Chapter 6: Carbohydrate Metabolism

carbohydrate metabolism

changes glucose to glycogen, breaks glycogen down into glucose

catabolic pathways of carbohydrate metabolism

- breakdown

- glycolysis, the citric acid cycle, and oxidative phosphorylation

anabolic pathways of carbohydrate metabolism

- synthesis

- gluconeogenesis and the pentose phosphate pathway

default pathway for carbohydrate metabolism

glycolysis

glycolysis

the breakdown of glucose by enzymes, releasing energy and pyruvic acid

glycolysis location

cytoplasm

glycolysis inputs

glucose, fructose, galactose

glycolysis products

2 pyruvate, 2 ATP, 2 NADH, 2 H2O, and 2 H+

glycolysis energy investment phase

- glucose -> fructose-1,6-bisphosphate

- 2 ATP -> 2 ADP (use ATP)

- preparatory phase

fructose-1,6-bisphosphate

- an intermediate of glycolysis

- glucose -> _______ in energy investment phase of glycolysis

- inflection point

glycolysis steps (# and phases)

10 steps in 2 phases

where pyruvate goes after output from glycolysis

into the mitochondria

glycolysis energy generation phase

- 2 ATPs formed for each 3 carbons

- fructose-1,6-bisphosphate -> pyruvate

- payoff phase of glycolysis

D-glucose-6-phosphate (G6P)

- phosphate group from ATP

- phosphate keeps glucose trapped because of charge & can't diffuse out of cell - for energy production via glycolysis or glycogen synthesis

- first intermediate of glycolysis

regulation of glycolysis

- investment or production of ATP molecules

- process in the balance of energy production and storage

what investment or production of ATP molecules does for glycolysis regulation

speeds up or slows down glycolysis

low energy in the body

activates glycolysis

high energy in the body

inhibits glycolysis; energy storage

3 glycolysis enzymes that regulate ATP

- hexokinase

- phosphofructokinase

- 1- pyruvate kinase (in liver only)

energy storage location in ATP

phosphate bonds (cleave bonds to liberate energy)

chelation

the process of binding metal ions to the same ligand at multiple points

ion that stabilizes bonds in ATP

Mg2+

citric acid cycle (CAC)

- completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide

- AKA Krebs cycle, tricarboxylic acid (TCA) cycle

CAC inputs

pyruvate from glycolysis via the acetyl-CoA molecule

CAC location

mitochondrial matrix

acetyl-CoA

acetyl coenzyme A; the entry compound for the citric acid cycle in cellular respiration, formed from a fragment of pyruvate attached to a coenzyme

CAC outputs

3 NADH, 1 FADH2, and 1 guanosine triphosphate (GTP); electron carriers

CAC steps (#)

8; starts and ends with oxaloacetate

CAC endpoint

oxaloacetate; can't start CAC without oxaloacetate

what pyruvate oxides to

acetyl-CoA

buildup of an intermediate can (slow down/speed up) CAC

slow down

citrate

- first regulatory point and primary regulator in CAC

- first intermediate of the cycle and is important in the control of the body's carbohydrate resources

- allosteric inhibitor of phosphofructokinase

regulation of CAC

- availability of enzymes and substrates

- stalled if energy levels are high (NADH, FADH2)

- citrate

allosteric inhibitor

binds to allosteric site and reduces enzyme activity

oxidative phosphorylation (OP)

- the process in which NADH + H+ and FADH2 are oxidized in the electron transport system and the energy released is used to synthesize ATP from ADP and Pi

-

OP inputs

electron carriers (NADH, ADP, succinate, FADH2) + O2

OP outputs

ATP, NAD+, FAD+ and H2O

OP location

inner mitochondrial membrane

OP steps

respiratory chain (i.e., oxidative) and ATP synthesis via ATP synthase

input that makes OP aerobic

O2

OP is (catabolic/anabolic)

catabolic

complexes I-IV

move electrons

complex V

- final enzyme

- all about making ATP

supercomplex

multiple complexes working together

ATP synthase

- complex V

- enzyme that catalyzes the reaction that adds a high-energy phosphate group to ADP to form ATP

- 2 subunits: F0 and F1

what F1 catalyzes

ADP phosphorylation (catalysis)

ADP phosphorylation (reaction)

ADP + Pi --> ATP

ATP synthase stator

holds rotor and knob in position

ATP synthase axle rotation

rotation caused by F0 spinning in response to H+ flowing down concentration gradient

what ATP synthase is triggered by

- H+ acting as an electrical generator

- conformational change

what F1 facilitates as it moves

ADP -> ATP

inner part of ATP synthase

hydrophilic inner amino acids interact with H+

full oxidation of any organic fuel (formula)

CO2 + H2O

lactic acid

- product of fermentation in many types of cells, including human muscle cells

- anaerobic process result

what CAC and OP require that glycolysis doesn't

O2

cellular respiration

process of converting carbohydrates to usable energy

gluconeogenesis

the formation of glucose from noncarbohydrate sources, such as amino acids

gluconeogenesis inputs

2 pyruvate + 4 ATP + 2 GTP + NADH

gluconeogenesis outputs

glucose + 4 ADP + 2 GDP + 6Pi

gluconeogenesis location

liver (some kidney) cytosol

gluconeogenesis steps (#)

11 (some reverse of glycolysis)

when gluconeogenesis occurs

when the body has low energy stores (e.g., starvation, lengthy periods of exercise) and elects to divert non carbohydrate molecules (protein, fat)

energy expensive pathway

4 ATP + 2 GTP + NADH (all energetic molecules)

insulin

turns on glycolysis (when blood glucose is high)

glucagon

turns on gluconeogenesis (when blood glucose is low)

phosphofructokinase-2

promoted by insulin and inhibited by glucagon

fructose-2,6-bisphosphate

promoted by glucagon and inhibited by insulin

pentose phosphate pathway (PPP)

- make new nucleotides

- DNA and RNA are 5C sugars (riboses)

PPP input

glucose-6-phosphate

glucose-6-phosphate

product of first step of glycolysis

PPP outputs

ribose-5-phosphate sugar used to make DNA and RNA, and NADPH

PPP location

cytoplasm

ribulose-5-phosphate

if PPP gets to this molecule and the cell needs glucose, it will undergo the nonoxidative phase

PPP nonoxidative phase

transketolase and transaldolase

PPP steps

2 - oxidative and nonoxidative

NADPH in PPP

regulates partitioning of glucose 6 phosphate between glycolysis & PPP

high [NADP+] in PPP (and low NADPH)

- PPP oxidative pathway is accelerated

- go to nucleotides

high [NADPH] in PPP (and low NADP+)

- PPP oxidative pathway is inhibited

- nonoxidative phase

- won't go to nucleotides

glycogen

- stores glucose

- polymer in animals

glycogen synthesis

- involves sugar nucleotides (UDP glucose)

- UDP glucose molecules are added to existing glycogen

- investment of 1 ATP per glucose

- involves protein called glycogenin

uracil diphosphate (UDP) glucose

activated form of glucose

glycogen synthase

rate limiting enzyme of glycogen synthesis; adds UDP glucose to glycogen to form glycogen + 1 glucose + UMP

uridine monophosphate (UMP)

precursor for pyrimidine nucleotides

glycogenin

core protein found in the glycogen protein

start of glycogen synthesis

glycogenolysis

- glycogen metabolism

- breakdown of glycogen to glucose

- found in liver & muscle

- stored in large cytosolic granules

- provides energy in aerobic & anaerobic metabolism

what glycogenolysis is signaled by

increased blood glucose liberates stored glycogen as needed

glycogen breakdown process

1) removal of glucose by phosphorylase G

2) glucan transfer moves residues to linear part of chain

3) debranching enzyme cleaves final 1,6-bond

4) process continues based on metabolic needs

what glycogen can be catalyzed to create

glucose

catabolism

breaking down

anabolism

building up

epinephrine (liver and muscle)

when rapid need for glucose in glycogen metabolism

glycogen phosphorylase

- first enzyme to liberate monomers from glycogen in glycogen metabolism

- permits rapid release of glucose from glycogen stores

insulin (liver and muscle; in glycogen metabolism)

turns on glycogen synthesis