Carbohydrate Metabolism

digestion

preparation of food or feed for absorption; includes physical, chemical, and microbial action

absorption

passage of food or nutrients from the GI tract into the blood stream and its distribution

metabolism is the sum of

all biochemical processes that nutrients undergo to furnish energy and build new tissues

anabolism means to build up; whereas catabolism means to 

break 

products of CHO metabolism are

energy; stored and immediate; amino acid precursor

energy - glucose catabolism (remeber → breakdown) yields ATP which is

adenosine triphosphate, energy currency, heat produced as byproduct,

energy currency

how the organism captures chemical energy to perform the work necessary for life processes 

heat produced as the by product of metabolism is important

for homeotherms → mammals, birds

brain cells and red blood cells rely exclusively on

glucose for energy

immediate energy of carbohydrate metabolism is

glucose

stored energy of metabolism is

glycogen (short term) and CHO used as precursors to synthesize fatty acids (fat) (long term) 

amino acid precursor is C in

CHO used to synthesize the non-essential amino acids

CHO metabolism is centered around the metabolism of glucose while

other monosaccharides (i.e. galactose and fructose) enter glycolysis

ATP: adenosine triphosphate;

“molecular unit of currency” of intracellular energy transfer

GTP: guanosine triphosphate is a source of energy or an activator of substrates in

metabolic reactions, like that of ATP (1 GTP = 1 ATP )

NADH: nicotinamide adenine dinucleotide (reduced), is a 

reducing agent (electron donor) (1 NADH = 2.5 ATP)

FADH2: flavin adenin dinucleotide is

reduced; an electron donor (1 FADH = 1 ATP )

glycolysis is

the break-down of one glucose molecule to two pyruvate

glycogenolysis is

break-down of glucogen into glucose

glucogenesis

process of storing glucose as glycogen

gluconeogenesis is the synthesis of

glucose from non-hexose sorces (non-6 C sugar)

anabolic pathways are

glucogenesis and gluconeogenesis

catabolic pathways are

glycogenolysis, glycolysis, pentose phosphate pathway, pyruvate dehydrogenase pathway, and tricarboxylic acid cycle

pentose phosphate pathway is an alternate pathway for 

breakdown of glucose to pyruvate 

pyruvate dehydrogenase pathway is the

breakdown of pyruvate to acetyl CoA

tricarboxylic acid cycle is the

breakdown of aetyl CoA to CO2 (continued breakdown of glucose)

glucose is metabolic fuel that most cells get from

blood

in muslce and fat, glucose is transported from the blood into the cell through

GLUT4

GLUT4 is insulin-dependent, meaning that when the insulin: glucagon ratio increases,

more glucose is transported into the cells

the liver is a central place for CHO metabolism; glucose uptake from the blood into the liver cells is

indepentdent of insulin through GLUT2, it is the only “exporter” of glucose

blood glucose concentration is tightly regulated through

homeostasis

blood glucose is regulated by pancreatic hormones

insulin and glucagon

insulin stimulates the uptake of glucose by insulin-responsive tissues such as

skeletal muscle and adipose tissue

glucagon stimulates the release of

glucose from the liver

as insulin increases, blood glucose

decreases

as glucagon increases, blood glucose

increases 

both storage and breakdown of glucose occur at all times regardless of

nutritional status

in the fasted state, insulin: glucagon ratio is low. glucose is mainly broken down to

yield energy

glucose is made from other compounds such as

glycogen, glycerol from triglycerides, the carbon skeleton of some amino acids

in the fed state, insulin:glucagon ratio is

high

glucose is stored as

glycogen in liver and skeletal muscles and triglycerols in adipose tissues

remember that energy is always needed by cells, so

some glucose is always broken down, even when the insulin:glucagon ratio is high

circulating glucose means that it is broken down to

produce energy; glycolysis and krebs cycle

circulating glucose is stored in the body as

glycogen (glycogenesis) and fat (lipid synthesis)

in stage one (endergonic) of glycolysis how much ATP does it yeild?

-2 ATP

in stage two (exergonic) of glycolysis how much ATP does it yield?

+4 ATP 

glycolysis yields a net production of how much ATP?

+2 ATP

2 moles of NADH are produced during glucolysis what happens?

NADH generates ATP in the mitochondria; ~2.5 ATP per NADH; one of these is used to transport the NADH into mitochondria; glycolysis yields ~6 ATP

Glycolysis controlled by enzymes (activation and inhibition) either

negative feed back regulation or allosteric regulation

regulation of glycolysis

enzymes, hexokinase, phosphofructokinase, pyruuvate kinase

regulating glucose using hexokinase (glucokinase) uses

activated by glucose, inhibited by glucose-6-phosphate

pyruvate → to acetyl CoA occurs in the mitochondrial matrix and links glycolysis with krebs’ cycle. pyruvate is transported from 

cytosol into mitochondrial matrix by a pyruvate carrier 

one carbon from pyruvate is lost as CO2, making the reaction irreversible and commiting acetyl CoA for entry into Krebs’ cycle where 

one NADH is produced and the other NADH is transported into the mitochondria, where it is used for ATP synthesis by oxidative phosphorylation

How much energy is produced from one pyruvate in the krebs cycle?

4 NADH, 1 FADH2, 1 GTP with a net energy production of ~12.5 ATP/Pyruvate 

glycolysis and krebs cycle =

energy

glycolysis →

4 ATP per mole of glucose

krebs cycle →

25 ATP equivalents per mole of glucose (2 pyruvate (12.5+12.5 ATPs) enering the krebs cycle) 

oxidation of one mole of glucose yields how much ATP

31 ATP

How is ATP formed from NADH and FADH2?

ATP is formed from NADH and FADH2 by donating electrons to the electron transport chain, creating a proton gradient that powers ATP synthase

Glucose catabolism efficciency of ATP capture is ~35% and refers to

the fraction of energy stored in glucose that is actually converted into usable ATP by the cell. Not all energy from glucose oxication is captured in ATP - some is lost as heat 

in excercising muscles and in red blood cells (no mitochondria) there might be a buildup of NADH because 

there is not sufficient amounts of O2 to oxidize NADH back to NAD+ in the mitochondria 

When NAD+ concentration falls too low for glycolysis to continue, to compensante, tissues

react pyruvate with NADH molecule and a free hydrogen ion to form lactate, thus regenerating NAD+ 

Lactate is released into the bloodstream and picked up primarily by the liver for

(re-) synthesis into glucose (via gluconeogenesis in the Cori cycle)

The re-formed glucose can enter the muscle or red blood cells again, under anaerobic conditions, the majority of

acetyl CoA does not enter Krebs cycle

Cori Cycle=

lactate shuttle between muscle and liver to regenerate glucose during anaerobic condtions

there is isome aerobic glucose metabolism in muscles during exercise, the anaerobic pathway just becomes

much more important in terms of ATP production

Pentose Phosphate Pathway (PPP)

hexose monophosphate shunt is an alternative pathway for glucose metabolism that runs parallel to glycolysis. Main function is to produce NADPH and ribose-5-phosphate rather than ATP 

Glucose + NADP+ → pentose phosphate + NADPH →

glyceraldehyde-3-phosphate

Functions of PPP

needed for biosynthesis of nucleotides, NADPH is needed for fatty acid synthesis and other biosynthetic reactions (remember NADH is used for ATP synthesis), NADPH is also needed to protect against oxidative damage via anabolic reactions 

2 major anabolic fates for glucose are

glycogenesis and lipogenesis

glycogenesis occurs in the liver and skeletal muscle during times of excess glucose supplies (supply>demand) and is stimulated by 

insulin (high blood glucose → release of insulin from pancreas) 

how much of the livers weight is glycogen?

20%

how much of skeletal muscle weight is glycogen?

0.5-1.0%

Lipogenesis is the synthesis of

fatty acids and tirglycerides from acetyl-CoA

lipogenesis is particularly important in monogastrics being fed a diet high in

carbohydrates (ex; grain fed pigs)

lipogenesis occurs in adipose tissue and liver and also 

in the mammary glands of lactating animals 

an increase of blood glucose → increase insulin secretion →

increase glucose uptake by insulin sensitive tissues (i.e. WAT)

Glycogenolysis is the breakdown of

glycogen to glucose

glycogenolysis, glycogen serves as a reservoir of glucose, readily converted to

blood glucose (sugar) for distribution to other tissues

in glycogenolysis, skeletal muscle glycogen is only used locally, therefore,

liver glycogen is only glycogen source that can contribute to blood glucose

gluconeogenesis is the synthesis of glucose from non-sugar sources and it utilized when

glycogen stores have been depleted

gluconeogenesis primarily occurs in the liver, the kidney cortex can also

produce glucose for its own uses

gluconeogenesis main precursors are

amino acids, lactate, propionate, and glycerol

3 main sources of blood glucose are 

absorption of dietary glucose, gluconeogenesis, glycogenolysis 

VFA metabolism in the liver leaves little acetate utilized in the liver → most appear in

peripheral blood (can enter into Krebs Cycle as acetyl CoA)

over 90% of VFA metabolism in the lin liver in the portal blood is used for

gluconeogenesis

over 90% of butyrate in the portal blood used by liver and converted to

beta hydroxybutyrate

circulating blood in ruminants will have glucose, acetate, beta hydroxybutyrate but

very little propionate or butyrate

microbes make propionate →

liver makes glucose from propionate

propionate enter krebs cycle post-oxidative,

acetate and butyrate do not

acetate enters the krebs cycle

via acetyl CoA

butyrate converted to keton (beta hydroxybutyrate) and is then

converted to acetyl CoA

Propionate enter kreb’s cycle via

succinyl CoA, also converted to pyruvate and back to glucose