Sec_3 - Amino Acid Catabolism 3-1 Lec#8 To date we have...

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3-1 A m i n o A c i d C a t a b o l i s m To date we have considered the catabolism of carbohydrates and lipids with the object of generating energy in the form of ATP. Both give rise to AcCoA which is fed through the TCA cycle for complete conversion to CO 2 . Amino acids are treated in much the same fashion except that removal and excretion of the amino group provides an additional level of complexity. Carbohydrates Lipids AcCoA TCA cycle Amino acids NH 3 Given that the first step in many of the degradative pathways is the removal of the amino group from the carbon skelton of the amino acid, we will first consider transamination and deamination reactions. 1. T r a n s a m i n a t i o n We will first consider a generic transamination which will illustrate the fact that glutamate and D -ketoglutarate are always involved in transaminations. CH COO H 3 N R CH COO H 3 N CH 2 CH 2 COO C COO O CH 2 CH 2 COO C COO O R + + amino acid D -keto acid glutamate D -ketoglutarate Amino acid transaminase pyridoxal phosphate CH COO H 3 N CH 3 CH COO H 3 N CH 2 CH 2 COO C COO O CH 2 CH 2 COO C COO O CH 3 + + alanine pyruvate glutamate D -ketoglutarate Alanine transaminase pyridoxal phosphate Lec #8
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3-2 CH COO H 3 N CH 2 CH COO H 3 N CH 2 CH 2 COO C COO O CH 2 CH 2 COO C COO O CH 2 + + oxaloacetate aspartate glutamate D -ketoglutarate Aspartate transaminase pyridoxal phosphate COO COO In these and all other transaminase reactions, glutamate and D -ketoglutarate are always involved and the enzyme is named for the other amino acid. Mechanism of transamination (involving pyridoxal phosphate) N H O 3 POH 2 C OH CH 3 C O H 2 CH COO NH 2 R H .. pyridoxal P N H O 3 POH 2 C OH CH 3 C OH H 2 CH COO N R .. N H O 3 POH 2 C OH CH 3 C H 2 O H 2 CC O O N R H H 1a 2a 3a 4a N H O 3 POH 2 C OH CH 3 C H 2 O O N R H .. alanine (R=CH 3 ) H
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3-3 N H O 3 POH 2 C OH CH 3 C H 2 CC O O N R H N H O 3 POH 2 C OH CH 3 C H 2 O O N R O H 5a 6a 7a 7b H O H H H H N H O 3 POH 2 C OH CH 3 C H 2 NH 2 H O O R' O N H O 3 POH 2 C OH CH 3 C H 2 O O N R' O H 6b H H H N H O 3 POH 2 C OH CH 3 C H 2 O O N R' H 5b H O H .. .. H N H O 3 POH 2 C OH CH 3 C H 2 O O NH 2 R O H .. pyruvate D -ketoglutarate (R=CH 2 CH 2 COO - )
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3-4 N H O 3 POH 2 C OH CH 3 C H 2 CC O O N R' H .. 4b N H O 3 POH 2 C OH CH 3 C H 2 O O N R' 3b H N H O 3 POH 2 C OH CH 3 C O H 2 CH COO N R' H 2b H N H O 3 POH 2 C OH CH 3 C O H 2 CH COO NH 2 R' pyridoxal P 1b For alanine transaminase, R = CH 3 (alanine to pyruvate) and R' = CH 2 CH 2 COO - ( D -ketoglutarate to glutamate). glutamate 2. D e a m i n a t i o n Oxidative (An oxidation-reduction reaction accompanies the deamination process.) The most common deamination is that of glutamate catalyzed by glutamate dehydrogenase . H 2 O H
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3 - 5 CH COO H 3 N CH 2 CH 2 COO glutamate C COO O CH 2 CH 2 COO D -ketoglutarate Glutamate dehydrogenase NAD + + H 2 O NADH + H + The reaction is reversible, but the enzyme has a high K M for NH 3 resulting in the enzyme operating as a deaminase at low [NH 3 ].
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Sec_3 - Amino Acid Catabolism 3-1 Lec#8 To date we have...

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