Methionine amino acid:

Essential amino acid

It is used as methyl donor The active form of methionine as methyl donor is S-adenosyl methionine (SAM)

+

S-adenosyl methionine (SAM)

Functions of methionine:

1- SAM act as methyl donor in :

A- synthesis of creatine (see synthesis of creatin from glycine).

B- conversion of norepinephrine into epinephrine

Synthesis of creatin

↓

↓

homoserine

2-Methionine participates with serine in the synthesis of cysteine amino acid as follow

Homocystinuria or hyperhomocysteinemia:

Is a genetic disease caused by genetic deficiency of cystathionine synthase leading to accumulation of homomcysteine in blood (hyperhomocysteinemia) and its excretion in urine (homocystinuria).

Symptoms: Homocysteine is toxic to CNS, cardiovascular system, kidney, connective tissues, etc. leading to:

1- Mental retardation2- at chilhood: osteoporosis, bone weakness and fracture (disturb collagen biosynthesis)3- Cardiovascular diseases: myocardial infarction and atherosclerosis, thrombosis

Treatment: restriction of methionine in diet

Cysteine amino acid

Non essential, formed from methionine (as before)

Glucogenic as it gives pyruvate

Functions:

1- Formation of hormones: insulin, oxytocin and vasopressin (ADH)

2- Formation of bile salts:Cysteine enter in the formation of taurine which combine with cholic acid giving taurocholate (see bile salts).

3- Formation of GSH: with glycine and glutamic acid

4- Formation of cystine (by forming disulfide bond which stabelize the protein)

Histidine amino acid

Glucogenic: it gives α-ketoglutarate

Functions:

synthesis of histamine which is local hormone, vasodilatotor substance released in allergic diseases.

Histidine Histamine

Serine amino acid

Glucogenic amino acid: gives pyruvate

Non essential amino acid: Synthesized from either glycine or 3-phosphoglycerate

Synthesis of serine from 3-phosphoglycerate

Serine is generated in a two-step reaction from 3-phosphoglycerate. The first step is an oxidation, the second a transamination.

Synthesis of serine from glycine:

Glycine is converted into serine by the enzyme serine hydroxymethyl transferase.

↔

Functions of serine:

1-Synthesis of sphingomyelin

Serine + palmitoyl CoA → sphingosine base → sphingomyelin

2- Synthesis of cysteine (with methionine, see before)

3- Synthesis of glycine amino acid

Acidic amino acids:

Aspartic acid and glutamic acid

Glutamic acid

Non essential: as it is synthesized either by:1-Transamination

2- glutamate dehydrogenase enzyme (GDH) (see before)

Glucogenic: as it gives α-ketoglutarate by transamination or by oxidative deamination by GDH (reversible reactions)

Functions of glutamic acid:

1- Formation of Folic acid together with p-amino benzoic acid and pteridine ring

•Folic acid is a member of vitamin B complex. It plays an important role in synthesis of purines and thymidine monophosphate (TMP) and so DNA synthesis. Deficiency of folic acid leads to diminished DNA synthesis and inhibit cell division resulting in meglaoblastic anemia (accumulation of large, immature RBCs -called megaloblasts- in blood and B.M.).•Folic acid is needed for rapid cell division and growth as in infancy and pregnancy. Many foods contain folic acid, including vegetables such as spinach, broccoli, green beans, and potatoes. Some food as bread and cereals (corn flakes) are fortified with folic acid.

Structure of folic acid illustrate participation of glutamic acid

1- Role of THF in purine biosynthesis:

THF is the carrier of formate group that constitute the carbons 2 and 8 of purine ring

2- Folic acid is important for biosynthesis of thymidine monophosphate (TMP) of DNA.

-3- Maturation of RBCs needs folic acid

3- detoxification of ammonia: by formation of glutamine in brain, muscles and liver by glutamine synthetase enzyme

Glutamic acid + ammonia → glutmine

4- N3 of pyrimidines and N3, N9 of purines are deived from glutamine

3

3 9

2- Formation of GABA in brain: by decarboxylation

GABA ( γ-amino butyric acid) is an important compound in brain. Drugs that act as agonist of GABA receptors (known as GABA analogues or GABAergic drugs) or increase the available amount of GABA typically have relaxing, anti-anxiety and anti-convulsive effects.Its deficiency leads to convulsion especially in children. The reaction is PLP-dependent. PLP is derived from vitamin B6, so deficiency of vitamin B6 → deficiency of GABA in brain → convulsion.

Aspartic acid

Non essential: formed through transamination

Glucogenic as it gives oxaloacetate by transamination

COOH-CH2-CH-COOH

NH2|

Functions of aspartate:

1- Enter in urea formation: see urea cycle

-

2- N1 of purines and N1 and C4,5,6 of pyrimidines are derived from aspartate

1

Role of aspartic acid in urea formation