M.Prasad NaiduMSc Medical Biochemistry, Ph.D,.

Proteins most abundant org.compound Major part of the body dry wt (10-12Kg) Perform wide variety of functions. Viz 1. Static functions ( Structural functions) 2. Dynamic functions( Enzy, hor,

receptors) Half of the body protein is (Collagen) is

present in supportive tissue (skeletan & connective) while the other half is intracellur.

Proteins are the N- containing macro molecules

Consists of L- AAs as repeating units Of the 20 AAs half can be synthesized Essential and non-essential AAs Proteins on degradation release AAs Each AA undergoes its own metabolism Proteins metabolism is more appropriately

learnt as metabolism of amino acids.

An adult has about 100 gm of Free AA which represent the AA pool of the body.

Glutamate and Glutamine together constitute about 50% and EAA 10% of the body pool.

The conc of intracellular AA is always higher than the Extracellular AA

AAs enter the cells againt Active transport The AA pool is maintained by the sources that

contribute ( input) and the metabolic pathways that utilize (out put) the amino acids.

1. Turnover of body protein 2. intake of dietary protein 3. synthesis of non- EAAs

The protein present in the body is in a dynamic state.

About 300-400 gm of protein per day is constantly degraded and synthesized which represent the body protein turnover.

There is wide variation the turnover of individual proteins.

Eg: plasma proteins & digestive enzymes are rapidly degraded ( half life is hrs/days)

Structural proteins have long half lives often months and years.

many factors 1. Ubiquitin : small PP – 8,500 – tags with

the proteins and facilitates degradation. 2. PEST Sequences: - Certain proteins

with Pro, Gln, Ser, Thr sequence are rapidly degraded.

Regular loss of protein due to degradation of AAs. About 30-50 gm protein is lost every day from the

body. This amount must be supplied daily in the diet to

maintain N Balance. There is no storage form of AAs unlike the

Carbohydrates and lipids (TG) The excess AAs – metabolised – oxidized –Energy

or glucose or fat. The daily protein intake by adults is 40-100gm

10 out of 20 naturally occurring AAs can be synthesized by the body which contributes to AA pool.

1. most of the body proteins (300-400g/D) degraded are synthesized from the AA pool. ( enzymes, hormones, immuno proteins, contractile proteins)

Many imp N compounds ( porphyrins, purines & pyrimidines) are produced from AA . About 30g of protein is daily utilized for this purpose.

Generally, about 10-15% of body energy requirements are met from the AAs

The AAs are converted to Car, fats. This becomes predominant when the protein consumption is in excess of the body requirements.

AAs undergo common reactions Transamination followed by Deamination for the liberation of NH3 The NH2 group of AAs is utilized for the

formation of urea (excretory end product of protein metabolism)

The C-skeleton of the AAs is first converted to keto acids (by transamination) which meet one or more of the following fates

Utilized to generate energy Used for the synthesis of glucose Derived for the formation of fat / ketone

bodies Involved in the production of non-EAAs

Transfer of an amino group from an AA to a keto acid

This process involves the interconversion of a pair of AAs and a pair of keto acids

Transaminases / aminotransferases

All transaminases require PALP Specific transaminases exist for each pair of amino and

keto acids However, only two namely Asp. transaminase & Ala.

transaminase make a significant contribution for transamination

There is no free NH3 liberated, only the transfer of NH3 group occurs

Reversible Production of non-EAAs as per the requirement of the cell Diverts the excess of AAs towards Energy generation

AAs undergo TAN to finally concentrate N in glutamate

Glutamate is the only AA that undergoes OD to liberate free NH3 for urea synthesis

All AAs except Lys, Thr, Pro & Hy.pro participate in TAN

TAN is not restricted to α-group only. (eg: δ-amino group of Ornithine is transaminated.

Serum transaminases are important for diagnostic and prognostic purposes

SGPT or ALT is elevated in all liver diseases SGOT or AST is increased in myocardial infarction

Occurs in 2 stages. 1. Transfer of the NH2 group to the

coenzyme PLP ( bound to the coenzyme) to form Pyridoxamine Phosphate.

2. The NH2 group of Pyridoxamine PO4 is then transferred to a keto acid to produce a new AA and the enzyme with PLP is regenerated.

All the transaminases require PLP , a derivative of Vit B6

The – CHO group of PLP is linked with έ-NH2 group of Lys, at the active site of the enzyme forming a Schiff’s base (imine linkage)

When an AA comes in contact with the enzyme, it displaces lys and a new Schiff base linkage is formed.

The AA-PLP-Schiff base tightly binds with the enzyme by non covalent forces.

Snell & Braustein proposed Ping-Pong Bi Bi mechanism involving a series of intermediates ( aldimines & ketimines) in transamination reaction.

The removal of amino group from the AAs as NH3

Transamination involves only shuffling of NH3 groups among the AAs

Deamination results in the liberation of NH3 for urea synthesis

Simultaneously, the C-skeleton of AAs is converted to keto acids

2 types (Oxidative & Non oxidative) Transamination & Deamination occurs

simultaneously, often involving glutamate as the central molecule (Transdeamination)

Liberation of free NH3 from the AAs coupled with oxidation

Liver & kidney Purpose of OD: to provide NH3 for urea

synthesis & α-ketoacids for a variety of reactions, including Energy generation

In the process of Transamination, the NH3 groups of most of the AAs are transferred to α-KG to produce glutamate

Thus , glutamate serves as a collection centre for amino groups in the biological system

Glutamate rapidly undergoes oxi.deamination by GDH to liberate NH3

GDH is unique in that it can use utilize either NAD+ or NADP+

Conversion of glutamate to α-KG occurs through the formation of α-iminoglutarate

GDH catalyzed reaction is imp as it reversibly links up glutamate metabolism with TCA cycle through α-KG

GDH is involved in both catabolic & anabolic reactions.

Zn containing mitochondrial enzyme Complex enzyme containing 6 identical units with

a mol.wt of 56000 each. GDH is controlled by allosteric regulation GTP , ATP, steroid & Thyroid hormones are

inhibitors of GDH GDP and ADP are activators After ingestion of protein meal, liver glutamate

level is ↑. It is converted to α-KG with liberation of NH3 Further , when cellular E levels are ↓low, the

degradation of glutamate is ↑ to provide α-KG which enters TCA cycle to liberate Energy

L- AAoxidase & D-AAoxidase are flavo proteins, possessing FMN and FAD respectively.

They act on corresponding AAs to produce α-Ketoacids & NH3

In this reaction, O2 is reduced to H2O2, which is later decomposed by catalase

The activity of L-AAoxidase is much low while D-AAoxidase is high in tissues (liver & kidneys)

L-AAoxidase does n’t act on Gly & dicarboxylicacids

D-AAs are found in plants & mos Absent in mammalian proteins But D-AAs are regularly taken in diet and are

metabolized D-AAoxidase converts them into α-ketoacids by

od. The α-ketoacids so produced undergo TAN to be

converted to L-AAs Ketoacids may be oxidized to generate energy or

serve as precursor for glucose & fat synthesis Thus D-AAoxidase is imp as it initiates the first

step for the conversion of unnatural D-AAs to L-AAs in the body.

Some of the AAs can be deaminated to liberate NH3 without undergoing oxidation

A) Aminoacid dehydrases: Ser,Thr,Homoserine α-ketoacids Catalyzed by PLP dependent dehydrases (dehydratases) B)Aminoacid desulfhydrases: Cys, homocysteine pyruvate Deamination coupled with desulfhydration C) Deamination of histidine: Histidine urocanate histidase