Cats don't Prefer Sweets

Cats taste the world very differently than humans

One thing that I found fascinating during my undergraduate studies was my Modern Biology class. It involved learning about and implementing numerous techniques that are rather new to the field of biology. Thanks to this class, I isolated a number of different cell components, ran various biological molecules through a gel electrophoresis machine, and performed polymerase chain reaction. I also took my own DNA and prepared it for sequencing, which involved refining and isolating the material, cutting out the gene we wanted to look at, then replicating it so that it could be sent off for the actual sequencing process to be performed.

The gene that we looked at was one of the two that go into making the receptors on your tongue that register sweetness. Specifically, the TAS1R2 (taste receptor, type 1, member 2) gene. TAS1R2 must join up with a second protein to signal that sweet flavor. Here is the TAS1R2 gene (specifically, my copy) in its three hundred fifty-five nucleotide entirety: 

GCTGCGTACCACACCCAGCGCCGACCACCACATCGAGGCCATGGTGCAGCTGATGCTGCACTTCCGCTGGAACTGGATCATTGTGCTGGTGAGCAGCGACACCTATGGCCGCGACAATGGCCAGCTGCTTGGCGAGCGCGTGGCCCGGCGCGACATCTGCATCGCCTTCCAGGAGACGCTGCCCACACTGCAGCCCAACCAGAACATGACGTCAGAGGAGCGCCAGCGCCTGGTGACCATTGTGGACAAGCTGCAGCAGAGCACAGCGCGCGTCGTGGTCGTGTTCTCGCCCGACCTGACCCTGTACCACTTCTTCAATGAGGTGCTGCGCCAGAACTTCACTGGCGCCGTGTGG

...not really that interesting.

This is your average TAS1R2 gene for Homo sapiens, nothing special. Just about every human out there has this exact sequence, with few exceptions (one of my classmates, for example, had a single point mutation). However, it is one gene crucial to why humans like sweets so much. This is what the gene looks like after it's been translated into its protein form (the letters are standard for amino acids):

Query  2    LRTTPSADHHIEAMVQLMLHFRWNWIIVLVSSDTYGRDNGQLLGERVARRDICIAFQETL  181
            LRTTPSADHHIEAMVQLMLHFRWNWIIVLVSSDTYGRDNGQLLGERVARRDICIAFQETL
Sbjct  82   LRTTPSADHHIEAMVQLMLHFRWNWIIVLVSSDTYGRDNGQLLGERVARRDICIAFQETL  141

Query  182  PTLQPNQNMTSEERQRLVTIVDKLQQSTARVVVVFSPDLTLYHFFNEVLRQNFTGAVW  355
            PTLQPNQNMTSEERQRLVTIVDKLQQSTARVVVVFSPDLTLYHFFNEVLRQNFTGAVW
Sbjct  142  PTLQPNQNMTSEERQRLVTIVDKLQQSTARVVVVFSPDLTLYHFFNEVLRQNFTGAVW  199 

This is a comparison showing my own protein versus the average human. It's a perfect match and codes for a fully functional receptor protein. This gene is seen in a very wide variety of different animals, and the shared genetic material is quite astounding, allowing for countless species to have the ability to taste sweetness.

A cat enjoying a fresh fish

So, why am I talking about humans a post about cats? Well, let's compare the sequence above to the equivalent gene in a cat. The "query"line is the human gene again, and the "sbjct" or subject line is the equivalent gene in Felis silvestris catus.

Query  2    LRTTPSADHHIEAMVQLMLHFRWNWIIVLVSSDTYGRDNGQLLGERVARRDICIAFQETL  181
            LRT P+ +H   AM  ++ +FRWNW+  + + D YGR   +   E    RDICI F E +
Sbjct  184  LRTIPNDEHQATAMADIIEYFRWNWVGTIAADDDYGRPGIEKFREEAEERDICIDFSELI  243

Query  182  PTLQPNQNMTSEERQRLVTIVDKLQQSTARVVVVFSPDLTLYHFFNEVLRQNFTGAVW  355
                 +Q    EE Q++V ++   Q STA+V+VVFS    L     E++R+N TG +W
Sbjct  244  -----SQYSDEEEIQQVVEVI---QNSTAKVIVVFSSGPDLEPLIKEIVRRNITGRIW  293

Cat tongue anatomy

Overall, the genes are fairly similar, but with one major difference. I'm not going to get too complicated with this, but the dashes you see indicate amino acids that are absent in the final protein found in cats but present in humans: a deletion. Changes in a protein don't necessarily result in a change in its functionality, but that deletion is enough to result in the cat's protein being non-functional. Since the protein cannot bond to molecules of sugars or sweeteners, cats simply cannot register the sensation of sweetness. This deletion seems to be unique to the cat family, since other carnivores, such as dogs and bears, have a functioning gene. From what I remember, big cats have this non-functioning gene as well, but it isn't known whether or not the cat allies (such as civets and genets) share this unusual feature.

Though cats aren't able to taste sweetness, this doesn't mean they will avoid it. They are, in fact, completely indifferent to the flavor. So, if your kitty likes something sweet, it's probably going after something other than the sugar. They're quite fond of certain amino acids, so it's possible that's what your kitty enjoys.

Sources Basic Local Alignment Search Tool (BLAST), PLOS Genetics, Journal of Nutrition, and GeneCards. Images are from Wikimedia Commons under Creative Commons licenses: one, two, three.

By the way, BLAST is an amazing tool. Feel free to plug in the ATCG sequence above into their databases to compare the gene for yourself. The programs I used for this post are nucleotide blast and blastx.