2 - Carbohydrates

Question 1

describe the general structures and functions of carbohydrates

Answer 1

general formula (CH2O)n
- may contain aldehyde (aldose) or keto groups
- aldehydes and ketones react with an alcohol to form cyclic groups
- monosaccharides are single sugar units with 3-9 carbons
- disaccharides are two sugar units
- oligosaccharies are 3-12 units of sugars
- polysaccharides are 10-1000s of sugar units

Question 2

name some common sugars and what they are in humans

Answer 2

• glucose = single 6C monosaccharide. Energy source and broken down to pyruvate in glycolysis
• fructose = fruit sugar. monosaccharide
• sucrose = table sugar. Glucose-Fructose disaccharide.
• lactose = milk sugar. Glucose-Galactose disaccharide.
• maltose = glucose-glucose disaccharide. Often found in processed food
• starch = polymer of β glucose. Carbohydrate storage molecule in plants
• glycogen = polymer of glucose. Carbohydrate storage molecule in animals. Highly branched

therefore the 3 main dietary monosaccharides are glucose, fructose and galactose

Question 2

name some common sugars and what they are in humans

Answer 2

• glucose = single 6C monosaccharide. Energy source and broken down to pyruvate in glycolysis
• fructose = fruit sugar. monosaccharide
• sucrose = table sugar. Glucose-Fructose disaccharide.
• lactose = milk sugar. Glucose-Galactose disaccharide.
• maltose = glucose-glucose disaccharide. Often found in processed food
• starch = polymer of β glucose. Carbohydrate storage molecule in plants
• glycogen = polymer of glucose. Carbohydrate storage molecule in animals. Highly branched

therefore the 3 main dietary monosaccharides are glucose, fructose and galactose

Question 3

how are the main dietary carbohydrates digested

Answer 3

starts extracellularly in the GI tract by glycosidase enzymes:
- in the saliva, amylase breaks down starch and glycogen into dextrins
- pancreatic amylase then breaks down further into monosaccharides
- any remaining disaccharides are broken down by disaccharidases (attached to the brush border membrane of epithelial cells)

lactase (lactose)
sucrase (sucrose)
pancreatic amylase (α 1-4 bonds)
isomaltase (α 1-6 bonds)

dextrins = small ogliosaccharides

Question 4

how are monosaccharides, such as glucose, absorbed in the body

Answer 4

1. the Na+/K+ pump uses ATP to pump 3Na+ from intestinal epithelial cell into the capillary, at the same time as pumping 2K+ out of capillary into intestinal epithelial cell
2. this maintains the Na+ gradient, creating a lower concentration of Na+ inside the intestinal epithelial cells
3. this allows the SGLT1 to bring in 2Na+ from the ileum lumen (down CG), bringing with one monosaccharide (against CG) from ileum lumen into the intestinal epithelial cell
4. glucose can then (facilitated) diffuse out of intestinal epithelial cell into the capillary via GLUT2 (passive transport down concentration gradient)
5. transport, via blood supply, to target tissues
6. glucose uptake into target cells via facilitated diffusion using transport proteins (GLUT1-5). Down CG (high→low concentration)

GLUT2 = glucose transporter 2
SGLT1 = sodium-glucose transporter 1

GLUTs have different tissue distribution and affinities, and can be hormonally regulated, ie by insulin

Question 5

explain why cellulose is not digested in the human GI tract

Answer 5

cellulose has β glycosidic linkages, whereas starch and glycogen hae α glycosidic linkages
- the β bonds in cellulose make it very planar, which are important for its structural function
- the α bonds present in starch/glycogen are much more flexible
- the bonds have different shapes in 3D space, so are cleaved by different enzymes
- humans don’t possess enzyme capable of cleaving β-1,4 glycosidic bonds

Question 6

what is lactose intolerance

what are the symptoms and mechansim behind symptoms?

difference between primary, secondary and congenital are on different card

Answer 6

where individuals have an inability to digest lactose
lactose is found in dairy products and many processed foods. Caused by different reasons (different card)

• as unable to digest lactose in small intestine, makes it’s way to large intestine
• in large intestine, bacteria start to digest it, and they produce H2 gas and methane etc
• basically a fermentation process
• environment where osmosis pulls water into the large intestine
• this causes dehydration and diarrhoea

other symptoms:
- bloating
- cramps
- flatulence
- vomiting
- rumbling stomach

needs to be treated carefully in infants, as untreated can lead to dangerous dehydration

Question 6

what is lactose intolerance

what are the symptoms and mechansim behind symptoms?

difference between primary, secondary and congenital are on different card

Answer 6

where individuals have an inability to digest lactose
lactose is found in dairy products and many processed foods. Caused by different reasons (different card)

• as unable to digest lactose in small intestine, makes it’s way to large intestine
• in large intestine, bacteria start to digest it, and they produce H2 gas and methane etc
• basically a fermentation process
• environment where osmosis pulls water into the large intestine
• this causes dehydration and diarrhoea

other symptoms:
- bloating
- cramps
- flatulence
- vomiting
- rumbling stomach

needs to be treated carefully in infants, as untreated can lead to dangerous dehydration

Question 7

what are the different types of lactose intolerance

Answer 7

primary lactase deficiency
- in most populations, we only express high levels of lactase during infancy
- by age 5-7, lose 90% of our ability to digest lactose
- however, in some populations, particularly NW Europe and USA, where milk is a major dietary component, the ability to digest lactose carries on (LACTOSE PERSISTENT PHENOTYPE)
- in those populations, primary lactose deficiency only refers to individuals who lack this ‘normal’ lactose persistent phenotype
- only occurs in adults

secondary lactase deficiency
- caused by injury to small intestine (gastroenteritis, coeliac, crohn’s, ulcerative colitis etc)
- occurs in both infants and adults
- generally reversible, once epithelial cells recover

congenital lactase deficiency
- extremely rare, autosomal recessive defect in lactase gene
- cannot digest breast milk
- evolution (where babies needed to be able to digest milk) means that this is rare
- give the babies formula instead

Question 8

describe the glucose-dependency of some tissues

Answer 8

all tissues can remove glucose, galactose and fructise from the blood
- all tissues metabolise glucose
- liver is the major site of fructise and galactose metabolism
- glucose is major sugar in blood, and concentration is relatively constant
- some tissues have an absolute requirement for glucose, and the rate of glucose uptake into these tissues is dependent on its concentration in the blood
- healthy adult on normal diet requires around 180g of glucose per day
- some tissues, such as RBCs, kidney medulla and lens of eye can only use glucose, requiring around 40g
- brain and CNS usually prefer glucose, requiring around 140g per day
- variable amounts are required by tissues for specialised functions (ie synthesis of triacylglycerols in adipose tissue requires glucose metabolism to provide glycerol phosphate)

Question 9

describe the key features of glycolysis

Answer 9

• central pathway of all carbohydrate (CHO) metabolism
• occurs in all tissues (cytosolic)
• exergonic (energy producing)
• oxidative
• irreversible pathway
• 6C molecules → 2 x 3C molecules (no loss of CO2)
• with one additional enzyme (LDH), it’s the only pathway that can operate anaerobically

Question 10

what is the purpose of phsophorylation in glycolysis

and enzyme that carries this out

Answer 10

phosphorylation of glucose to glucose-6-phosphate by hexokinase (glucokinase in liver)
- makes glucose negatively charged (anionic)
- prevents passage back across the plasma membrane
- increases the reactivity of glucose to permit subsequent steps
- this idea (initial priming) to get the pathway going is a principle seen in many catabolic pathways

glucose is transported passively into cell by glucose transporters. Glucose could also leave cell pathway. Once its phosphorylated by hexokinase, then glucose is charged and it can no loner go through the glucose transporter ⇢ committed to the cell

Question 11

what are the main functions of glycolysis

Answer 11

• oxidation of glucose
• NADH production (2 per glucose)
• synthesis of ATP from ADP (net gain is 2 ATP per glucose)
• provides biosynthetic precursors for fatty acids, amino acids and nucleotides

Question 12

why are there so many steps + enzymes involved in glycolysis?

Answer 12

1. chemistry is easier in small stages
2. efficient energy conservation
3. allows for fine control
4. gives versatility:

• allows interconnections with other pathways
• allows production of useful intermediates
• allows part to be used in reverse

Question 13

describe how some important intermediates are derived from glycolysis

Answer 13

2,3-Bisphosphoglycerate
* produced in red blood cells from 1,3-bisphosphoglycerate (product of phase 2 glycolysis)
* via the bisphosphoglycerate mutase enzyme
* important regulator haemoglobin O2 affinity (promotes release)

glycerol phosphate
- produced in adipose tissues and liver from dihydroxyacetone-P (product of phase 2 glycolysis)
- via the glycerol 3-phosphate dehydrogenase enzyme
- important to triglyceride and phospholipid biosynthesis
- lipid synthesis in adipose tissue requires glycolysis
- however, liver can also phosphorylate glycerol directly

Question 14

explain how fructose is metabolised

Answer 14

• dietary sucrose is hyrdolysed by the digestive enzyme sucrase → glucose + fructose
• these are absorbed into the bloodstream
• fructose is metabolised largely in the liver
• fructose is phosphorylated by fructokinase so that it is retained in the cell → fructose -1P
• Adolase enzyme catalyses fructose-1P to form glyceraldehyde 3-phosphate which is an intermediate of glycolysis

Question 15

essential fructosuria + fructose intolerance

what are the clinical links to issues with fructose metabolism

and what enzymes are missing

Answer 15

essential fructosuria

• fructokinase (enzyme that phosphorylates fructose → fructose- 1P)
• fructokinase is missing
• therefore unable to carry out first step of fructose metabolism
• fructose is accumulated and excreted
• fructose in urine, no other clinical signs
• benign condition, just means that there is an energy source that cannot be utilised

fructose intolerance aka hereditary fructose intolerance, HFI

• adolase (enzyme that converts fructose-1P → glyceraldehyde, which is an intermediate of glycolysis)
• adolase missing
• accumulation of fructose-1P substrate in liver
• fructose-1P uses up inorganic phosphates, and therefore inability to have normal ATP control
• leads to liver damage and possible death
• managed by removing fructose and sucrose from diet

Question 16

explain how galactose is metabolised

Answer 16

dietary lactose is hydrolysed by the digestive enzyme lactase to release galactose and glucose
- these products are absorbed in the bloodstream
- galactose is metabolised largely in the liver
- galactose is phosphorylated by galactokinase → galactose-1P
- galactose-1P is catalysed by uridyl transferase → glucose-1P which goes on to form glucose-6P which is then used in glycolysis
- galactose-1P can also be catalysed by UDP-galactose epimerase → UDP-galactose, which can the form UDP-glucose and then form glycogen

UDP Galactose is used in many metabolic pathways for sythesis of glycoproteins and glycolipids. Production of this is really important for biosynthesis

Question 17

explain why lactate production is important in anaerobic glycolysis

Answer 17

• total NAD+ and NADH is constant in cell
• therefore glycolysis would stop if all NAD+ is → NADH
• normally, NAD+ is regenerated from NADH in a later stage of metabolism
• in some cells/conditions (ie cells with no stage 3+4 of metabolism or lack of O2), insufficient NAD+ is regenerated
• therefore need to regenerate NAD+ via a different route
• pyruvate → lactate (reduction) by lactate dehydrogenase (LDH)
• reversible enzyme and reaction
• uses NADH and H+ to reduce pyruvate
• regenerates NAD+ for glycolysis to continue

this is important for anaerobic respiration, as glycolysis is the only source of ATP, and therefore the only chance to generate energy for the body

Question 18

explain how the blood concentration of lactate is controlled

Answer 18

• lactate is transported from anaerobic tissues (with high lactate concentration) to heart, liver and kidney via the bloodstream

in the heart
- due to the high O2 blood supply coming from lungs
- LDH can work in both directions
- lactate can be used to produce pyruvate (oxidation) which can be used to generate energy (+ CO2)

in liver and kidney
- lactate can also be oxidised to form pyruvate by LDH
- pyruvate may be converted to form glucose (gluconeogenesis), which can be taken up by other tissues
- pyruvate can also be used directly by liver and kidney cells to produce energy and CO2

Question 19

elevations of plasma lactate concentration

clinical link

Answer 19

plasma concentration determined by relative rates of
- production
- utilisation (liver, heart and muscle)
- disposal (kidney)

high levels of lactate can cause problems as it is acidic in the blood

hyperlactaemia
- 2-5mM ie more production than disposal
- below renal threshold
- no change in blood pH due to buffering capacity

lactic acidosis
- above 5mM
- aboe renal threshold
- blood pH lowered
- alter protein function, loss of cardiac contractility and loss of circulatory control
⇢ critical marker in the acutely unwell patient

Question 20

explain the biochemical basis of the clinical conditions of galactosaemia

mechanism for cataracts on different card

Answer 20

a person cannot break down the sugar present in milk into glucose and is due to a lack of one of: galactokinase, uridyl transferase, or UDP galactose epimerase

• lack of uridyl transferase (most common) causes the most severe symptoms as it is due to accumulation of galactose and galactose-1P
• galactose build-up causes it to enter alternative pathway and get reduced → galactitol (by aldose reductase). This reaction depletes some tissues of NADPH.
• accumulation of galactose-1P affects liver, kidney + brain (could be due to sequestration of Pi, making it unavailable for ATP synthesis)
• causes cataracts and glaucoma in some patients (mechansim on next card)

treatment is the removal of lactose from the diet

evidence is galactose and galactitol present in the urine

Question 21

why do galactosaemic patients develop eye problems?

Answer 21

• galactose builds up
• enters alternative pathway, gets reduced to form galactitol by aldose reductase
• involves NADPH → NADP+
• this depletes some cells of NADPH
• cross linking of lens proteins by disulfide bond formation causes cataracts
• in addition, accumulation of galactose and galactitol in the eye lead to increased intro-ocular pressure which causes glaucoma
• if goes untreated, glaucoma can cause blindness

Question 22

explain why the pentose phosphate pathway is an important metabolic pathway in some tissues

Answer 22

important pathways in tissues such as the liver, RBCs and adipose tissue

the major functions of the pathway are to…
produce NADPH
- in cytoplasms
- provision of reducing power for anabolic processes such as lipid synthesis (liver and adipose).
- in RBCs, maintains free -SH groups on cysteine residues in certain proteins
- involved in various detoxification mechanisms that protect cells against toxic chemicals

produce the 5C sugar ribose
- for synthesis of nucleotides (for DNA and RNA)

Question 23

describe the pentose phosphate pathway

Answer 23

• glucose 6P (after first step of glycolysis) is converted → 6-phosphogluconactone by glucose-6-P dehydrogenase
• that is then further converted to form ribose-5-P
• steps produce NADPH from NADP+
• an oxidative pathway, but no ATP generation and CO2 is produced
• pathway is regulated by controlling the activity of glucose 6-phosphate dehydrogenase (controlled by NADP+:NADPH)
• NADPH inhibits, NADP+ activates

Question 24

describe the clinical condition of glucose 6-phosphate dehydrogenase deficiency (G6PD deficiency)

symptoms on different card

Answer 24

• common X linked gene defect
• caused by point mutations in gene coding for glucose 6-phosphate dehydrogenase (see pentose phosphate pathway)
• results in reduced activity for the enzyme, and therefore lower levels of NADPH
• NADPH is required to reduce oxidised glutathione back to its active reduced form
• reduced glutathione protects cells against oxidative damage (by mopping up free radicals)
• RBCs are particularly affected, as PPP is only source of NADPH in these cells
• Haemoglobin and other proteins become cross-linked by disulphide bonds, resulting from oxidative damage
• form insoluble aggregates called Heinz bodies
• leads to premature destruction of the RBCs and causes haemolysis
• causes haemolytic anaemia

found mainly from males originating from Mediterranean region and Black USA males

Question 25

describe the clinical symptoms of glucose 6-phosphate dehydrogenase deficiency (G6PD deficiency)

biochemistry on different card

Answer 25

• causes haemolytic anaemia
• this is where RBCs are broken down faster than they are made
• paleness (in darker-skinned individuals, this is seen on lips and tongue) due to decreased blood supply to the skin
• fatigue and dizziness (due to low oxygen in tissues)
• tachycardia (as heart needs to pump faster to compensate for less O2 being delivered to tissues)
• increased respiratory rate and shortness of breath
• splenomegaly (due to spleen working harder than usual due to decreased RBCs)
• jaundice (byproduct of haemolysis, bilirubin, is not excreted by hepatic cells quickly enough)
• dark coloured pee

Question 26

glycolysis regulation of hexokinase

Answer 26

hexokinase (not glucokinase in liver)
- glucose → glucose - 6P
- is regulated by it’s product, glucose 6P
- inhibited by high glucose 6P levels
- negative feedback mechansim

Question 27

glycolysis regulation of phosphofructokinase (PFK)

Answer 27

fructose 6P → fructose 1,6 bis P

allosteric regulation (muscle)
- inhibited by high ATP
- inhibited by high citrate, which is a product much further down the pathway (negative feedback)
- stimulated by high AMP
- stimulated by high F2,6,BP
- therefore it is ATP:AMP ratio that inhibits/stimulates enzyme
- high AMP levels suggest that ATP levels are low

hormonal regulation (liver)
- stimulated by insulin (insulin levels are high if blood glucose levels are high, therefore lots of glucose available for glycolysis)
- inhibited by glucagon

Question 27

glycolysis regulation of pyruvate kinase

Answer 27

involved in the last step of glycolysis, where the intermediate forms pyruvate

hormonal regulation
- irreversible reaction
- high insulin relative to glucagon causes activation of the enzyme
- ie when glucose levels are high in blood ⇢ insulin released ⇢ high insulin relative to glucagon ⇢ activate pyruvate kinase ⇢ allows pathway to continue
- in conditions where glucose levels are low, flux through the pathway will be limited