Biochemistry: Hexose Monophosphate Pathway

hexose monophosphate pathway aka

pentose phosphate pathway

hexose monophosphate pathway is an important component of _____

biosynthesis

hexose monophosphate pathway provides ___ __and is the main source of__ ___

ribose-5-phosphate; nicotinamide adenine dinucleotide phosphate (NADPH)

the oxidative phase produces ___ __and__ ___

NADPH and ribulose-5-phosphate

the non-oxidative phase can use glycolytic intermediates to make ribose-5-phosphate without making _____

NADPH

the non-oxidative phase involves reversible _____

carbon shuffling

glucose-6-phosphate dehydrogenase (G6PD) is the first step and _____ enzyme of the pathway

rate-limiting

G6PD deficiencies:

1. common
2. mostly in males
3. mainly pose a problem with oxidative stress triggers

NDPH is important as a reducing agent for:

1. fatty acid and nucleotide biosynthesis
2. free radical reactions
3. other processes

Ribose 5-P is used for the synthesis of nucleotide-based molecules such as:

1. DNA
2. RNA
3. ATP
4. coenzymes (NAD+ and FAD)
5. coenzyme A (CoA)

some cells have greater requirement for NADPH over ribose 5-P and vice versa and that will determine the use of ___ __and__ ___ non-oxidative HMP pathway utilization

oxidative and non-oxidative

glucose 6 phosphate is the _____ substrate of the hexose monophosphate pathway (HMP)

starting

HMP is the main producer of _____ in the body

NADPH

no ATP is ___ __or__ ___ in the pathway

generated or consumed

the initial, regulated, and rate-limiting enzyme is _____

glucose 6 phosphate dehydrogenase (G6PD)

is a G6PD deficiency relatively common?

yes

two phases of reactions in the hexose monophosphate pathway:

1. oxidative
2. nonoxidative

the oxidative phase has 3 irreversible reactions that produce:

* 2 NADPHs
* 1 ribulose 5-phosphate
* 1 CO2 per 1 molecule of glucose 6 phosphate

nonoxidative involves

1. reversible carbon shuffling
2. interconversion of sugars

NADPH is the source of _____ for many processes in the cell

reducing equivalents

location in the cell of the hexose monophosphate pathway

cytosol

location in the body of the hexose monophosphate pathway

can occur in all cells (but more active in those that need NADPH and/or ribose 5-phosphate)

glucose, once phosphorylated to glucose 6 phosphate, instead of proceeding through ___ __or to__ ___, can take an alternative path through hexose monophosphate pathway (HMP)

glycolysis or to glycogen synthesis

there is no _____ generated or required in any of the reactions of the HMP

ATP

the oxidative phase consists of 3 _____ reactions

irreversbile

the oxidative phase produces _____ (unlike the non-oxidative phase)

NADPH

oxidative phase: HMP is most active in tissues that require:

1. fatty acid and steroid hormone biosynthesis
2. as well as in RBC which use NADPH for detoxifying reactions

oxidative phase: overall reaction

Glucose 6-P + 2 NADP+ + H2O → Ribulose 5-P + CO2 +2NADPH + H+

oxidative phase: 3 irreversible steps

1. Glucose 6P → 6-phosphoglucono-&-lactone by


1. **dehydrogenase (G6PD): rate limiting- produces the first NADPH**
2. 6-phosphoglucono-&-lactone → 6-phosphogluconate by gluconolactonase
3. 6-phosphogluconate→**ribulose 5-P** (5 carbon sugar); + **NADPH**, + CO2

regulation: G6PD catalyzes the initial committed _____ of the HMP

rate limiting step

regulation: _____ is a competitive inhibitor of G6PD

NADPH

regulation: _____ is the strongest inhibitor of the G6PD step

NADPH/NAP+ ratio

regulation: increased demand for _____ will lower the NADPH/NADP+ ratio (since NADPH will be utilized more) which will promote the oxidative phase

NADPH

regulation: insulin increases expression of the G6PD gene in the liver

1. so, insulin promotes the HMP
2. fatty acid biosynthesis occurs in the liver which requires high amount of NADPH

nonoxidative phase occurs in cells that produce ___ __and__ ____

nucleotides and nucleic acids

nonoxidative phase does NOT produce _____

NADPH

nonoxidative phase involves interconversion of sugars (3-7 carbons) and the reactions are completely _____ steps (do NOT need to memorize steps)

reversible

nonoxidative: conversion of ribulose 5-P either:

1. to ribulose 5-P for nucleotide biosynthesis for DNA, RNA, and other nucleotide-based molecules
2. to fructose 6-P and glyceraldehyde 3-P, intermediates of glycolysis

nonoxidative: _____ transfers 2-carbon units

transketolase

nonoxidative: required coenzyme for transketolase

thiamine pyrophosphate (required coenzyme) so it requires the vitamin thiamine

nonoxidative: _____ transfers 3-carbon units

transaldolase

nonoxidative: if the demand of the cell is greater for NADPH- ___ __and__ ___ enzymes will convert ribulose 5-P glycolysis intermediates that can be utilized for the glycolysis pathway

* greater need for reductive biosynthesis

transketolase and transaldolase

nonoxidative: if the demand or the cell is greater for ribose 5-P- the glycolytic intermediates will be converted to ribose 5-P

1. greater need for nucleotides
2. can be done without the oxidative steps

although they are both high-energy molecules, the additional phosphate group on one of the ribose units for NADPH allows its involvement in ___ __and__ ___ like hydrogen peroxide

biosynthetic reactions and reduction of reactive oxygen species (ROS)

biosynthesis: NADPH provides the reducing equivalents for ___ __and__ ___

fatty acid and cholesterol biosynthesis

biosynthesis: NADPH for fatty acid and cholesterol biosynthesis are important in the following organs/tissues

1. liver
2. mammary gland
3. adipose tissue

biosynthesis: NADPH are also the reducing equivalents for steroid hormone synthesis biosynthesis and are important in the following:

1. testes
2. ovaries
3. placenta
4. adrenal cortex

reduction or reactive oxygen species: hydrogen peroxide (H2O2) is a ROS formed from _____

partial reduction of oxygen

reduction of reactive oxygen species: hydrogen peroxide is a byproduct of _____ when reacted with drugs or toxins

anaerobic metabolism

reduction of reactive oxygen species: ROS can cause chemical damage to:

1. DNA
2. proteins
3. unsaturated lipids

reduction of reactive oxygen species: ROS can cause chemical damage- linked diseases

1. inflammatory disease
2. cancer
3. aging

reduction of reactive oxygen species: ROS can cause chemical damage- antioxidants

antioxidants can inhibit their (ROS) formation as a protective mechanism

reduction of reactive oxygen species: reduced glutathione (G-SH) is present in most cells and can detoxify ___ __reducing it to__ ___

detoxify H2O4 reducing it to H2O

reduction of reactive oxygen species: glutathione is a peptide consisting of:

1. glycine
2. cysteine
3. glutamate

reduction of reactive oxygen species: glutathione peroxidase catalyzes the reaction and forms _____

oxidized glutathione

reduction of reactive oxygen species: in the oxidized form, G-S-S-G , glutathione can no longer _____

detoxify

reduction of reactive oxygen species: **NADPH donates its electrons in the reaction to _____ which is catalyzed by glutathione reductase**

regenerate G-SH

NADPH therefore _____ the electrons for reducing ROS

indirectly provides

other enzymes that convert ROS to harmless products:

* superoxide dismutase
* converts superoxide radical to H2O2
* catalase
* converts H2O2 to water and oxygen

superoxide dismutase and catalase are enzymes that are NOT present in RBC, making them dependent on the HMP for _____

detoxifying capabilities

examples of antioxidant chemicals that are intracellular reducing agents can also detoxify ROS:

1. vitamin C
2. vitamin E
3. B-carotene

foods rich in ______ have been correlated with reduced risk of certain ROS related diseases like cancer and other health problems

antioxidants

however, antioxidant supplements have NOT shown _____ in cancer trials

inhibitory effects

it is likely a complex interaction with these compounds and other health-promoting components of ___ __and__ ___ that provide health benefits

fruits and vegetables

cytochrome P450 monooxygenase system: overall reaction

R-H + O2 + **NADPH** +H+ → R-OH + H2O + NADP+

(R group represents a steroid/drug/chemical)

monooxygenase incorporates one atom from oxygen into a substrate which creates a ___ __and one molecule of__ ___

hydroxyl group and one molecule of water

cytochrome P450 (CYP) is a family of enzymes containing heme (as a cofactor) that catalyze the reaction while _____

**NADPH provides the reducing equivalents**

mitochondrial CYP monooxygenase system processes are located:

1. steroidogenic tissues


1. placenta/ovaries/testes/adrenal cortex: involved in the conversion of cholesterol to steroid hormones
2. liver


1. bile acid synthesis
3. kidney


1. hydroxylation of vitamin D3 to the active 1,25-dihydroxylated form hydroxylated

microsomal CYP monooxygenase system is a membrane of teh _____ (mostly in the liver)

smooth endoplamsic reticulum

microsomal CYP monooxygenase system: detoxification of foreign compounds (xenobiotics)

1. drugs/pollutants/pesticides


1. CYP enzymes hydroxylate the toxins which can activate/inactivate the drug and make it more soluble (when the drug is more soluble, it can be excreted in the urine/feces

phagocytosis is ingestion by receptor mediated endocytosis of:

1. microorganisms
2. foreign particles
3. cellular debris by leukocytes

oxygen-independent phagocytosis involves pH changes in phagolysosomes to destroy the pathogens, while

**oxygen dependent involves NADPH requiring reactions**

___ __and__ ___ are the main enzymes in oxygen dependent microorganism destruction

NADPH oxidase and myeloperoxidase (MPO)

oxygen dependent microbial killing steps

1. the leukocyte recognizes the bacterium, and it is phagocytosed
2. NADPH oxidase (on the leukocyte membrane) reduces oxygen to superoxide (O2-, a ROS) utilizing NADPH
3. superoxide is converted to H2O2 by superoxide dismutase or spontaneously
4. MPO converts H2O2 plus chloride ions to hypochlorous acid (a component of bleach) which kills the bacteria

nitric oxide synthesis

* Nitric oxide (NO) is a free radical gas that has many roles
* relaxing factor
* relaxes vascular smooth muscle causing vasodilation
* NTM
* in the central and peripheral nervous system
* prevents platelet aggregation
* involved in macrophage function

nitric oxide synthesis: NO synthase (NOS) (cytosolic) requires _____

* Requires NADPH
* substrates
* Arginine/oxygen/NADPH
* coenzymes
* flavin mononucleotide/Fad/heme/tetrahydrobiopterin
* products
* NO and citrulline

nitric oxide synthesis: there are 3 NOS isoenzymes

1. endothelium NOS (eNOS)
2. neural tissue NOS (nNOS)
3. inducible NOS (iNOS)

endothelium NOS (eNOS)

constantly synthesized (for vasodilation purposes)

neural tissues NOS (nNOS)

constantly synthesized for neurotransmission purposesin

inducible NOS (iNOS)

1. expressed in many cells (macrophages and neutrophils) for early defense against pathogens
2. usually iNOS activity is low, but is synthesized in response to infection/inflammation
3. inducers can include proinflammatory cytokines and bacterial endotoxins

NADPH requiring processes:

1. biosynthesis


1. fatty acids/steroid hormones
2. reduction of reactive oxygen species


1. glutathione
3. cytochrome P450 monooxygenase system


1. detoxification of xenobiotics
4. white blood cell phagocytosis and microbe killing


1. oxygen dependent destruction

_____ is the most common disease-producing enzyme abnormality

G6PD genetic deficiency

not all mutations of the gene cause _____

physical symptoms

most enzyme function lowering mutations are _____ point mutations

missense

mutations can result in:

1. **decreased catalytic activity**
2. decreased stability
3. alter binding affinity for NADP+ or glucose 6 phosphate

severity is based on the level of _____

residual activity

there are more than _____ that can result in enzyme deficinecy

400 G6PD gene variants

X-linked disorder makes it more common in males/females

males

G6PD deficiency is most common in the:

1. middle east
2. tropical Africa
3. Asia
4. parts of the Mediterranean

without an oxidative stress, cells without G6PD are typically _____

fine

certain **oxidant stresses** (food and drugs) can lead to _____ in people with G6PD deficiency

**hemolytic anemia**

oxidant drugs are categories that begin with A (some, NOT all drugs in each group)

1. antibiotics
2. antimalarial

favism

1. fava
2. broad bean ingestion (common in Mediterranean diets)

infection is an inflammatory response b/c of an infection generates _____ in macrophages which can diffuse in RBC

free radicals

another cyclical symptom of G6PD deficiency

neonatal jaundice

hemolytic anemia is caused by the inability of the RBC to detoxify oxidizing agents b/c of the lower amounts of _____ due to the G6PD deficiency

NADPH

although all cells are affected by the deficiency, the most severe effects are on the ___ b/c the HMP is the only way NADPH is generated in RBC

red blood cells

RBC do not have a ___ __or__ ___ so they can’t renew supply of G6PD

nucleus or ribosomes

other cells have NADP+ dependent _____ that provides an alternative production of NADPh

malate dehydrogenase

sulfhydryl groups in proteins like hemoglobin, if oxidized can form insoluble masses called _____

heinz bodies