfer potential that can transfer their phosphate group to ADP to form ATP (substrate level phosporylation)
Why are step 1 and 3 of glycolysis irreversible?
Large -‘ve gibbs free energy changes
Why is reaction three known as the commiting step?
Commits flucose to metabolism via glycolysis as it is irreversible due to large negative delta G.
Which reactions in glycolysis involve substrate level phosphorylation?
7 + 10
Give seven key features of glycolysis
Starting material, end products and intermediates are c6 or c3
There is no loss of CO2
Some of the C3 intermediates are used by the cell for specific functions
Glucose is oxidised to pyruvate and NAD is reduced to NADH
Over it is an exergonic process with -‘ve delta G
ALl intermediates are phosphorylated, some able to undergo substrate level phosphorylation
Net yield of 2ATP
What is the role of DHAP in the body
Can be converted to glycerol phosphate, which is required for synthesis of tags and adipose tissue
Why is liver less dependent of glycerol phosphate produced from glycolysis than adipose tissue?
Glycerol kinase enzyme can phosphorylate glycerol directly
What is 2,3 BPG, and what is the name of its precursor formed in glycolysis?
Important regulator of oxygen affinity of haemoglobing and is produced from 1,3 BPG in red blood cells
What enzyme is used at step 3 of glycolysis?
Phosphofructokinase
Give two ways in which PFK is stimulated and inhibited, and give the locations of each
Allosteric regulation (msucle) - Inhibited by high ATP, stimulated by high AMP
Hormonal regulation (liver) -Insulin stimulates and glucagon inhibits
What enzyme is used in step 10 of glycolysis? What is it activated by?
Pyruvate kinase
High insulin:glucagon ratio
What enzyme is used to convert pyruvate into lactate, and what is the co-factor used?
Lactate dehydrogenase
NADH+H+ –> NAD+
What enzyme is used to convert lactate to pyruvate, and what co-factor is used?
Lactate dehydrogenase
NAD+ –> NADH+H+
What happens to pyruvate produced from anaerobic respiration?
Goes to liver, converted into glucose
Goes to heart, converted into CO2
When does lactic acidosis occur?
When lactate levels in blood are >5mmol/L
What is galactosaemia?
An inability to utilise galactose obtained from diet due to a lack of galactose kinase or transferase enzyme
What is the most serious enzyme defiency in galactosaemia, and why?
Absence of galactrose 1-phosphate uridyl transferase enzyme, as both glucose and galactose 1-phosphate build up in the tissues
What happens to galactose when it accumulates in the tissues? What is the product, the enzyme used and the cofactor?
Galactose –> galacticol
NADPH –> NADP+
Aldose reductase
What are the effects of galactosaemia, and what is their biochemical explanation?
Lens structure is damaged, due to cross linking S-S bonds in the proteins of the lens. This is because of absence of NAPH, which maintains -SH SH- structure. CATARACTS.
Non-enzymatic glycosylation of lens proteins, due to high concentration of falactose. CATARACTS.
Accumulation of glactose may cause intra-ocular pressure, and thus cause BLINDNESS.
Accumulation of galactose 1-phosphate in tissues causes damage to the liver, kidney and brain.
Give two purposes of pentose phospate pathway, and name the monomer which enters it
Glucose 6-phosphate
- Produces NADPH in the cytoplasm
- Produces the C5-sugar ribose for the synthesis of nucleotides.
What enzyme is used in the pentose phosphate pathway, and how is it regulated?
Glucose 6-phosphate dehydrogenase
Controlled by NADP+/NADPH ratio in the cell. NADPH inhibits.
What type of genetic condition causes G6PD defienc, and what does it cause low levels of? What cells does it mostly effect?
x-linked genetic condition, causing low levels of NADPH. Effects red bblood cells.
What happens in the red blood cells of someone with G6PD defiency, and why does this occur?
Haemoglobin and other proteins become cross-linked by disulphide bonds to form insoluble aggregates called heinz bddies, leading to haemolysis. This is caused by low levels of NADPH, which usually maintains free -SH HS- groups.
What becomes satured in G6PD defiency?
Glutathione system for reducting NADP back to NADPH.
What enzyme is responsible for the conversion of pyruvate to acetyl coA?
Pyruvate dehydrogenase
Why is pyruvate very sensitive to vitamin B defiency?
Because it is a multi-enzyme complex involving four separate B vitamins.
What are the two main consequences of PDH reaction?
Loss of CO2 from pyruvate is irreversible
Acetyl CoA cannot be converted to pyruvate, and therefore cannot be converted to glucose
When is PDH reaction activated?
When lots of low energy signals, such as ADP and NAD+
When hormones such as insulin are present in high concs.
What is lactose intolerance?
Low activity of the enzyme lactase, resulting in a decreased ability to digest lactose. Is instead fermented, causing diarrhoea and cramps.
Give three reasons NADPH is important
Reducing power for anabolic processes such as lipid synthesis
- In RBCs maintain free SH groups on cysteine residues
- Used in various detoxification methods
How does pentose phosphate pathway differ from glycolysis? (2)
No ATP produced
CO2 produced
How are the following regulated
Hexokinase
Phosphofructokinase - 1
Pyruvate kinase
Hexokinase
Inhibited by high G 6-P
Phosphofructokinase - 1
Inhibited by high ATP:AMP ratio (muscle)
Activated by high insulin:glucagon ratio (liver)
Pyruvate kinase
Increased insulin:glucagon
DEPHOSPHORYLATION
What is the chemical reaction for the reaction catalysed by pyruvate dehydrogenase?
Pyruvate + CoA + NAD+ –> Acetyl-CoA + CO2 + NADH + H+
