Carbohydrate metabolism

• 60% of food carbohydrate

• Starch ,glycogen,sucrose,lactose and cellulsoe are chief.

• Hydrolysed to hexose sugar (glusose, galactose and fructose) in gastrointestinal tract before they are absorbed.

• Hydrolysis of glycosidic bonds by glycosidases monohexose components.

In the mouth

• Salivary α- amylase (Ptyline).

• Salivary gland

• Opt.pH 6.1

• Activated by chloride ions (Clˉ).

• Acts on starch and glycogen breaking α(1-4)bond maltose

• Isomaltose (α1-6 linkage).

Cont.

• Food remains for a short time in the mouth patial digestion of starch ,dextrins(amylodextrins,erthrodextrins,and achroodextrins).

• So digestion in the mouthmaltose , isomaltose and starch and dextrins.

• In stomach high acidity inactivates the salivary α-amylase.

Digestion by intestinal enzymes

• The final digestive process occur at the mucosal lining and include the action of several disaccharides.

• These enzymes are secreted through and remain associated with the brush border of the intestinal mucosal cells

enzymes

• Lactase (β-galactosidase)which hydrolyse lactose into two molecules of glucose and galactose.

• Lactose lactase Glucose+ Galactose• Maltase(α-glucosidase)• Maltose maltase Glucose + glucose• Sucrase(α-fructofuranosidase)• Sucrose sucrase Glucose + frutose

Cont.

• Αlpha- dextrins (oligo-1,6 glucosidase)

• Isomaltose α-Dextrins Glucose+ Glucose

Digestion of cellulose

• Cellulose contains β(1-4)

• In human no β-(1-4) glucosidase.

• Cellulose passes as such in stool.

• Cellulose helps water rentention during the passage of food along the intestine ,producing larger ,softer feces.

Absorption of carbohydrates

• End products of carbohydrates are monosaccharides .

• Active transport .

• Passive trasnport.

Active transport

• Sugar should have:

• 1. hexose ring

• OH group at postiton 2 at the right side

• Present in glucose and galactose.

• Fructose does not contain OH to the right side at position 2 is absorbed more slowly than glucose and galactose by passive transport.

Mechanism of active transport

• Carrier proteins

• Two separate sites Na and glucose.

• Transport them from the intestinal lumen across cell membrane to the cytoplasm.

• Glucose and Na are released in the cytoplasm.

• Na is transported from high to low concentration and at same time causes the carrier to transport glucose against its concentration gradient.

• Enzyme adenosine triphosphatase.(ATPase)

Inhibitors of transport

• Ouabain(cardiac glycosidase): inhibit adenosine triphosphate necessary of ATP which produces energy of sodium pump.

• Phlorhizin:inhibits the binding of sodium in the carrier proteins.

• Passive transport (simple diffusion): Sugars passes with concentration gradient .No energy required.Fructose and pentoses are absorbed by this mechanism.

Defects of carbohydrates

• Lactase deficiency: Lactose intolerance

• 1. congenital : which occurs very soon after birth (rare).

• Acquired: which occurs later on life.

• Effect : presence of lactose causes.

• 1.increased osmotic pressure: water will be drawn from the tissues into the large intestine(osmotic diarrhoea).

Cont.

• 2. increased fermentation of lactose by bacteria .: Intestinal bacteria ferment lactose with subsequent production of CO2 gas..This causes distention and abdominal cramps.

• Treatment: 1.By removing lactose form diet.

• Tablets containing lactase enzyme.

Fate of absorbed sugar

• Uptake by tissues(liver): after absorption of sugars are taken up by the liver ,where galactose and fructose are converted by glucose.

• Utilization by tissues: glucose under go

• Oxidation : through

• 1. Major pathway: glucolysis and kreb cycle. Energy production

Cont.

• Hexose monophosphate shunt: production of ribose , deoxyribose , and NADPH + H+

• Uronic acid pathway: production of glucoronic acid which is used detoxication and enter in the formation of mucopolysaccharide.

Storage

• Glycogen : glycogenesis• Fat : lipogenesis• Conversion:• 1. Ribose,deoxyribose: RNA,DNA• 2. Lactose; Milk• 3. Glucosamine,Galactosamine;M

ucopolysaccharides• Glucuronic acid; Mucopolysaccharides,conjugation.• Fructose : semen

Glucose oxidation

• Conversion of glucose to CO2 ,H2O and energy .

• Occurs in cytoplasm(Glucose to pyruvate)

• Mitochondria (Pyruvate to acetyl CoA).

• Acetyl CoA enters a series of reaction known as Kreb cycle to be oxidised completely.

Embden-meyerhof pathway

• Oxidation of glucose or glycogen to give pyruvate (in the presence of oxygen) or lactate (in the absence of oxygen)

• Site .

• Muscle during exercise : due to lack of oxygen

• RBC’s :due to absence of mitochondria.

• One mol of glucose gives 2 mol of glyceraldehyde-3- phosphate and 2 ATP utilized..

Glycolysis…

• What happens when you burn sugar?• How do we extract the energy from the

food (fuel) that we eat without burning ourselves up?

• The answer: Glycolysis and the TCA/Krebs Cycle!

• Glycolysis and the TCA cycle is what we use to convert sugar (glucose) into usable chemical energy for our cells.

Cellular Respiration

• All organisms on this planet need energy to survive. As we already know, almost all most use trapped energy from the sun for their metabolic needs. Animals that can trap the energy themselves are called autotrophs; they make their own food. The rest of us are called heterotrophs; we need to eat other organisms for our energy.

What is Glycolysis?

• Glycolysis is a series of chemical reactions that make a little bit of ATP from the partial breakdown of sugar into energy.

• Organisms usually choose one of two paths after glycolysis: Fermentation or Aerobic Respiration.

Glycolysis:

• In glycolysis, organisms take in glucose, or some other 6 carbon sugar, and turn it into two three carbon molecules. It makes a little ATP in the process, which the organism can use for it’s metabolic needs.

The Net Equation:

• Glycolysis’ net equation is this:

Glucose + 2 ATP + 2 NAD+ ------------->

2 Pyruvic acid + 4 ATP + 2 NADH + 2 H+

• Pyruvic acid is 3 carbon molecule. NAD+ is a compound that accepts electrons so that they may be used elsewhere (similar to NADP+ in photosynthesis).

Fermentation

• Once the organism has performed glycolysis it can metabolize the pyruvic acid in several pathways, the first being fermentation.

• Fermentation occurs in the absence of oxygen. Fermentation does not make energy, but it does regenerate NAD+ so that glycolysis can continue.

Fermentation

• Our own bodies carry out fermentation. When you are working out really hard and your legs start to burn, that burning is the breakdown of glucose into pyruvic acid and then the fermentation of pyruvic acid into lactic acid. When your body is working hard, the oxygen that your lungs intake is inadequate to carry on normal aerobic respiration. Your body, in an attempt to keep things going (just in case it’s a bear chasing you) starts lactic acid fermentation.

Why Fermentation?

• Well, if your body couldn’t ferment pyruvic acid into lactic acid and NAD+, then you would quickly run out of NAD+ and glycolysis would stop. If you couldn’t perform glycolysis, then not only would those muscles of yours stop, but your heart and brain would run out of fuel as well and you would pass out or pass out and die.

As a note…

• When you eventually stop running, or when you are in good or great physical condition, you body can take that lactic acid that you made while working out and convert it back into pyruvic acid and send it off to the TCA cycle as normal. Well conditioned athletes can maintain aerobic conditions longer than untrained athletes, thus they can “go” longer and harder than those who are out of shape.

Some other kinds of Fermentation…

• Lactic acid fermentation is not the only game in town. There are thousands of kinds of fermentations. Some make ethyl alcohol, like in beer, wine, vodka, gin, etc. Ethyl alcohol (EtOH) is drinking alcohol. Some other organisms make rubbing alcohol, cheese, nasty petroleum distillates, etc.

Glycolysis is Not Very Efficient

• Glycolysis is not very efficient. It does not do a good job of extracting energy from sugar. In fact, it only obtains about 3.5% of the available energy from sugar.

Energy gain of glycolysis

• In the absence of O2• ATP produced• 2 ATP from 1,3

biphosphoglycerate• 2 ATP from

phophoenolpyruvate• ATP lost • 1 ATP from glu-6 p• 1 ATP from fru-1,6

biphoshatte

• In the presence of O2• ATP produced• 2ATP

1,3biphosphoglcerate• 2 ATP

phosphoenolpyruvate • 4 ATP or 6 ATP form

oxidation of 2NADH +H+ • ATP lost• 1 ATP – gluose 6 –p• 1 ATP- fru 1,6-p

Importance of glycolysis

• Only the source of contracting muscle during muscular exercise due to lack of O2 and to RBC due to absence of mitochondria.

• Gives 8 ATP • 2,3 biphosphoglycerate which may be

produced in glycolysis decrease the affinity of haemoglobin to O2. O2 is delivered in tissues especially in case if hypoxia.

•

Cont.

• Glycolysis provides the mitochondria with pyruvic acid ,which gives acetyl CoA (kreb cycle).

• Dihydroacetone phophate which may be converted to α-glycerophophate.Later is an important compound in lipogenesis.

• The amino acid serine may be formed from 3 phosphoglycerate.