Carbohydrates Flashcards
Give some properties of carbohydrates
Highly oxidisable
Storage form of energy
Structure and protection
Cell to cell communication (ABO blood group antigens)
What are monosaccharides?
Sugars that can’t be hydrolysed to form a simple sugar.
Most commonly hexoses (6carbons)
What are the 3 main hexoses in human biochemistry?
Glucose
Galactose
Fructose
What are disaccharides?
2 monosaccharides linked together by glycosidic bonds
What is a glycosidic bond?
Covalent bond from when hydroxyl group of one monosaccharide reacts with an anomeric carbon of another monosaccharide.
What are the 3 main disaccharides of human biochemistry?
Maltose
Sucrose
Lactose
What is an anomeric carbon?
Mirror images of each other
It is carbon 1 on the glucose residue. Stabilises glucose and is the only residue that can be oxidised
What is the difference between homo- and heteropolysaccharides?
Homo- single monomeric species
Hetero- have 2 or more monomer species
What makes glycogen more extensively branched than starch?
It has alpha 1-6 bonds every 8-12 residues instead of every 24-30
What are glycoproteins?
Proteins that have carbohydrates covalently attached
What are the functions of carbohydrates attached to proteins?
Increase protein solubility
Influence folding and conformation
Protect it from degradation
Act as communication between cells
What are GAGs?
Glycosaminoglycans
Unbranched polymers made from repeating units of hexuronic acid and an amino sugar which alternate through the chains
What are Proteoglycans?
GAGs that have a protein covalently attached.
They form part of many connective tissues in the body.
What are Glycoproteins?
Very similar to proteoglycans but there is more protein present than carbohydrate.
Found in ECM, cell membrane, blood and within cells
What catalyses the final digestion of carbohydrates in the jejunum?
Mucosal cell surface enzymes
What are the main products of carbohydrate digestion?
Glucose
Galactose
Fructose
What type of carbohydrates cannot be digested by the gut?
Oligosaccharides
E.g cellulose and hemicellulose
What are the benefits of oligosaccharides not being digested?
Increase faecal bulk and decrease transit time
What are some causes of disaccharide deficiencies?
Severe intestinal infection
Inflammation of gut lining
Drugs injuring gut wall
Surgical removal of intestine
What characterises disaccharide deficiencies?
Abdominal distension and cramps
What is the most common disaccharide deficiency?
Lactose intolerance
Why does the lack of lactase give disaccharide deficiency symptoms?
Undigested lactose is broken down by gut bacteria causing a build up of gas and irritant acids
Lactose is osmotically active so draws water from gut into lumen causing diarrhoea
Why is glucose immediately phosphorylated when it reaches the liver or certain cells?
Glucose-6-phosphate can’t diffuse out of cells as GLUT transporters can’t recognise it
What enzymes catalyse the phosphorylation of glucose into glucose-6-phosphate?
Glucokinase (liver)
Hexokinase (other tissues)
What does the low Km and Vmax of hexokinase mean for tissue cells?
Low Km- even at low glucose levels tissues can grab glucose effectively.
Low Vmax- tissues are easily satisfied so don’t keep grabbing glucose
Give some fates of glucose-6-phosphate?
Pentode phosphate pathway
Glycolysis
Glycogen storage
What is the role of glucose-6-phosphatase?
Catalyses the release of glucose into the blood by converting glycogen into glucose-6-phosphate and then glucose.
What is von Gierke’s disease?
Liver, kidney and intestine glucose-6-phosphate deficiency
What is McArdle’s disease?
Skeletal muscle phosphorylase deficiency
Describe the process of glycolysis?
A catabolic pathway that saves potential energy from glucose by forming ATP through substrate level phosphorylation
What is the first committing step of glycolysis?
Phosphorylation of F-6-P to F-1,6 bisP. F-1,6bisP is solely destined for glycolysis. Irreversible step
Where does glycolysis occur?
In the cytosol
What does substrate level phosphorylation require?
Soluble enzymes and chemical intermediates
Why does NAD+ need to be replenished?
NAD+ needs to be regenerated as it is limited within the cell. It’s comes from niacin an essential vitamin.
Pyruvate replenishes this supply by oxidising NADH.
NAD+ required for metabolising various intermediates within glycolysis
What is the Cori cycle?
When the liver repays the oxygen debt muscles run up by converting lactate into glucose during gluconeogenesis
What are the 4 bypass reactions of gluconeogenesis?
1 - pyruvate to oxaloacetate
2- oxaloacetate to PEP
3- F-1,6bisP to F-6-P
4- Dephosphorylation of G-6-P to Glucose
Where does the last step in gluconeogenesis take place?
In the lumen of the ER.
Requires G-6-P to be shuttled in and glucose is then shuttled out
What is the name of the pathway that produces NADPH for all organisms?
Pentode phosphate pathway
