From CellBiology

5-Aminolevulinic Acid Synthase (ALAS)

The mitochondrion is a dynamic organelle which contains many proteins and enzymes required for various essential metabolic purposes. One such important enzyme, within eukaryotic cells, is 5-aminolevulinic acid synthase (ALAS). ALAS has the crucial role of initiating the first step in heme synthesis, that is, the production of 5-aminolevulinate acid[1]


The active enzyme is found on the inner surface of the outer mitochondrial membrane and is synthesised on the cytosolic ribosomes of the cell.

There are two isomers of this enzyme, ALAS1 and ALAS2, which have regional specificity. ALAS1 is found in all non-erythroid cells whereas ALAS2 is found explicitly in erythroid cells. It has been found that the GATA-1-binding site and CACCC boxes located within the human ALAS2 intron 8 part of the gene is what is responsible for the tissue restricted expression of this gene[2]

Crystal structure of ALA synthase.


Crystallization methods have allowed the structure of ALAS to be deciphered. ALAS is described as a tightly interacting homodimer[3] which has:

• Residues 1 - 52: N- Terminus which contains an alpha helix and anti-parallel beta sheets.

• Residues 53 – 296: Central catalytic domain consisting of seven stranded parallel beta sheets

• Residues 297 – 401: C- Terminus consisting of three stranded anti-parallel beta sheets


It is involved in heme synthesis. ALAS has the crucial role of initiating the first step in heme production, that is, the production of 5-aminolevulinate acid. The reaction is as follows:


The above reaction is the rate limiting step of the entire process and requires prydoxin as a co factor.

Heme mediated feedback inhibition of ALAS occurs in hepatic cells. There are three main ways in which heme inhibits ALAS activity[4]

  • Causes a decrease in enzyme production
  • Interferes with the transport of ALAS into the mitochondrion
  • Heme also causes allosteric inhibition of ALAS activity.

Interesting Research

An interesting feature regarding ALAS is that heme synthesis and the circadian clock seem to be reciprocally regulated. Kaasik K and Lee CC (2004) summarise the relationship as “Neuronal protein NPAS2 has a heme binding motif; heme controls activity of the BMAL1–NPAS2 transcription complex.Heme differentially modulates expression of the mammalian Period genes mPer1 and mPer2 in vivo by a mechanism involving NPAS2 and mPER2. mPER2 positively stimulates activity of the BMAL1–NPAS2 transcription complex and, in turn, NPAS2 transcriptionally regulates Alas1.” This has clinical significance when it comes to problems such as insomnia[5] .

Another area of research regarding ALAS is concerned with its relationship with steroids. Recent studies have shown that ALAS can be induced by steroids. A study by Sassa S. et. al. confirmed that the presence of 5β and 5α steroids significantly increases the synthesis of ALAS.

This relationship between steroid hormones and ALAS catalysed heme biosynthesis is clinically important. It was found that patients with acute intermittent porphoria, a genetic liver disease, “display a disproportionate generation of 5beta metabolites from radio-labelled precursor hormones on comparison with normal subjects.”[6]


People with X-linked sideroblastic anaemias have been shown to have mutations within the ALAS2 enzyme caused by mutations in the gene encoding for this enzyme[7] .


  1. Molecular Biology of the Cell Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter New York and London: Garland Science; c2002 Bookshelf Link.
  2. Identification and characterization of a conserved erythroid-specific enhancer located in intron 8 of the human 5-aminolevulinate synthase 2 gene.Surinya KH, Cox TC, May BK.J Biol Chem. 1998 Jul 3;273(27):16798-809.PMID: 9642238.
  3. Crystal structure of 5-aminolevulinate synthase, the first enzyme of heme biosynthesis, and its link to XLSA in humans. Astner I, Schulze JO, van den Heuvel J, Jahn D, Schubert WD, Heinz DW. EMBO J. 2005 Sep 21;24(18):3166-77. Epub 2005 Aug 25. PMID: 16121195
  4. Frederick J. Suchy, R. J. (2001). Liver Disease in Children. Lippincott Williams & Wilkins. Click here to see original work.
  5. Reciprocal regulation of haem biosynthesis and the circadian clock in mammals.Kaasik K, Lee CC.Nature. 2004 Jul 22;430(6998):467-71.PMID: 15269772
  6. Steroid induction of delta-aminolevulinic acid synthase and porphyrins in liver. Structure-activity studies and the permissive effects of hormones on the induction process.Sassa S, Bradlow HL, Kappas A.J Biol Chem. 1979 Oct 25;254(20):10011-20.PMID: 114519
  7. The molecular biology and pyridoxine responsiveness of X-linked sideroblastic anaemia.May A, Bishop DF.Haematologica.1998 Jan;83(1):56-70. Review.PMID: 9542324


Nucleous Lecture

It was intereting to learn about chromosome territories, this was something I was not aware of before regarding the nucleous.

Exocytosis Lecture

--Mark Hill 15:19, 25 March 2009 (EST) The morphology of the ER is quite dynamic, further away from the nucleus and in the SER region it tends to be tubular (like a pipe) while close to the nuclear envelope this is "squashed flat" to form a "flat sheet". The 'docking' mechanisms are quite complex at the vesicle level and I was trying to describe several different models as too how fusion between vesicles and the plasma membrane occurred. Note that vesicles can also sit quite close to the plasma membrane without actually docking.

Thankyou for your feedback. The structure of the ER became clear to me as I slowly read through your explanation. I will be sure to revisit the exocytosis lecture via ilec to confirm the various processes you mentioned about the vesicle docking.

Mitochondria Lecture

--Vibha Patil 18:44, 6 April 2009 (EST)

There are 3 main types of work cells need to do for survival.

1. Mechanical work

  • This involves precesses such as reprodution (that is, mainly chromosome movement), physical movement of cell (which involves beating of cilia)as well as cell contractions (such as in smooth muscle cells).

2. Chemical work

  • This is mainly to do with drivign reactions which cannot occur spontaneously due to the thermodynamics; so things such as making certain polymers.

3. Transport work

  • This mainly involves working to push certain molecules across the cells membrane against their concentration gradient.

The main way cells achieve thier work goals is by energy coupling which requires ATP (adenosine triphosphate). It is here that mitochondria play a key role. Mitochondria, is also reffered to as the 'power house' of the cell because it produces ATP via ATPase.

More information can be found in the following:

Cell Junctions Lecture:

ICAM1 stands for "inter cellular adhesions molecules one

  • It is a human gene also known as CD54
  • The gene codes for intecellular adhesion proteins present on leukocyte and endothelial cell membranes
  • More information can be found at: [[1]]

NCAM stand for "nueral Cell Adhesion Molecule"

  • It is found on neurons, glia, skelatal muscle as well as on natural killer cells
  • These molecules are also involved in cell-cell adhesion
  • more information can be found at: [[2]]

NgCAM stands for Neuro glia cell adhesion molecules

--Vibha Patil 19:18, 10 April 2009 (EST)

Intermediate Filaments Lecture

In thick skin, the layer betwen the stratum basale and granulosum is the STRATUM SPINOSUM. It is the thickest layer of the epidermis. This layer consists of a large number of intermediate filament bundles and the cells are attached to each other via desmosomes. On thier way up to the surface of the skin, the cells in the spinosum become enlarged, differentiated and accumulate more keratin in the skin.

More detailed information on the different layers of the skin can be found here:

Histology By Ray C. Henrikson, Gordon I. Kaye, Joseph E. Mazurkiewicz Edition: 3, illustrated Published by Lippincott Williams & Wilkins, 1997 ISBN 0683062255, 9780683062250

Good diagram can be found here:

Lab 6

Genotype A = Tm4 Genotype B = Wild Type


Phenotype ‘B’ is significantly higher in the wild type cells whereas phenotype ‘C’ is significantly higher in the Tm4 treated cells.

Phenotype B: “Broken Fan”

Phenotype C: “Stumped”

Lab 7: Confocal Microscopy

The two forms of confocal microscopy are:

1. laser scanning confocal microscope

2. two photon confocal microscope

--Mark Hill 23:48, 29 April 2009 (EST)A good try, but not the answer I am looking for, both use a line scanning laser, try again. look at the lab page Confocal Microscopy.


The two forms of confocal microscopy are:

1. laser scanning confocal microscopy

2. spinning disk confocal microscopy

Lab 8: Cell Culture