HEME SYNTHESIS AND PORPHYRIAS

HEME PROTEINo Heme is ferroprotoporphyrin IX, it is the prosthetic group of a number of

heme proteinso Heme proteins:

Mb, Hb, mitochondrial chromosomes, microsomal cytochrome, catalase and peroxidase, tryptophan pyrrolase

o Heme is usually associate noncovalently with its apoprotein through the coordination of its iron with an aa side chain N

The heme in cytochrome C derivatives are an exception in being covalently bound to the apoprotein through thioether linkages

STRUCTURE OF PORPHYRINSo Heme is the iron chelate of protoporphyrin IXo Porphyrins-metal free cyclic tetrapyrroles which are named according to

the nature and arrangement of their side chain groups Positions 5,10,15,20 are C atoms that serve as bridges bt the 4

pyrrole rings (aka alpha, beta, gamma, delta) These C bridges are unsaturated in porphyrins and are

called methene groups (-CH=)o Because they are unsaturated, the entire structure is

conjugated, therefore, porphyrins are colored (usu a shade or red or brown)

o Porphyrin Biosynthesis First the molecule goes through a stage where the bridge carbons

are saturated=methylene bridge Its presence in the molecule disrupts the conjugation of the

molecule and therefore, the tetrapyrrole is colorless Porphyrinogen=when all the carbon bridges are methylene

groups Poryphrins are named by the groups attached to them

Acetic Acid Proprionic acid Methyl Vinyl Occur as:

o AP, MP, MV The porphyrins of interest to us contain 4 pyrroles in the

combinations below:o Uropophyrin- 4AP (most soluble)o Coproporphyrin-4 MPo Protoporphyrin-2MP, 2MV (least soluble)

It is possible to draw a number of isomers of these porphyrins depending on how different the pyrroles are put together

o Type III is the naturally occurring isomer or uroporyphyrin and coproporphyrin (APAPAPPA and MPMPMPPM respectively)

o Protoporphyrin IX is the naturally occurring isomer (MVMVMPPM)

These compounds can occur with methane or methylene bridges

PROPERTIES OF PORPHYRINSo Color

usually a shade of red or brown with a distinctive light absorption spectrum

major absorption is at 400nm (the Soret band) with several smaller bands between 500-630nm

metal-free porphyrins emit an intense red fluorescence when excited by light at ~400nm, but heme does not

Porphyrin-sensitized injury aka photosensitivity-the fluorescence in vivo of porphyrins in the presence of O damages cells

Based on injury to plasma and lysosomal membranes Photosensitization occurs when abnormally large quantities

of porphyrins are present either by experimentation or by a derangement in porphyrins synthesis called porphyria

o Symptoms=itching, edema, erythema and ulceration

Another problem due to elevated porphyrins levels is in regards to water solubility

In general, porphyrins solubility increases as the number of carboxylic acid side chains increases

o Uro and coproporphyrins can be excreted in urine whereas protoporphyrin is so insoluble it must be secreted in feces via bile

THE PATHWAY OF HEME SYNTHESISo The cells of most tissues can synthesize heme compounds but the liver and

bone marrow produce the most In bone marrow, heme is inc. into Hb, the liver normally produces

15% of the total heme synthesized in the body and the bone marrow does the rest

In rats, 65% of heme synthesized in the liver is used for the formation of microsomal cytochrome P450 and 6% for mitochondrial cytochromes

o The porphyrins ring is synthesized entirely from glycine and succinate (as succinyl CoA)

2 key intermediates=aminolevulini acid ALA and porphobilinogen PBG

o #1-8 enzymes are involved from glycine; the first and last three are in the mitochondria requiring the cooperation of the mitochondria and cytostolic compartments for heme synthesis

ALA IS FORMED IN THE FIRST STEP PBG (the pyrrole precursor of heme) IS FORMED IN

THE SECONDo 4 molecules of PBG are condensed in the

pathway to hemeo #2-ALA synthase, the first enzyme of the pathway, is dependent on

pyridoxal phosphate for activity This coenzyme functions to form a Schiff base with the glycine

before is reats with succinyl CoAo #3-ALA is the initial intermediate=committed to heme synthesis and

rate controlling step!o #4-from PBG until protoporphyrin IX, the precursors of heme are

synthesized reactions involving reduced ridge C’s of the porphyrinogen type

Only the final enzyme, ferrochelatase, uses a porphyrins instead of a porphyrinogen

o #5-Reduced iron (Fe2+) is incorporated into heme by ferrochelataseo #6-PBG deaminase catalyzes the condensation of PBG molecules to form

a linear tetrapyrrole When PBG acts in the presence of a 2nd enzyme, uroporphyrinogen

III cosynthase, proper closure of the tetrapyrrole occurs In the absence of this enzyme, type I is formed which is unusable

o #7-uroporphyrinogen III is the first porphyrinogen formed in the biosynthesis of heme

It is converted to protoporyphyrin IX by enzymatic reaction of its side chains

4 acetic groups methly groups 2 proprionic acid groupsvinyl groups

REGULATION OF HEME BIOSYNTHESISo In liver:

ALA synthase (the 1st enzyme of the pathway) catalyzes the rate-limiting step and is the major point of regulation of the pathway in the liver (but not in erythroid cells)

level of ALA synthase is regulated by the concentration of “free” heme, the product of the pathway, and the autooxidation product hemin (heme with Fe+3)

o There are 3 mechanisms which might regulate the rate of heme synthesis 1. Feedback inhibition of the isolated enzyme by hemin

Does not seem to be physiologically important 2. Hemin represses the synthesis of ALA synthase 3. Hemin inhibits the transport of cytosolic ALA synthase into the

mitochondria

THEREFORE, HIGH LEVELS OF HEME PREVENT THE SYNTHESIS OF HEME!

o Many compounds increase ALA synthase and result in overproduction of liver ALA, PBF and porphyrins

Ie. Phenobarbital-related barbiturates, naturally occurring steroids (including estrogens) and other drugs

o There are tissue-specific ALA synthase isozymes which are encoded by 2 separate genes

ALAS1-housekeeping ALA synthase gene expressed in liver and non-erythyroid tissues

ALAS2-expressed only in erythroid cellso In erythroid cells:

The major regulation of heme synthesis in these cells is no via the repression of ALA synthesis by heme as in the liver

In fact, hemin stimulates heme formation in several types of erythroid cells

There is an erythroid-specific ALA synthase gene which has a distinct mode of regulation tied to cell differentiation

ALAS-E is not inducible by the drugs that induce ALAS1 PORPHYRIA

o A group of related but distinct diseases, they are inherited and acquired diseases characterized by:

Defects in specific enzymes in heme biosynthesis Increased accumulation and excretion of intermediates of the

pathway Most dramatic clinical symptoms=neurological disorders and

cutaneous photosensitivity Excess ALA and PBF=peripheral neuritis, abdominal pain

and intestinal spasms Excess porphyrins in plasma(rather than in blood

cells)=skin lesionso Porphyrins transported by albumin

CLASSIFICATION OF PORPHYRIASo Classified as erythropoietic or hepatic, symptoms are essentially the same

bc overproduced porphyrins can enter plasma from both sources COMMENTS ON PORPHYRIAS

o Erythropoeitic porphyrias Congenital erythropoietic porphyria (CEP) is very rare

Severe photosensitivity, red cell hemolysis Erythropoietic porphyria protoporphyria (EPP)

Usually exhibits milder clinical symptoms with little or no hemolysis

o Hepatic Porphyrias 5-aminolevulinic acid dehydratase-deficient porphyria (ADP),

acute intermittent porphyria (AIP), porphyria cutanea tarta (PCT), hereditary coproporphyria (HCP) and variegate porphyria (VP)

Variable degree of clinical expression Hormonal, drug and nutritional factors predispose patients

to full expression of the diseaseo These agents precipitate clinical expression in

asymptomatic patients carrying the genetic defecto Most act by inducing higher levels of ALA synthaseo PCT is fairly common

More frequent in males over 40 who drink and women on birth control

o ADP is extremely rare! One would expect in some cases the accumulation of the ‘gens in

the urine, but porphyrinogens oxidize spontaneously to the porphyrins form during and after excretion; thus, a urine specimen will gradually darken if left standing

The enzymatic defect in various porphyrias is genetic Enzymatic activity is REDUCED

LEAD POISONINGo Usually occurs from the ingestion of peeling leaded paints or other

exposureo Symptoms are similar to those AIPo Lead tends to concentrate in erythroid cellso Erythrocyte ALA dehydratase and ferrochelatase are particularly sensitive,

resulting in a kind of acquired porphyria o Inhibits erythrocyte heme synthesis more than liver heme synthesiso Excess excretion of ALA and proto IX and Copro IIIo Possible photosensitiviy and noticeable anemiao Abodominal and neurological problems